Elastic materials include metal plates, rubber, foam, airbags and so on, which have a good buffer effect, toughness and strong recovery ability. In this paper, the deformation and thermal diffusion of  Nowadays, more and more new materials have entered the field of vision of scientists. The study of deformable materials has become a hot topic in this field, and it is also very important in the design and application of materials. The mechanical changes such as deformation, tension, torsion and fracture of materials are the inherent mechanical properties of objects. The application of deformed materials in real life and industry is also relatively common. The biggest problems facing the deformation of materials are the accurate description of deformation variables and the physical deformation modeling containing temperature or material with crack defects. In addition, the deformation adaptability of different materials is low when the material has defects. Many scientific experiments show that temperature and deformation are closely related, and a multiphysics theoretical model of mechanical and thermal coupling is expected to be established. However, the theory of studying deformation problems in multiphysics models is still in the exploratory stage. Therefore, the current status also reflects the research value of our paper and the problems that need to be solved urgently. In this paper, the coupled physical model is used to solve the material deformation. The purpose of the research is to improve the accuracy of the model and expand the application range, so that the model we established can solve the deformation of the thin plate material in the high temperature environment. Our method provides a better theoretical basis for mechanical performance changes of plates in the thermo-mechanical coupled Fields and single fields.t = 1000 Recently, the 3D printing technology , robot smaterial . The supTemperature affects material deformation in many cases: at 250–450 eference  used theeference . With theference . The meceference  establisThere are many ways to study the influence of temperature on material deformation. One of them is the multi-physical field of mechanical-thermal coupling by FEM method to solve material deformation ,19,20. FThin plate bending is the most common physical phenomenon in engineering and life, which belongs to the knowledge of elastic mechanics ,25. WhenIn addition, the bending of the plate is related to material properties, external load and ambient temperature. On the analysis of the influence of thermal load on the dynamic bending response of the clamped plate, it can also be considered (WBM). The wave-based model of the dynamic plate bending problem under thermal load is established, and its convergence rate is higher than that of FEM . ThermalThe single-layer single deformation research in this paper also provides a good foundation for the deformation research of composite laminates. The deformation of laminates with cracks is a very challenging subject. There is sufficient experimental evidence that the propagation of cracks in brittle, isotropic and homogeneous materials makes the crack tip maintain the pure type I condition. The competition between the crack propagation in the interface and the bending out of the interface depends on the relative toughness of the material. For homogeneous isotropic solids, the role of the interfacial stress intensity factor is exactly the same as that in elastic fracture mechanics . We needE, load F, plate length L and deformation factor This paper mainly studies the deformation of 2D and 3D thin plates. We established a separate mechanical field, a separate temperature field and a thermal-mechanical coupling multi-physical model. This paper has five contributions: Our first contribution is that we analyze the causes of plate deformation, which is related to Young’s modulus The framework and content layout of this paper is composed of five sections. Thin plate deformation is an important branch of elastic mechanics, and an accurate model can be used to analyze the deformation of elastic body, which can be applied to practical material assembly design, safety assessment of industrial construction, mechanical products and so on . For theThe model generally needs to consider the basic Kirchhoff assumptions, the control equation of thin plate deformation, the boundary and the effect of thin plate deformation under different shapes and loads. Plate classification is generally based on the ratio of plate thickness to the minimum side (length or width). Specially thick plate and very thin plate, secondly, large deflection and small deflection corresponding to the physical model is different. At the same time, the ratio of deflection to plate thickness can determine which can determine the magnitude of the change. Small deflection is Of course, different deflections need to establish different physical models, and they also have different application scenarios. This paper provides the application range of common different deflections.(1) The application range of small deflection.The deformation range is relatively small, which can be generally used for the dust removal method of vibration, the auxiliary calculation of some buckling loads, and the calculation of small deflection of the recovery valve plate of automobile shock absorber. Based on the small deflection bending theory of thin plate, the buckling of shell and thin plate is analyzed. For the polar coordinate disk, the semi-analytical solution of the deflection equation can be obtained by using the generalized Fourier series solution.(2) Application of mid-deflection.The problem of mid-deflection calculation and modeling is encountered when the vehicle load is long-term acting on the bridge. The reasons for the formation of long-term deflection are analyzed. The cracks in the beam and the control of prestress play a key role. The numerical prediction of mid-deflection and the stability analysis of reinforced concrete under the action of mid-deflection are commonly used in the design of factory building structures.(3) Large deflection application.A large deflection of the elastic Timoshenko beam is a nonlinear PDE, and the soft soil tunnel analysis will be applied to the large deflection model. For the large deflection experiment of bridge, laser scanning can be used to obtain relatively accurate data. The small film material also has a large deflection phenomenon, such as the pressure sensor made of graphene. The numerical model belongs to the large deflection problem. With the increase of load, the elastic modulus and the number of graphene layers show a decreasing effect on the deflection deformation, but the natural frequency and modal characteristics still depend on the prestress, and are less affected by the film parameters.y. The displacement is commonly used to replace the stress tensor in the elastic equation, and the deflection is commonly used to replace the stress tensor in the thin plate bending model . Of. Of48]. E is the elastic modulus of the material, h is the thickness of the plate, v is the Possion’s ratio, and Therefore, the governing equation of bending deformation of elastic thin plate is a fourth-order bending equation, which is as follows:∇4w=−qDbe∇4w=−qDbeFor the deformation problem of a rectangular thin plate, it is necessary to specify at least two boundary conditions ,52. The (1) Clamped boundary (C).If the edge of the plate (2) Free boundary (F).If the elastic thin plate is completely free at According to the thin plate theory, these three boundary conditions can be combined and simplified into the following two boundary conditions, where Before solving the deformation of the 2D thin plate, we need to give the equation for the bending of the plate. As for the fourth-order bending differential equation of thin plate, this follows in Equation .(10)∂2wExample 1a.Considering a square rectangular metal plate, the edge length of the plate is a = 200 mm, the elastic modulus of the material E = 206 Gpa, and Poisson’s ratio is v = 0.3, assuming that the thickness of the thin plate h = 6 mm, the metal plate belongs to the isotropic material which satisfies the Kirchhoff assumption, and that the left side is fixed by the fixed boundary condition, which can be expressed as the following form:The right end of the square metal sheet subjected to vertical downward external load.Example 1b.Considering the bending deformation of a rectangular plastic strip (High pressure polyethylene), the elastic modulus of the material, the isotropic materials, the boundary conditions of length b = 160 mm, width d = 30 mm are also fixed at the left end, the initial condition for deflection is Formula .The right end is subjected to vertical upward force The deformation experiment for an elastic plate under the constant temperature Then, Equation  is discrEquation  is conveEquation :(15)DbenThe first right term of Equation  is furthThen, using the partial integration method and finishing the second item of Equation , Γ is thEquation .(17)∫∫ΩBring Equation  into EquFinally, we can obtain the final Galerkin integral formula as follows.The transverse shear force vector is denoted as: Equation .(20)DbeThe basis function Then, the linear equation can be obtained:The bending stiffness matrix of the linear equation element is denoted as:The right hand items of algebraic equations can be written as:Finally, the right term N represents the total number of elements, and i-th element. In Equation , N repreV of the steel plate on the scale mesh y direction of the rectangular elastic plate. Here, it should be emphasized that our red mesh is the initial mesh, and the green is the deformation mesh. There is a scale factor in the program, which can adjust the deformation according to the need.Two deformation numerical examples are solved by the model established in this paper. The numerical results of Example 1a are Considering the deformation of the circular plate under normal load, the polar coordinate r is the distance from the center of the circular plate, and Radius rotation ,54. UsinThen Formula  can be oSince the bending of thin circular plates also conforms to the fourth-order differential equation of bending in polar coordinates, it is necessary to apply the biharmonic operator to the deflection function L function in the formula is as follows:The specific form of the a is the radius of the plate, constant f is uniform on the whole circular plate, there is a suitable special solution:The analytical solution of the Fourth-Order deflection equation in polar coordinates can be solved semi-analytically method of Fourier series approximation, and the detailed solution process and derivation, please refer to The corresponding second-order homogeneous general solution is:The following contents will provide two specific numerical examples to solve the bending problem of circular plate. The semi-analytical Fourier series method and the other selective finite element numerical method are used respectively. We also compare the advantages and disadvantages of these two methods.a = 1 m, thickness h = 0.05 m, Young’s modulus E = 206 Gpa, Poisson’s ratio v = 0.3, density The geometry size of thin circle steel disc selected in this example: radius of disc is Equation  is to beThe side of the circular thin plate is fixed, the displacement boundary conditions are:Normal load The surface area The difference between Examples 1a and 1b is that the loading method is different. In this Example, the direction of external force load is parallel to the plate, and it can also be understood as loading external force load tangentially along the right boundary of the rectangle. The direction of external force load in this example is along the normal direction of the plate plane, that is, perpendicular to the plate plane.N of Fourier series, the higher the approximation accuracy is.The numerical results are as follows: N = 25. In addition, the larger the load, the larger the error is slightly larger, mainly because the deflection equation applicable to the small deformation problem, when encountering medium deflection and large deflection, the solution error will be large. U is basically close to 3, and the convergence order of stress The error is basically stable when the approximation term exceeds In order to study the variation law of internal temperature of two-dimensional elastic plate, this problem is equivalent to solving a 2D transient heat conduction equation. For some regular rectangular domains, they have analytical solutions, but this paper presents a special superposition region, and the analytical solution is difficult to obtain . TherefoIn order to study the adiabatic condition and temperature convection of plate problem. It is generally necessary to set the geometric solving domain and material parameters, and then make the mesh division of the geometric region. The selection of the mesh type is closely related to the characteristics of the solution domain. If the geometric region is a regular region, the mesh should choose uniform mesh as far as possible. If the calculation domain is an irregular region, it is necessary to use an adaptive mesh division. The reason is that the gradient value of the temperature field in the irregular region increases quickly, and the appropriate refinement of the mesh is beneficial to improve the numerical accuracy. Finally, it is due to the discretization of the heat conduction equation by FDM-FEM and the solution of the algebraic equation. In order to facilitate the comparison of the results, the numerical solution of the temperature field will be visualized. The following contents will give the theory of finite element combined with finite difference method for discretization of the two-dimensional transient heat conduction equation. The two-dimensional heat conduction equation can be expressed as:c is the specific heat capacity of the material (unit J/kg · °C). The materials studied in this paper are isotropic materials, so the thermal conductivity coefficient is the same as In Equation , ρ is th(1)The first boundary condition:The temperature function of the boundary is a known function:(2)The second boundary condition is the heat flux function:(3)Adiabatic boundary conditionsThree different temperature boundary conditions are expressed as follows: h is the heat exchange coefficient, the corresponding unit lis W/mk is the thermal conductivity. The above three cases contain the boundary conditions of different scenarios. The third type of boundary is also one of the focuses of our study. In addition, adiabatic conditions are applied in industrial insulation panels, insulation walls, cold chain vehicles, etc.Among them, The solution domain of any element can be denoted as Similarly, we selected the test function to be denoted by (1)Displaying Euler(2)Implicit Euler method(3)Crank–Nicholson methodBecause Equation  is a timComparison of three numerical methods: Although Displaying Euler is relatively simple and easy to calculate, the implicit scheme is more stable. The accuracy of the explicit and implicit descriptions of the time term is first order, and the CN scheme is second order. The accuracy of the numerical solution of the spatial term is related to the selection of the basis function and to the smoothness of the accurate solution CN is unconditionally stable. In general, the spatial term distribution of temperature is the focus of our study, and the time term error should be within an acceptable range. Ultimately, the implicit Euler scheme is used to discrete the time term, and the weak scheme formed by Q4 element discretization for the spatial term. The discretization process is as follows:x, y integrals on the region According to the variational method, the two sides of the equation are multiplied by the test function Using the first Green formula, combined with the second and third boundary conditions, it is easy to get the following Equation :(51)∮ST*After moving the right term of Equation , we can Equation  is simplThen we use the matrix Equation  to repreAmong them, The matrix form of the right term is This example mainly simulates the 2D transient thermal conduction of single layer graphene thin film material; graphene has a wide range of application: such as, supercapacitance (graphene battery has large storage capacity), flexible electronic screen (graphene has better conductivity), loudspeakers , seawater purification (graphene has a good adsorption effect) ,60. In ah = 5000 W/mE = 0.74 Tpa of monolayer graphene. The high strength graphene film (tensile strength is about 400 MPa) also has an in-plane negative Poisson’s ratio, and the Poisson’s ratio is at v = −0.25∼0.55. After understanding the basic physical parameters of graphene films, according to the law of heat transfer, the heat transfer law of monolayer graphene can also be described by two-dimensional transient heat conduction:This example simulates the temperature field distribution inside the film when the single-layer graphene material is subject to different boundary conditions. The geometric appearance of the graphene film designed in this example is two square nested patterns, and there is no overlapping area in the middle. It can also be seen as a regular shape with a side length of The graphene solution region designed in this example has eight boundary conditions, involving three types of boundary conditions.(1) Convection conduction:The boundary (2) Dirichlet boundary conduction:Boundary (3) Adiabatic boundary condition:Boundary t = 0.45 s. It can also be seen from the figure that the boundary According to the FDM-FEM numerical method given in this paper, the solution steps are also drawn according to the geometric region, and the material parameters are set. Then, the mesh is divided. This example uses the LST mesh element type, which can improve the accuracy of approximation. Compared with the CST element, the convergence speed of LST is faster. The standard details of mesh and boundary information are shown in t = 0.245 s, we can see in t = 0.8 s. In this figure, only 0.8 s time can be seen, and the whole film temperature has been heated to about 600 When the two-dimensional transient heat conduction equation is used to solve the temperature change of graphene film, the temperature distribution functions corresponding to different time are different. When the time This example mainly considers the deformation of three-dimensional monocrystalline silicon non-metallic thin plate at different temperatures. Since monocrystalline silicon is the most important raw material in electronic products, it plays a huge role in chip manufacturing, semiconductors, solar panels and other aspects. This example mainly studies the influence of temperature on the deformation of monocrystalline silicon, which will change the internal material structure and mechanical properties of monocrystalline silicon semiconductor materials when they work at high temperature . TherefoL = 3[in] = 0.0762 m, the width is W = 1[in] = 0.0254 m and the height is h = 275 um = 2.75 × 10The common wafers of monocrystalline silicon are 8 inches and 12 inches. In this example, a rectangular 3D monocrystalline silicon thin layer is given as the research object. The specific geometric parameters and material parameters are given below. For this rectangular monocrystalline silicon, the length is x axis direction. If the angular velocity A = 1, and the initial phase In this numerical example, the change of temperature field is simply considered, and the single crystal silicon wafer is heated by a laser probe ,65. The In addition, the temperature of the laser center is generally the highest, and the temperature heat flux of the surrounding coordinate points is Gaussian distribution. The radius of the laser radiation area is calculated as follows:Laser temperature heat flux formula:x, y and z directions is the same, which satisfies the The following 3D transient heat conduction equation of single crystal silicon thin plate is given. Through this equation, the temperature change at any point in the solution domain can be calculated ,67. The Further processing Equation , we can c is the specific heat capacity J/(kg·k) of the material, and the plate needs energy when it is heated. Q is the heat generated by the internal heat source. x, y, and z directions, respectively. x, y and z directions in a unit time, respectively. The heat transferred into the object is balanced with the heat transferred out of the object when the object is heated. In addition, the temperature field distribution of the solution domain For Equation , ρ is th(1) The initial temperature of solid surface is 298.15 K, which can be expressed as follows.(2) Adiabatic conditions:In this example, considering that the laser prohibits heat loss while heating the silicon sheet and does not allow heat loss around the sheet, we can consider limiting a computational physical area where Equation :(67)k∂T∂The solving domain of monocrystalline silicon thin plate is denoted as The physical region of adiabatic calculation of single crystal silicon plate is set as (3) If Then, this example also studied the deformation of single crystal silicon wafer at different temperatures under the same load, which is the varying elastic model is considered in the experiment. With the increase of temperature, the elastic modulus decreases, and then the deformation becomes larger. The three-dimensional transient elastic equation can be expressed as:Stress Equation , shows:, the strain in a certain direction is simulated, and the strain In Equation , Ly is tEquation , we can According to the above theory, we can obtain the temperature and deformation of the single crystal silicon thin plate. This physical coupling model belongs to one-way coupling, that is, the increase of temperature has a great influence on the deformation of the material thin plate, but the temperature change caused by the deformation can be ignored. The temperature affects the deformation mainly through the elastic modulus, and the thermal strain is related to the expansion coefficient of the material . At prest= 30 s, and the two temperature vortices are mainly the reciprocating motion of the probe. The temperature did not form uniformly in time. t = 45. It can also be seen from the figure that the laser heating probe is at the right end of the thin plate, and the thermal diffusion phenomenon is occurring in the high temperature region at the left end.Through the three-dimensional transient heat conduction equation established above, combined with the geometric parameters and material parameters of single crystal silicon material, the corresponding temperature and shape variables at different times are calculated. Finally, the numerical results of thermodynamic coupling calculation are shown in E of the material will be directly affected. According to the numerical conclusion mentioned above, the Young ‘s modulus E will decrease with the increase of temperature, and then the deformation of the thin plate will also change differently. Meanwhile, x direction, which acts on the stress field with the same amplification factor E increases and the deformation increases in theory. The above numerical realization also reflects the idea of the coupling of temperature field and solid mechanics, the advantages of simulation experiments can reduce the cost and quickly simulate the material.Then, the coupling model of temperature and force field is considered in our numerical experiment to study the influence of temperature on the deformation. The numerical implementation limits the single crystal silicon thin plate to be fixed for one section, and loads 100 N downward force for one section. Then, the temperature ranges from 300 K to 600 K. By increasing the temperature around the thin plate, the Young’s modulus Under the combined action of loading conditions and environmental factors, micro-holes, micro-cracks and other micro-defects will occur in the material plate. In the continuous damage mechanics theory, the representative volume element (RVE) is taken as the research object, and it is considered that the total cross-sectional area in a certain direction of RVE is denoted as S, and the total micro-defect area is c is the equilibrium concentration of interstitial atoms at high temperatures will change the grain boundary energy, different grain boundary energy corresponding to different grain boundary damage degrees, the damage degree of material caused by temperature can be represented by the volume fraction of grain boundary damage.In the high temperature environment, the internal crystal structure of the sheet material deviates from the microscopic region of the complete lattice arrangement, forming point defects, line defects and surface defects ,72,73. cG of the material, Poisson’s ratio v and the Birth’s vector Then, we estimate the damage degree of temperature to the material by defining different grain boundary energy. P and the damage of temperature The effective bearing area E is the elastic modulus with damage. The elastoplastic constitutive model used in this paper can consider isotropic and kinematic hardening [In addition, the elastic modulus of damaged RVE is defined as ardening ,75. In tv is Poisson’s ratio, Among them, F of the damage coupling is:In addition, the yield function N is the number of back stress components, In the above formula: Griffith, a British scientist, first applied the energy principle method to analyze the fracture of brittle materials such as glass and ceramics, and established the relationship between brittle stress, crack size and material properties, which laid the theoretical foundation of fracture mechanics. There are microcracks in brittle materials, and the stress concentration at the tip of microcracks greatly reduces the fracture strength of materials. For a certain size crack a, there is a certain critical stress fracture ,77. The (1) Type I cracks (Opening displacement).r from the crack tip, the angle with the x-axis is This crack model is established under the condition of tensile force The stress can also be written as:(2) Type II cracks (Sliding displacement).Three elements of fracture are crack size and shape, stress size (necessary) and fracture toughness of a material  . A measur and The third type of crack has an intensity factor is Equation .(99)σijType I and Type II are for plane cracks, and Type III for three-dimensional cracking problems. Similarly, the third type of crack belongs to tearing, and the corresponding crack stress fields are Equations  and 101101, TypeIn Cartesian coordinates, the following relations for the stresses In spatial mixed crack problems, including surface, edge and internal cracks, general loading can lead to overlapping of all three crack loading types . In thisAs opposed to the plane solution, now an additional stress intensity factor fijIII(φ). The 1/rFurthermore, the crack tip stress intensity factors are defined by crack tip stress The establishment of the above model lays the foundation for the analysis of material plates with damage under high temperature. The final coupled stress field should be formed by the coupling of temperature damage stress, crack stress and elastic equation stress.The main reason for the discussion in this theory is that the range of some parameters of the model need to be determined by experiments, so the obtained model is more worthy of promotion and is also in line with scientific rigor. However, the proposed coupling model theory provides a good foundation for the subsequent study of the deformation of the material plate with defects in the coupling field. The model can basically reflect two scales, the damage stress caused by temperature is established from the micro crystal, and the crack and elastic stress belong to the mesoscopic scale. We will further complete the numerical deformation of the material with defects in the future.This paper is mainly focused on solving the bending deformation and thermal diffusion problems of 2D and 3D thin plates. For the 2D thin plates, we have derived the bending deformation equation of rectangular and circular plates. We calculated the deformation process of square and rectangular thin plates at different mesh scales. The results show that the factors affecting the deformation are related to the Young’s modulus, load, plate length and deformation factor of the material. As for the semi-analytical solution of the deflection function, In order to accurately describe the relationship between temperature and variable force, a 3D dynamic thermal-mechanical coupling model (3D-DTMCM) is proposed. We found that Young’s modulus decreases with increasing temperature, and a model of the laser heating monocrystalline silicon sheet with a periodic motion formula is given. The temperature radiation of the laser heat source can be reflected by the Gaussian distribution characteristics. Under the condition of constant external force load and laser heating for 1 min, the numerical results show that the Young’s modulus decreases with the increase of temperature, but the strain energy density increases slightly with the nonlinear increase of temperature. In addition, the deformation amplitude of the plates in the coupling field is larger than that in the single mechanical field. Finally, a deformation model of the material plate with defects is established, and we also consider comprehensive factors such as high temperature, load, internal mixed cracks, etc. This model will be our future work. In short, our research provides theoretical support for the deformation of different plates and also reflects the value of the coupled model in practical applications. This paper promotes the basic theory and numerical simulation of the mechanics of elastic materials, which also expands the application range of elastic deformation. Our study will provide a new insight into the influence of temperature on the deformation of 2D and 3D materials. Meanwhile, the 3D deformation theory of a multi-physical field model is given, which is advocated for by the physics community and industry currently, which is worth further discussion and study.


Nonribosomal peptide synthetases (NRPSs) employ multiple domains, specifically arranged in modules, for the assembly‐line biosynthesis of a plethora of bioactive peptides. It is poorly understood how catalysis is correlated with the domain interplay and associated conformational changes. We developed FRET sensors of an elongation module to study in solution the intramodular interactions of the peptidyl carrier protein (PCP) with adenylation (A) and condensation (C) domains. Backed by HDX‐MS analysis, we discovered dynamic mixtures of conformations that undergo distinct population changes in favor of the PCP‐A and PCP‐C interactions upon completion of the adenylation and thiolation reactions, respectively. To probe this model we blocked PCP binding to the C domain by photocaging and triggered peptide bond formation with light. Changing intramodular domain affinities of the PCP appear to result in conformational shifts according to the logic of the templated assembly process. Translocation of the peptidyl carrier protein (PCP) between two catalytic domains in nonribosomal peptide synthetases (NRPSs) has been studied for the first time. Monitoring PCP interaction with A and C domains during catalysis using two FRET sensors of an elongation module revealed conformational population shifts that correlate with the directionality of synthesis. The first light‐controlled NRPS was created to further probe and support this model. Nonribosomal peptide synthetases (NRPSs) are involved in the biosynthesis of many pharmaceutically important bioactive peptides. The engineering of their modular structure holds great potential for the generation of new products.N and small AC subdomains relative to one another along the adenylation and thiolation reactions of the aminoacylation pathway, thereby adopting at least an open (O), an adenylation (A), and a thiolation (T) conformation.One of the intriguing questions about the protein‐templated NRP biosynthesis is how the catalytic and conformational interplay of NRPS domains and modules is facilitated in space and time to result in the directional synthesis of a specific product. NRPSs domains and modules are known to undergo some significant conformational changes. Static snapshots in crystal structures have shown that the PCP has to translocate over long distances to reach the different catalytic domains.We have previously explored intramolecular FRET spectroscopy and HDX‐MS analysis to study conformational changes in NRPSs.n)( substrate as extender unit at its acceptor site and the incoming upstream aminoacyl‐ or peptidyl‐S‐PCPn−1)( from the preceding module at its donor site.The C domain is part of an elongation module (domain composition: C‐A‐PCP). It binds its aminoacyl‐S‐PCPIn this study, we addressed the question how a PCP shuttles between the A and C domains as competing binding partners. We designed a set of novel FRET sensors of an elongation module to report on the PCP interactions with both the A and the C domain. Corroborated by an HDX‐MS analysis and a novel approach to control the PCP‐C interaction by light, we identified a conformational shift of the aminoacylated module towards the C conformation. This study expands our understanding of the dynamics of conformational changes in NRPS during catalysis.N subdomain organize two well‐separated binding sites for the PCP domain at the A and C domains  that are associated with the aminoacylation reaction. We envisaged FRET spectroscopy as an in‐solution technique to unravel domain interactions and selected the second module of the tyrocidine NRPS, TycB1, for our studies . We designed a C‐A*‐PCP* sensor to report on the transfer conformation, representative for the PCP‐A interaction . Thiol‐bioconjugation with maleimide dyes Alexa Fluor®555 (AF555) and Alexa Fluor®647 (AF647) as FRET donor and acceptor dyes,The protein mutants were expressed in The TycB1 sensors were biochemically characterized and shown to exhibit enzymatic activities of virtual wild‐type levels. Both the holo‐C‐A*‐PCP* and holo‐C*‐A‐PCP* sensors were indistinguishable from the unmutated holo‐TycB1 module in the virtually quantitative ATP‐dependent aminoacylation reaction with the substrate L‐Pro, which rapidly occurred within ≤1 min Figure C and S4CIA/ID) and normalized it to the respective ratio of the buffer‐only conditions in the absence of substrates .To investigate the interaction between PCP and A domains in the TycB1 elongation module, we performed time‐dependent bulk FRET experiments with the holo‐C‐A*‐PCP* sensor. The formation of the prolyl‐TycB1 thioester triggered by addition of the substrates ATP and L‐Pro . Second, upon completed aminoacyl thioester formation, the transfer conformation is depopulated, i.e. PCP detaches from the A domain .increase to plateau P1, indicative of increased proximity between the Pro‐S‐Ppant‐PCP and the C domain  ATP as a single substrate, 2) L‐Pro+AMP, and 3) ATP+L‐Pro+PPi disrupts the transfer conformation, this finding suggests that desulfo‐A*‐PCP* does not populate the transfer conformation in the absence of substrates, in contrast to desulfo‐C‐A*‐PCP*.The first two conditions represent ligand combinations expected to mostly favor the closed A conformation of the A domain. Therefore, they should concomitantly disfavor the transfer conformation, which requires the A domain in T conformation, and which shows a basal population in the absence of ligands (see above). As a consequence, PCP is released from the A domain and therefore more likely to bind to the C domain, thus favoring the C conformation. These considerations explain the decreased and increased FRET ratio responses of the C‐A*‐PCP* and C*‐A‐PCP* sensors, respectively. In contrast, the A*‐PCP* sensors showed no significant responses to the first two conditions. This latter finding may be explained by the missing C domain, because without the C domain also the C conformation is removed from the conformational space of these sensors. The remaining conformations like open, A and transfer conformations are thus likely to adopt altered populations compared to the C‐A‐PCP proteins, both in the absence and presence of ligands, resulting in different FRET sensor readouts. In support of these considerations, an altered conformational equilibrium in the absence of the C domain is obvious from the experimental data for the desulfo‐A*‐PCP* sensor. Here, P2 is about the same as P0, whereas P2<P0 for desulfo‐C‐A*‐PCP*. Considering that excess PPi is present at excess concentrations when the enzyme has the L‐Pro‐S‐Ppant thioester. As reported previously, PPi binding reverses the domain alternation mechanism and disfavors the transfer conformation by competing with a conserved Glu‐Arg salt bridge between the AC and AN subdomains (E761 and R873 in TycB1).[i by adding pyrophosphatase (PPase) showed the reversibility of this effect . In cases of the holo‐sensors, the PCP may still bind to the A domain owing to the affinity of the aminoacyl thioester to the active site. However, it can do so only in the postulated, half‐open Ppant‐threading conformation[i . The observation of a FRET ratio increase (P2>P1) in these cases for the transfer conformation sensors (holo‐C‐A*‐PCP* and holo‐A*‐PCP*) and of a decrease for the C conformation sensor (P2<P1 for holo‐C*‐A‐PCP*) suggests a higher stability or higher FRET signaling capability of the Ppant‐threading conformation compared to the transfer conformation. The TycB1 sensors labeled at PCP and A domains thus report on both the transfer and the Ppant threading conformations, which is consistent with the overall quite similar positioning of the two domains in these conformations.[In the third condition, PPn TycB1).4, 5 Rwithin these conformations between different modules can influence the FRET read‐out .We performed HDX‐MS experiments to independently investigate the proposed conformational shift towards the C conformation upon aminoacylation of TycB1. The HDX rate depends on the accessibility of the amide protons within the peptide chain. Differences in HDX between two experimental settings may reflect perturbations in conformation. Briefly, we incubated wildtype holo‐TycB1, pre‐incubated for 20 min with or without ATP/L‐Pro, in DN and AC subdomains. Notably, both the A8 and A10 motifsC subdomain and represent the key catalytic motifs for the A domain's T and A conformations, suggesting that both conformations are simultaneously favored compared to the buffer condition. This finding further supports the ideaWe observed differences in HDX between the two settings for 22 sequence stretches, all of which became less accessible for HDX in the presence of ATP/L‐Pro compared to the buffer control without substrates ,We next sought to probe our model by specifically and reversibly perturbing the system on the conformational level and by monitoring this artificial control by the FRET readout. We envisioned to sterically block the PCP entry to the C domain's active site with a photolabile caging group, thereby preventing or reducing the ability of the elongation module to adopt the C conformation required for dipeptide formation with an incoming initiation module Figure A. Remova#‐PCP#(ONBY) mutants showed a relative reduction in activity of about 98±0.2 % and 52±10 %, respectively, and similar values were obtained for the mutants of holo‐C#‐A‐PCP# .3b, 19decrease upon treatment with 365 nm. This finding was consistent with an ONB‐caused increased population of the transfer conformation before the light‐induced restoration of the native conformational equilibrium under these conditions  sensors under aminoacylation conditions with ATP and L‐Pro a FRET ratio increase upon UV irradiation Figure D, consisFinally, these findings provided even further corroboration of our FRET sensor design and associated conformation assignments.We report the first in‐solution conformational analysis of a full‐length NRPS module under catalytic conditions to reveal the multidomain interplay. The investigated TycB1 construct is a canonical elongation module composed of C‐A‐PCP domains. To investigate the interactions of the PCP domain with the A and C domains, we designed two sets of FRET sensors to reciprocally report on the proximity of the respective protein pairs in bulk FRET experiments. With further corroboration from HDX‐MS measurements and defined pertubations using steric blocking with photolabile caging groups, we could correlate the FRET sensor read‐outs with conformational changes and interpret the data. Previous findings from a simpler didomain A‐PCP sensor showed that several, if not all, possible conformations can exist in a dynamic mixtureIn general, one would assume that substrate binding or a catalytic step would trigger a conformational shift towards the conformation for the next reaction to be catalyzed. However, in principle it would also be conceivable that the required conformation was dynamically populated already beforehand at sufficient or even dominating levels, in which case only a marginal or no overall such conformational shift would be observable in bulk experiments. In the present work on the C‐A‐PCP elongation module we unraveled two conformational shifts associated with the two‐step aminoacylation reaction that indeed were consistent with the expected NRPS logic. The transfer conformation of the PCP‐A interaction was favored upon formation of the aminoacyl adenylate and subsequently the C conformation of the PCP‐C interaction was favored upon formation of the aminoacyl‐S‐Ppant‐PCP thioester. Aminoacylation of the PCP appears to increase its affinity for the C domain's acceptor position.Our findings of the conformational shift towards the C conformation also provide direct experimental support for the hypothesis that the interaction between the C domain and aminoacylated PCP helps to prevent mis‐initiation at internal modules, as this binding event would disfavor the premature translocation of the PCP to the donor site of the downstream C domain. However, to fully understand this control of directionality in an elongation module, the downstream C domain of the following module as the remaining interaction partner of the PCP must be included in future experimental designs.We could further refine our proposal of fine‐tuned affinities of the aminoacylated PCP by obstructing the entry site at the C domain with a sterically demanding photolabile caging group and thereby modulating the affinity of the PCP‐C interaction. Using FRET measurements, we confirmed the decreased binding of the aminoacylated PCP at the blocked C domain. Concomitantly, we could monitor an increased population of the transfer conformation, in which the aminoacylated PCP binds to the A domain. This finding demonstrates that the A domain retained the affinity for the aminoacyl‐PCP. The emerging model of shuttling the aminoacylated PCP between two competing catalytic domains based on affinities thus appears to be self‐sufficient in the sense that it does not require additional allosteric communications between the domains, although we cannot rule these out at present as an additional layer of regulation.More generally, our insights highlight the importance of the domain‐domain interactions when aiming to construct hybrid NRPS by domain or module swapping experiments. They may help to explain why such experiments can fail and how they could be rescued by optimizing domain contacts.Finally, our novel design of a photoactivatable NRPS assembly line may spur novel applications in synthetic biology approaches.The authors declare no conflict of interest.1As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.


The photoluminescence studies demonstrate that the AM PD is amplified in the presence of excipients and an alkaline medium. In this last case, the PD process of AM involves the emergence of new compounds, whose presence is highlighted by: (i) the emergence of the isosbestic point at 300 nm in the UV-VIS spectra; (ii) a change in the ratio between the absorbance of IR bands situated in the spectral ranges 1200–1660 and 3250–3450 cm−1; and (iii) a change in the ratio between the intensities of the Raman lines localized in the spectral ranges 1050–1800 and 2750–3100 cm−1. A chemical mechanism of the PD processes of AM in an alkaline medium is proposed.New aspects concerning the photodegradation (PD) of ampicillin are reported by photoluminescence (PL), Raman scattering and FTIR spectroscopy. The exposure of ampicillin in the absence (AM) and in the presence of the excipient (AMP) to UV light leads to an intensity diminution of the photoluminescence excitation (PLE) and photoluminescence (PL) spectra and the emergence of a new IR band at 3450 cm Ampicillin is one of the earliest antibiotics discovered in 1958 , respira3O4 nanoparticles decorated with Au [2 [2nanofibers [Photocatalytic degradation of ampicillin was studied in the presence of various compounds such as Fe with Au , magneti with Au , TiO2 [1th Au [2 , polylacnofibers , and so According to −1, they are attributed to the vibrations of deformation in plane of phenyl + torsion HNCCl in latame/amide + stretching Cl in aliphatic group; torsion CNCO in lactam + deformation COO− + stretching SCl form II; deformation COO− + stretching C–C in aliphatic + deformation out-of-plane in phenyl; deformation C–H–C(CH3) + stretching C–C(CH3) + stretching C–N in lactam/amide + deformation H–C–N in lactam; streching C–C + deformation H–N–C(NH3+) + deformation in-plane C–H in phenyl + deformation H–C–N in lactam; symmetrical stretching COO− + stretching C–Cl in phenyl + deformation in plan C–H in phenyl + stretching CN in amide/lactam + deformation H–C–C(CH3) + deformation H–N–C in amide + deformation NH3+; deformation CH2; deformation C–H in phenyl + amide II + deformation NH3+ + asymetrical stretching COO−; amide I + deformation N–H; stretching C=O in lactamic ring; stretching C–H in aliphatic; and stretching N–H, respectively [The changes in the intensity of PL and PLE reported in ectively .−1; (ii) the change in the absorbance ratio of the IR bands peaked at 1774, 1693–1695, 1525 and 1308 cm−1  from 1.18, 1.39 and 1.42  the emergence of a new IR band with a maximum at 3445 cmand 1.42 a to 1.31and 1.42 b; and  the PL spectra intensity is changed to 1.51 × 104 counts/s and 8.2 × 105 counts/s. Knowing these changes, The PLE spectra of AM and AMP show a band at 351–352 nm with an intensity equal to 8.55 × 10counts/s . The UV counts/s : (i) an (i)1), 361 nm (1) and 348 nm (1), its intensity is equal to 3.7 × 107 counts/s (1), 4.46 × 107 (1) and 6.85 × 107 (1); the UV exposure of the samples induces a shift of the PLE band to 375 nm (1), 352 nm (1) and 356 nm (1); the intensity is equal to 3.44 × 107 counts/s (1), 3.17 × 107 counts/s (1) and 5.09 × 107 counts/s (1);The PLE spectra show a band at 363 nm a1, 361 n, 361 nm b1 and 34d 348 nm c1, its icounts/s a1, 4.46 46 × 107 b1 and 6.85 × 107 c1; the Uo 375 nm a1, 352 n, 352 nm b1 and 35d 356 nm c1; the icounts/s a1, 3.17 counts/s b1 and 5.counts/s c1;(ii)2), 430 nm (2) and 439 nm (2) with an intensity equal to 2 × 106 counts/s (2), 1.6 × 106 counts/s (2) and 7.69 × 105 counts/s (2); after the UV exposure of the three samples, the PL spectra intensity is equal to 1.64 × 106 counts/s (2), 1.11 × 106 counts/s (2) and 2.12 × 105 counts/s (2).The PL band is peaked at 431 nm a2, 430 n, 430 nm b2 and 43d 439 nm c2 with acounts/s a2, 1.6 ×counts/s b2 and 7.counts/s c2; aftercounts/s a2, 1.11 counts/s b2 and 2.counts/s c2.Careful analysis of 1), 362 nm (1) and 360 nm (1), the intensity is equal to 1.08 × 107 counts/s (1), 1.65 × 107 counts/s (1) and 2.5 × 107 counts/s (1); after the UV exposure, the PLE bands are localized at 373 nm (1), 362 nm (1) and 353 nm (1) with the intensity equal to 2.17 × 107 counts/s (1), 2.95 × 107 counts/s (1) and 3.67 × 107 counts/s; (ii) the PL spectra show an emission band peaked at 430 nm (2), 444 nm (2) and 450 nm (2) with the intensity equal to 1 × 106 counts/s (2), 6.85 × 105 counts/s (2) and 6.02 × 103 counts/s (2). The UV exposure of the samples induce an intensity increase in the PL band up to 1.71 × 106 counts/s (2), 1.09 × 106 counts/s (2) and 2.15 × 104 counts/s (2)., 362 nm b1 and 36d 360 nm c1, the icounts/s a1, 1.65 counts/s b1 and 2.counts/s c1; aftert 373 nm a1, 362 n, 362 nm b1 and 35d 353 nm c1 with tcounts/s a1, 2.95 counts/s b1 and 3.t 430 nm a2, 444 n, 444 nm b2 and 45d 450 nm c2 with tcounts/s a2, 6.85 counts/s b2 and 6.counts/s c2. The Ucounts/s a2, 1.09 counts/s b2 and 2.counts/s c2.These changes are induced by the emergence of a hydrolysis compound under UV light. In order to sustain this sentence, As observed in −1; these are associated to the vibrational modes of out-of-plane deformation of benzene ring, deformation in the plane of phenyl + stretching C–C aliphatic + deformation of the C–N–C bond in lactam structure; deformation of N–C–O in lactam + rotation C–N–C–O in lactam + deformation in plan of phenyl; stretching C–C in aliphatic + deformation in plan O–C–O–C+ deformation C–C–C in lactam chain + deformation out-of-plan C–H in phenyl; deformation C–N–C in amide, lactame + stretching C–C in lactam+ deformation out-of-plan C–H in phenyl + deformation H–C–C–H(CH3); deformation H–C–C in phenyl + deformation out of plan C–H in phenyl + stretching C–C in phenyl + deformation H–C–C–H(CH3) + deformation NH3+; deformation in plan H–C–C in phenyl + deformation C–H + stretching C–C; deformation H–N–C in lactam/amide + stretching C–N in amide + deformation H–C–C + deformation NH3+; stretching C–C + deformation H–N–C(NH3+) + deformation H–C–N in lactam; deformation in plan C–H in phenyl + deformation H–C–C(CH3) + stretching C–N in amide/lactam; stretching C–Cl in phenyl + deformation C–H in phenyl + deformation CH3/CH2; stretching C–Cl in phenyl + deformation C–H in phenyl +deformation NH3+; amide I + deformation N–H; stretching C=O in lactamic ring; stretching C–H in aliphatic; stretching C–H in aromatic ring and stretching N–H [In order to explain the chemical processes assisted of UV light, hing N–H . −1; (ii) the emergence of new Raman lines peaked at 1080, 1394 and 1630 cm−1; (iii) a down-shift of the Raman line from 1188 cm−1 to 1184 cm−1; (iv) a significand intensity decrease in the Raman line at 2939 cm−1 so that the intensities ratio of the Raman lines from 2941 to 2939 and 3064 to 3062 cm−1 (I2941–2939/I3064–3062) varies from 16.16  and NaOH were purchased from Sigma-Aldrich. The drug marked under the name Ampicillin (abbreviate AMP) was bought from a local pharmacy. The composition of an AMP tablet was 500 mg AM, magnesium stearate and talc. Both AM and excipients were stocked in two capsules of yellow and red color containing quinoline yellow, TiOThe aqueous solutions of AM and AMP with the concentration of 50 mg/mL and NaOH 0.3 M were prepared and mixed in the volumetric ratio equal to 2:1, 1.5:1.5 and 1:2. The AMP PD process was studied by removing the two capsules, the powder inside them consisting of AM, magnesium stearate (MS) and talc was dispersed/dissolved in water by the ultrasonication and then removing of talc and MS by filtration was achieved. Elimination of MS and talc allows a correct assessment of the influence of NaOH on aqueous solutions of ampicillin prepared from compounds purchased from Sigma-Aldrich (AM) and the local pharmacy (AMP).The UV-VIS spectra of AM interacted to NaOH were recorded with a UV-VIS-NIR spectrophotometer, Lambda 950 model, from Perkin Elmer, and the scan speed was 266.75 nm/min.The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of AM and AMP were recorded with an FL3-22Fluorolog spectrometer, from Horiba Jobin Yvon, in right-angle geometry, with a Xe lamp as excitation source with the power of 450 W. The excitation and emission wavelengths used for the recording of the PL and PLE spectra were equal to 330 and 500 nm, respectively, and the integration time was 0.5 s.−1. Other experimental conditions used in this order consist of: (i) a laser power of 50 mW; (ii) a scan number of 200.Raman spectra of AM were recorded with a Bruker MultiRam FT Raman spectrophotometer, with a YAG:Nd laser as the excitation source (the wavelength excitation was 1064 nm), in backscattering macroscopic geometry, with a resolution of 1 cm−1.IR spectra of AM were recorded with a Vertex 80 FTIR spectrophotometer from Bruker; in the attenuated total reflection geometry, the scan number was equal to 64, and the resolution was 2 cm2 and then dried at 100 °C, under vacuum, for 1 h. Crystallized powders were collected and used in the studies by the Raman scattering and the IR spectroscopy.The samples analyzed by the IR spectroscopy and the Raman scattering were prepared using 1 mL from the solutions of AM and NaOH-reacted AM, which were previously photodegraded; these were deposited onto a Si plate with a length x width of 1 cmDiffractogram of AM was recorded with a D8 Advanced Bruker X-ray diffractometer, using CuKα radiation (λ = 1.54056 Å).−1.In this article, new results by photoluminescence, FTIR spectroscopy, Raman scattering and UV-VIS spectroscopy were reported for the PD of AM and AMP induced by NaOH. These results allow us to conclude that: (i) AM used in AMP shows a trihydrate crystalline structure; (ii) the variation in the intensity of the PL and PLE spectra of the AM and AMP has suggested that a PD took place; (iii) the interaction of AM with NaOH induces the emergence of new compounds highlighted by the appearance of an isosbestic point in UV-VIS spectra; (iv) according to Raman scattering, the PD products of the NaOH-reacted AM contain the functional groups of the type -COONa, C–N–C in amine and C=O, which were highlighted by the Raman lines at 1080, 1394 and 1630 cm


In the initial stage of compression (elastic deformation), the stress and strain are proportional, and the stress–strain curve is a straight line. In the plastic deformation stage, the flow stress decreases significantly with the increase of deformation temperature, while the strain rate has no significant effect on the flow stress during dynamic compression. A constitutive model has been established to predict the flow stress, and the relative error is 2.32%. It is shown by observing the microstructure that when the deformation temperature is 293 °C, and the strain rate reaches 1600 s−1, a shear band with an angle of about 45° to the axial direction of the specimen appears, and the severe shear deformation makes the α phase in the shear band fibrous and contains high-density dislocations. The formation process of the shear band and its influence on fracture are analyzed and discussed.The quasi-static and dynamic impact compression tests of the TA31 titanium alloy were conducted at the strain rates from 0.001 s TA31 is a near α titanium alloy, and its nominal composition is Ti-6Al-2Zr-1Mo-3Nb. TA31 titanium alloy has good weldability and plastic toughness in addition to the common excellent properties of titanium alloys, making it an ideal titanium alloy material for ships. In the application process of these fields, titanium alloy as a structural material may bear complex dynamic loading. The practice shows that under the impact load, the mechanical behavior of the material is quite different from that under quasi-static and low strain rates conditions ,5, and u−1. Ran et al. [There have been some reports on the dynamic response and deformation mechanism of titanium alloys under dynamic shock loading. Liu et al.  studied n et al.  found thn et al.  studied In this paper, the experimental technique of split Hopkinson pressure bar was applied to TA31 titanium alloy, and the deformation behavior of TA31 titanium alloy under high-speed impact loading was analyzed and modeled, especially the microstructure evolution and fracture behavior of TA31 titanium alloy during impact deformation were studied.The TA31 titanium alloy used in this study was a bar with a diameter of 25 mm. The manufacturing process of the bar was as follows: first, the ingot was prepared by three times of vacuum consumable melting, then the ingot was heated to the β-phase field and forged, and the final forging temperature was in the α + β phase field. The chemical composition of the TA31 titanium alloy is given in −1 on an electro-hydraulic servo universal testing machine MTS-370-250 KN. The dynamic impact tests were performed using a split Hopkinson pressure bar apparatus. Because the service temperature of TA31 titanium alloy components generally does not exceed 773 K and the maximum speed that the equipment can reach is 4000 s−1, the strain rates of 700 s−1, 1600 s−1, 2800 s−1, and 4000 s−1 and temperatures of 293 K, 373 K, 573 K, and 773 K were selected for the dynamic impact tests. The technical method and principle of the split Hopkinson pressure bar experiment are the same as those in reference [The quasi-static compression tests were performed at a room temperature of 293 K and strain rate of 0.001 seference .3:H2O2:H2O = 1:2:7:10. Optical microstructure observation and photographing were carried out on Leica DMI 5000 M microscope. The compressed specimens were sliced along the axial direction with a slice thickness of 0.5 mm, and then ion-thinned to prepare a transmission electron microscope sample. A transmission electron microscope experiment was carried out on CM200. A VMH-104 microhardness tester was used to test the microhardness of different regions of the compressed specimens under the load of 200 N and the holding time of 10 s.After compression, the compressed specimens were cut along the axial direction by wire cutting, and the cut surfaces were prepared into a metallographic sample. The metallographic samples were ground, polished, and etched for microstructure observation. The ratio of etchant was HF:HNO−1 to 4000 s−1 and temperatures ranging from 293 K to 773 K are shown in The stress–strain curves of TA31 titanium alloy compressed at strain rates ranging from 0.001 sIt can be seen from The deformation temperature has a significant effect on the flow stress, and the flow stress decreases with the increase in temperature, which indicates that the dynamic impact deformation of the TA31 titanium alloy is a thermal activation process. Compared with the deformation temperature, the effect of strain rate on flow stress during dynamic compression is insignificant.σ and ε are flow stress and strain, respectively, −1) respectively, T * represents (T − rT)/(mT − rT) in which T, m,T and rT is the deformation temperature, the melting temperature (about 1923 K) and the room temperature (293 K) respectively, m and n represent thermal softening exponent and strain hardening exponent respectively, A represent yield stress at rT and B and C is strain hardening coefficient and strain rate hardening coefficient respectively [The J-C model can accurately describe the deformation behavior of metal materials in the process of dynamic impact deformation, and its form isectively ,14,15.The value of A can be determined to be 893 MPa by a quasi-static compression test.−1, Equation (1) isAt 293 K and 0.001 sEquation (2) is equivalently transformed intoB and n obtained from The value of C is the slope of the linear fitting line in At 293 K, Equation (1) is simplified tom is the slope of the linear fitting line in Transform the form of Equation (1) intoAll parameters in the constitutive model have been determined. The corresponding parameters are substituted into Equation (1), and the constitutive model of TA31 alloy can be obtained as−1 to 4000 s−1. The prediction accuracy of the model is verified at deformation temperatures from 293 K to 773 K and strain rates from 0.001 s−1, there is no shear failure in the specimen after deformation, but when the strain rate is increased to 1600 s−1 or more, the specimen appears shear fracture. At the same strain rate of 1600 s−1, when the deformation temperature is 373 K, 573 K, and 773 K, there is no shear failure in the deformed specimens, which indicates that the increase of deformation temperature can improve the plasticity of TA31 alloy under dynamic impact condition.Photos of the deformed specimens under dynamic impact load are shown in Low and high-magnification optical microscope images of the deformed specimens without shear fracture are shown in −1 are shown in Optical images of the specimen deformed at 293 K and 1600 s−1 and 4000 s−1, shear bands similar to that in −1, four shear bands appeared inside the deformed specimen. The increase in the number of shear bands may be due to the fact that more shear bands can absorb the large energy input instantaneously under faster impact deformation conditions.When the deformation temperature was 293 K, and the strain rates were 2800 sDuring the impact deformation process of titanium alloys, shear bands can often be observed. The microstructure characteristics of shear localization caused by flow instability of near α Ti-6Al-2Zr-1Mo-1V titanium alloy under impact load were studied, and it was found that two factors, geometry and structure, affected the nucleation and propagation of shear band . Yang etRecrystallization is usually observed in titanium alloy shear bands. Liu et al.  When stuThe inside of the shear band has experienced a large plastic deformation and obtained significant work hardening, while the metal matrix outside the shear band is less deformed and has not been work hardened  the TA31 titanium alloy shows typical elastic-plastic characteristics. In the initial stage of compression (elastic stage), the flow stress increment is proportional to the strain increment, and the stress–strain curve is a straight line. In the plastic deformation stage, deformation temperature has a significant effect on the flow stress, and the flow stress decreases with increasing deformation temperature. The effect of strain rate on flow stress during dynamic compression is insignificant.−1 to 4000 s−1, and the average relative error is 2.32%, which can accurately describe the deformation behavior of TA31 titanium alloy under dynamic impact loading.(2) The J-C constitutive model of TA31 alloy is established and verified at deformation temperatures from 293 K to 773 K and strain rates from 0.001 s−1, a shear band with an angle of about 45° to the axial direction of the specimen appears. The plastic deformation is mainly concentrated in the shear band. The severe shear deformation makes the α phase in the shear band fibrous and contains high-density dislocations. Increasing the strain rate can promote the generation of shear bands. When the strain rate reaches 4000 s−1, four shear bands appear inside the sample. However, increasing the deformation temperature will inhibit the generation of shear bands.(3) When the deformation temperature is 273 °C, and the strain rate reaches 1600 s(4) The instantaneous plastic deformation during the impact deformation process is mainly concentrated on the edge of the shear band, and the continuous plastic deformation makes the shear band gradually widen.(5) During the shear fracture process, the crack propagates along the boundary of the shear band, which eventually leads to the fracture of the specimen.


The entropic nature of elasticity of long molecular chains and reticulated materials is discussed concerning the analysis of flows of polymer melts and elastomer deformation in the framework of Frenkel–Eyring molecular kinetic theory. Deformation curves are calculated in line with the simple viscoelasticity models where the activation energy of viscous flow depends on the magnitude of elastic entropic forces of the stretched macromolecules. The interconnections between deformation processes and the structure of elastomer networks, as well as their mutual influence on each other, are considered. E is the Young’s modulus describing the relative stiffness of a material, which is measured by the slope of elastic of a stress and strain graph, and by Newton’s flow law,B is a pre-exponential factor, b is Eyring’s coefficient of viscous volume [molecular kinetic units (MKU) [In the vacancy mechanism of self-diffusion in liquid bodies and deformation of solids proposed in the early works of Frenkel  and Eyrie theory , used inr stress ,4,5,6,7, stress [η=BτexpE0low law,τ=ηγ˙,st stress [η=BτexpE0 stress [η=BτexpE0ts (MKU)  and in vts (MKU) ,12. The  stress [η=BτexpE0ts (MKU) ,14.In our paper, we review and analyze another explanation for viscosity anomalies in polymer flows, taking into account the entropic nature of elasticity emerging in macromolecules. In our approach, activation energy Equation ) takes ientropic elastic forces (EEF) acting along the stretched macromolecular chains. For example, the rotor of a viscometer immersed into polymer melts rotates backward after the engine shuts down, testifying to highly elastic reversible deformation present in the flow of polymer melts [Statistical theory of rubber elasticity ,20 stateer melts ,21. The er melts ,22,23,24er melts ,27,28,29In In the present section, we discuss a possible change in the jump activation energy fostered by the effect of EEF reflecting the molecular structure of polymers. The chain and network molecular structure of polymers manifests in viscosity anomalies registered in polymer flows, a hysteresis phenomenon of tensile curves in elastomers, and the creeping behavior in silicon rubber.N is the number of monomeric links in the chain, and l is the length of each unit. The emerging entropic force  (PMVS) subjected to tensile deformation against the relative strain Empirically observed tensile curves for rubber fit the behavior predicted by the theoretical model  only forterature  and desiterature ,29,44. I relaxed . The def relaxed .standard linear solid (SLS) model schematically presented in E, and (ii.) the viscous stress H associated with the viscous element Before explaining the Mullins effect, let us consider the ke’s law , σe=Eε, A is some pre-exponential factor, independent of the applied stress The differential equation for the SLS model , expressAs the strain grows linearly over time, Equation  takes thquation (σSLS(t)=EThe hysteresis behavior in elastomers is entirely due to the action of EEF emerging in the molecular network undergoing deformation. Namely,(1)increasing deformation over the stretching phase, the EEF reinforces the resistance of molecular network against the tensile stress increase of the jump activation energy S pertains to the stretching phase of deformation.With  formula  by an amation ση ,14,30, v(2)decreases the jump activation energy coincides with the retraction direction of the specimen deformation, and, therefore,retraction phase of deformation.In a retraction phase of deformation, the EEF By equating the expression for viscosity following from the SLS model  to Eyrinlinear relation between the quantity in the left-hand side of 20) in thThe analytical solution of the modified SLS model taking into account the EEF effect predicts a linear stress–strain relation for the first round of specimen stretching, followed by the non-linear tensile curves for the further rounds of deformation.The proposed model accounting for the effect of the EEF on elastomer deformation predicts the linear relation between the value ally see .Kelvin–Voigt model (KVM), viz.,Kelvin–Voigt differential equation (KVDE) describing strain dynamics in the stretching phase, viz.,Creeping reflects the tendency of a material to deform plastically over time at any level of compressive, tensile, and shear mechanical stress applied. The creep behavior of elastomers subjected to the total stress , viz.,2σ=σe+ση=E), viz.,σ=σe+ση=Eng phase , and divApplying logarithm on the latter equation, we obtain the linear relation between ents see , viz.,, estimating the amount of time the product of rubber is likely to operate before it requires repair or replacement, viz.,The analytic solution  of the KThe linear dependence of In the present section, we report on the experimental verification of the EEF-related effects on non-Newtonian fluids, deforming polymers, and creeping silicon rubber.®RN4  as a rotational viscometer. The viscosity measurements were carried out using a cone and plate system with the cone diameter 36 mm and angle The linear and log-linear relationships predicted by (–8)) bet) bet8)) For the low shear rates, after the application of default shear stress on melted polystyrene, the reversible deformation of recovery a. The loTo verify the relationships –8), fol, fol8), ate, γe˙ a; shear ar rates b; finallation γe c.All empirically observed patterns presented in E in  equipped with a MultiXtens extensometer pursuant to DIN EN ISO 527-2/S2. The dumbbell-shaped specimen of a silicone PMVS rubber (ISO 527-2) used in the experiment had rectangular sections that were 2 mm thick, 4 mm wide, with a rectangular narrow section length of 43 mm. The test length of a specimen measured with the extensometer was 15 mm. The molecular weight of a chain segment used to calculate Young’s modulus E in  was takeHooke’s modulus, onograph ). The seThe dependence of true stress, taking into account the change in cross-sectional area of the specimen as it is stretched versus applied strain, is shown in ontinues .g, calculated over the maximum correlation of linear regression  specimen smoothly at a given rate, in a testing chamber with temperatures ranging from room temperature up to In The creep curves shown in KV model . The linKV model b. All exThe slopes of creep curves, g, for the creep curves with initial stresses from g has high values at low initial stress, but low at high stress. Apparently, a large number of physical cross-links characterized by long relaxation times are not destroyed during deformation under low loads in elastomers. In In g depend on neither temperature nor stretching rate. We, therefore, assume that the value of g may characterize the tendency of polymers to maintain a stable structure in mechanical deformation.The experimentally recorded values of the Flory correction coefficient Flory’s corrections measured for the repeated hysteresis loops were close to each other. Only the first hysteresis round seems to differ substantially from the others, as also predicted by the analytic solutions of the model Equations –20). Th. Th20). Up to five stable segments can be identified visually on the experimental hysteresis curves. In particular, there are three regions of increasing deformation and two regions of reversible deformation. The measured values of Flory corrections exhibited sufficient reproducibility for all tested samples. For the tested PMVA specimen, the recorded Flory correction factor was 5–7 units.A quantitative description of elastomer deformation can be obtained, using the basic equations of the statistical theory of rubber elasticity and Eyring’s equation modified to take into account the entropic nature of deformation in polymers. For the stretching phase of rubber deformation, the elastic forces increase the activation energy, while they decrease during the retraction deformation.The small segments at the beginning and at the end of tensile curves  show a stress growth slowdown, which may be associated with a decrease in the activation energy. There was a significant change in the values of Flory corrections in these segments at the same time. The final section of the return hysteresis curve has a particularly sharp increase in the Flory correction factor. This can be interpreted as a result of a strong increase in the number of physical cross-links at the final stage of the elastomer chain folding process.From the analysis of experimental data reported in The elastomer molecular structure is a major factor influencing the deformation behavior, as it determines how much and how quickly stress increases, as well as how strong the macromolecules would resist the external force when stretched. The molecular structure of a network formed by chemical and physical bonds changes under tensile and compressive strain, forming a reproducible pattern of deformation behavior that is consistently repeated in a series of measurements of the hysteresis loops in elastomer. The sequential hysteresis rounds are expressed in the consecutive decrease and increase of the jump activation energy required for the migration of MKU into vacancies over material deformation. The changes in energy barriers manifest an increase and decrease in the viscosity of the material during stretching and plastic flow. The anomalous viscosity affects the slowing down and acceleration of stress growth observed in the stretching and contraction curves measured in elastomers.We profoundly thank our reviewer for the inspiring questions about extending our model to branched macromolecules. One of us (V.K.) considered the possibility of evaluating the rheological behavior of melts of linear polymers under the influence of EEFs on the activation energy decrease during the folding of macromolecules in the course of plastic flow. This process is easily observable when the rheometer’s rotor stops, i.e., when it is free from the engine and rotates backward due to the reversing flow of the polymer melt . In thisFor branched  polymers, it would be very interesting to study the flow processes in rheometers. We would expect to observe a decrease in viscosity with increasing flow speed or shear stress. However, due to the action of physical cross-links between the entangling sufficiently long branches of chains that are associated with the main chain, the possible deviations towards increasing viscosity at the flow onset, as well as at the end of the flow, should be observed. When stretching branched polymers at a given rate on tensile machines, the dependence of stress on strain should have a large plastic flow region, with growing initial and final sections of the stretching curve . Instruments for MFI measurements are often used by industry technologists. Moreover, this approach can also be used for measuring the viscosity of melts of the rod extruded from the production extruder when obtaining polymer granules.Further research is needed to focus on possible applications of the molecular models to the MFI measurements.reversible properties under mechanical stress (pertaining to zero entropy production), in contrast to low-molecular solids and liquids with an atomic or low-molecular structure. However, polymers, of course, exhibit irreversible deformation as well, resulting in violation of their initial structure. Therefore, the irreversible deformation processes discussed in our work should be intimately tied with the IEP processes, although a unifying theory of entropy production that is valid for general thermodynamic processes, especially describing deformations in different media, has not yet been formulated [cannot be universal, as it ultimately depends on the underlying physical system and its governing dynamical laws [interpreting ongoing experimental observations in the framework of experimentally mastered statistical mechanics focused at possible corrections close to equilibrium—and it is a natural “constructive” limitation of our approach, indeed.We also profoundly thank another reviewer for pointing us at the potential problem of irreversible entropy production (IEP)  in polymrmulated . Indeed,cal laws . Presentcal laws  and do nFurther decades of intensive experimental research will be needed to adopt the relatively novel theoretical concepts of irreversible entropy production [oduction ,50 to thIn our work, we study the relations between the mechanical properties of elastomers and their molecular network structure. Based on the basic equations of the classical statistical theory of rubber elasticity, we have shown that it is possible to quantify the effects of labile physical bonds on the deformation behavior in elastomers.The role of physical bonds in rubber deformations was evaluated based on the Flory correction factor, which takes into account how bonding defects in polymers can affect the course of the deformation process in hysteresis.The use of Eyring’s equation, considered in the spirit of Frenkel’s ideas about the vacancy mechanism of flow in condensed media, made it possible to eliminate the primary inconsistency found in the previous quantitative descriptions of tensile curves for rubber, especially at large strains. The conceptual improvement was achieved by modifying the exponential viscous flow equation for polymers to take into account the decrease or increase in the activation energy of deformation by an amount proportional to the entropic elasticity force excited by macromolecule structures subjected to tensile stress.A and b are the parameters calculated in accordance with the theory of Eyring et al.When macromolecular materials, such as synthetic polymers, natural plants, or other biological materials are exposed to external stress, the entropic nature of viscosity should always be taken into account as follows: A modified SLS model was used for quantifying the stress–strain dependence and calculating the isothermal Young’s modulus. The value of Flory’s correction factor was included in the equations as a measure of the number of physical bonds that influences the deformation behavior in elastomers. The study of consecutive hysteresis loops revealed distinctive, consistently repeating segmentation patterns in the deformation behavior of silicone rubber. The data obtained showed good reproducibility for the numerical values of Flory corrections proportional to the number of possible physical cross-links present in reticulated elastomeric materials when they are subjected to mechanical stress.Our study suggests that the network structure of polymers is coherent with their mechanical properties and deformation mechanisms.


Lipid asymmetry, the difference in the lipid composition in the inner and outer lipid monolayers (leaflets) of a membrane, is an important feature of eukaryotic plasma membranes. Investigation of the biophysical consequences of lipid asymmetry has been aided by advances in the ability to prepare artificial asymmetric membranes, especially by use of cyclodextrin-catalyzed lipid exchange. This review summarizes recent studies with artificial asymmetric membranes which have identified conditions in which asymmetry can induce or suppress the ability of membranes to form ordered domains (rafts). A consequence of the latter effect is that, under some conditions, a loss of asymmetry can induce ordered domain formation. An analogous study in plasma membrane vesicles has demonstrated that asymmetry can also suppress domain formation in natural membranes. Thus, it is possible that a loss of asymmetry can induce domain formation in vivo. Many natural membranes have strong lipid asymmetry in which the headgroup and acyl chain compositions are different in the inner and outer lipid monolayers (leaflets) that form the lipid bilayer . The mosIn addition to random transverse diffusion of lipids across the bilayer of a membrane (see below), asymmetry is controlled by the sidedness of lipid biosynthesis and breakdown, plus the presence of lipid translocases, which include proteins that transport lipids inwardly  (flippases) or outwardly (floppases), and scramblases, which facilitate random diffusion of lipids in both directions .2+, which increases in concentration during some types of signal transduction [Loss of lipid asymmetry has biological impacts. Loss of PS asymmetry is associated with apoptosis and triggering of blood clotting, in which the appearance of PS in the outer leaflet promotes interactions with phagocytic cells and blood clotting factors, respectively . It has sduction .) Cancersduction . In addisduction ,7. Loss sduction . In addisduction . Thus, tDespite the above discussion, our knowledge of how asymmetry and changes in asymmetry control membrane structure and function remains rudimentary. This is largely due to the difficulty of measuring membrane asymmetry for lipids other than PS, and limitations in experimental control of membrane lipid composition and asymmetry. Lack of good methods for preparation of asymmetric artificial membranes in which the biophysical effects of membrane asymmetry can be studied has been one major stumbling block. Advances in the preparation of artificial and natural membranes with controlled asymmetry are starting to change this situation ,11. For Importantly, the cyclodextrin-catalyzed exchange method has been extended to mammalian cells. Efficient lipid exchange can be achieved in erythrocytes and cultured cells ,19. ThisAnother method proving highly useful for preparation of asymmetric giant artificial lipid vesicles involves hemifusion between giant vesicles and a lipid covered surface .An important parameter is the stability of lipid asymmetry. When lipids undergo transverse diffusion across the bilayer  asymmetry decays. In cell membranes the stability of asymmetry is often controlled by proteins. However, spontaneous lipid flip across the bilayer also occurs. In both symmetric and asymmetric membranes the spontaneous loss of lipid asymmetry in flip across the bilayer unaided by protein is generally very slow for the most commonly occurring phosphatidylcholines or sphingomyelins, on the order of hours to days in the liquid disordered state, and even slower in the gel state  . In contThe consequence of slow lipid flip is that it is easy to prepare asymmetric vesicles with lipid compositions that maintain lipid asymmetry for prolonged periods, including cases in which lipid composition mimics that in natural membranes, and this allows a wide range of experiments to be carried out.However, it should be noted that in some  cases, inclusion and/or addition of membrane-inserting peptides to lipid bilayers can greatly increase the rate of flip-flop ,28,29,30The formation of membrane domains in different physical states is a key membrane property believed to be of biological importance. It has been widely studied using lipid vesicles and has been a major subject of studies using asymmetric vesicles. The phospholipid/sphingolipid bilayer of a membrane often exists in one of three physical states. The most common is the liquid disordered (Ld) state, which is characterized by fast lateral diffusion of lipids and lipids having disordered acyl chains. Lipids with at least one of their hydrocarbon (acyl) chains having a cis double bond  tend to form bilayers in this state at physiological temperatures. At low temperatures, or for lipids with two hydrocarbon chains lacking cis double bonds , the solid-like, highly ordered gel state (G) forms at both low and higher temperatures. In the gel state, lipid hydrocarbon chains are tightly packed against each other and lateral diffusion is very slow. It should be noted that when temperature is varied a lipid bilayer can transition between liquid disordered and gel states at a characteristic transition (melting) temperature (Tm). As the discussion above indicates, lipids with at least one of their acyl chains being cis unsaturated tend to have low Tm values, while lipids with only saturated acyl chains (often SM or a saturated PC) tend to have high Tm values .In membranes with saturated phospholipids (or sphingolipids) and sterol , the liquid ordered (Lo) state can form. The liquid ordered state is characterized by a high level of order but relatively fast lateral diffusion. Recent studies indicate that the liquid ordered state is composed of gel/solid-like phospholipid or sphingolipid subdomains laterally separated by sterol-rich layers ,33. An aA consequence of basic physical chemistry principles is that for some lipid compositions in membranes composed of a mixture of lipids with different tendencies to form different physical states, two or more of these physical states will co-exist, forming lipid domains with different physical states and lipid compositions . The forHow lipid composition controls membrane physical state and domain formation has been studied extensively in symmetric artificial membranes. Far fewer studies have been carried out in asymmetric artificial membranes, but those already carried out have provided a number of novel insights. One is that asymmetry can have a wide variety of impacts upon the physical properties of each leaflet when only one leaflet contains lipids that would spontaneously form ordered domains in a symmetric membrane . For exaIn the most common cases, the location of a domain in one leaflet matches the location of a domain having the same physical state in the opposite leaflet. In this case, the domains are said to be in register. However, there can be exceptions . It shouIn asymmetric small vesicles without cholesterol, exchanging in SM to replace almost the entire outer leaflet of vesicles containing dioleoyl PC (DOPC), which has two unsaturated acyl chains, or palmitoyl oleoyl PC (POPC), which has one unsaturated acyl chain, resulted in vesicles that, at ambient temperature, had a highly ordered outer leaflet, but with only a modest increase in inner leaflet order relative to that in symmetric DOPC or POPC vesicles ,36, suggHowever, it was found in another study that the difference between inner and outer leaflet curvature could affect coupling greatly in asymmetric vesicles containing palmitoyl oleoyl PE (POPE) and POPC, with strong coupling of ordered state formation in both leaflets when the relatively high-Tm lipid POPE was enriched in the inner leaflet, but not when it was enriched in the outer leaflet .In a study using large vesicles containing saturated dipalmitoyl PC (DPPC) plus POPC in one leaflet and POPC in the other, partial coupling was observed by neutron scattering, such that the DPPC rich ordered domain in the outer leaflet packed more loosely than it would in symmetric vesicles . OrderedIn diffusion studies in which SM was introduced into the outer leaflet of giant vesicles composed of various PCs, decreased lateral diffusion of lipids in each leaflet was detected after introduction of SM even though domains were too small to visualize. The extent of coupling of diffusion, as judged by the relative decrease in lateral diffusion in each leaflet after SM was introduced, depended on the choice of SM and PC. Upon introduction of brain SM into the outer leaflet, lateral diffusion of inner leaflet PCs with one saturated and one unsaturated acyl chain  was more strongly coupled to that of the outer leaflet than when the PC in the vesicles was DOPC, as judged by the decrease in inner leaflet lateral diffusion relative to the decrease in outer leaflet lateral diffusion after SM introduction. Coupling of diffusion in the two leaflets was also strong in DOPC containing membranes when milk SM was introduced into the outer leaflet instead of brain SM. Penetration of long milk SM acyl chains into the opposite leaflet (interdigitation) may have increased the extent of interleaflet coupling in those vesicles.Because domain formation in cells involves membranes containing cholesterol or an analogous sterol, the behavior of cholesterol-containing artificial membranes is of most interest. Natural sterols present at high concentrations, as in the plasma membrane, generally promote ordered domain formation. However, sterol effects depend strongly on their structure, and some sterols can even suppress ordered domain formation ,39,40,41Early pioneering studies using unsupported or cushioned asymmetric planar bilayers containing cholesterol identified situations in which domains in one leaflet induced domains in the opposite leaflet, as well as those in which a lack of ordered domains in one leaflet appeared to suppress ordered domain formation in the opposite leaflet ,44. ThesFirst, we consider cases in which ordered domains formed by SM or saturated PC in one leaflet  induce ordered domains in an opposite leaflet enriched in Ld-forming lipids, i.e., lipids that do not by themselves form ordered domains in symmetric membranes. In cholesterol-containing asymmetric giant vesicles prepared by cyclodextrin-catalyzed exchange, large (visible by light microscopy) outer leaflet ordered domains and interleaflet coupling-induced ordered domains in the inner leaflet were observed in vesicles composed of egg SM, DOPC and cholesterol in the outer leaflet and DOPC plus cholesterol in the inner leaflet . It was It was hypothesized that the induction of inner leaflet DOPC-containing ordered domains was aided by the migration of cholesterol out of the inner leaflet Ld domains and into the inner leaflet DOPC-containing ordered domains . This waIt should be noted that no domains were seen when egg SM was replaced by brain SM. However, it is likely that ordered submicroscopic nanodomains were present, as shown by later experiments using FRET, which detected nanodomain formation in asymmetric vesicles containing brain SM, DOPC and cholesterol (see below) .Studies using the hemifusion method have extended these results . Domain Interestingly, the SM forming large domains in the hemifusion studies was brain SM, for which large domains were not observed in the studies using cyclodextrin-exchange. The likely explanation is that the hemifusion studies used a lower concentration of cholesterol, and a lower cholesterol concentration can promote the formation of larger SM domains, at least in symmetric vesicles .Suppression of ordered domain formation in asymmetric membranes has attracted less attention. In addition to early examples in planar bilayers showing suppression of at least large size ordered domains, noted above, an important study in which cyclodextrin-catalyzed exchange was used to prepare asymmetric planar supported bilayers detected changes in domains upon loss of asymmetry . In planThe question of the effect of varying lipid structure upon induction or suppression of ordered domain formation in asymmetric large lipid vesicles prepared by cyclodextrin exchange was investigated in a FRET study in which saturated PC acyl chain length was varied . VesicleAs saturated PC acyl chain length was decreased, the behavior of asymmetric and symmetric vesicles diverged, with the inner leaflet becoming increasingly dominant in asymmetric vesicles as outer leaflet saturated PC acyl chain length was decreased. In other words, decreasing saturated PC acyl chain length increasingly allowed the inner leaflet DOPC to induce a disordered outer leaflet in the asymmetric vesicles even though the outer leaflet composition could still form ordered domains in symmetric vesicles. This showed up as a lower Tm for ordered domain formation in asymmetric vesicles relative to symmetric vesicles. For the saturated PC with the shortest acyl chain length studied (di C14:0 PC) no ordered domain formation could be detected in the asymmetric vesicles at any temperature (the lower temperature limit measured was about 15 °C), although ordered domains formed over a considerable temperature range in the symmetric vesicles. Intuitively, the effect of decreasing saturated PC acyl chain length in asymmetric vesicles is consistent with the decreased thermal stability of the ordered gel state in vesicles composed solely of saturated PC as acyl chain length is decreased .Comparison of the behavior of symmetric and asymmetric vesicles containing the same total amount of saturated PC in the vesicles (as opposed to the same amount in their outer leaflets) showed that loss of asymmetry without a change in membrane lipid composition could induce ordered domain formation. In other words, the lower amount of high Tm lipid in the outer leaflet after asymmetry was lost, which destabilizes domain formation, was more than compensated by the stabilization of ordered domain formation when lipids with a strong tendency to form ordered domains were present in both leaflets. Because the assay involved FRET, this involved domain formation, not formation of large domains from preexisting nanodomains.The effect of the amount of high-Tm lipid upon domain formation in symmetric vs, asymmetric vesicles was also examined . This waThe observation that loss of asymmetry can trigger domain formation raises the interesting possibility that loss of asymmetry might be a trigger for raft formation in biological systems. However, the effect of loss of asymmetry noted above was detected for lipids (a short acyl chain saturated PC and DOPC) far from physiological. Therefore, a study was carried out in asymmetric vesicles using SM in place of saturated PC, and comparing DOPC to the more physiological POPC . VesicleThe mechanisms behind the domain-suppressing effect of POPC (relative to DOPC) when in the same leaflet as SM (cis leaflet) and the domain-suppressing effect of POPC that is in the opposite leaflet as SM (trans leaflet) may not be the same. When in the same (cis) leaflet, the key property may be that POPC mixes more readily with SM than DOPC, i.e., the mutual miscibility of SM and POPC may be somewhat greater than for SM and DOPC. However, it should be noted that in symmetric vesicles the difference between POPC and DOPC in this regard seems small . For theAn issue that may influence which leaflet dominates domain forming behavior in asymmetric vesicles is the degree of lipid balance between the leaflets, i.e., the degree to which exchange does not change the ratio of lipid molecules in the outer leaflet to that in the inner leaflet, and the resulting differential stress under conditions of lipid imbalance ,52,53.  ,56. DespHowever, several issues complicate interpretation of gpmv behavior. First, formation of gpmv is associated with a loss of cytoskeletal interactions, which can perturb their behavior. Second it is not clear if the lipid composition of gpmv is exactly equal to that in the bulk plasma membrane. Third, domain formation is affected by the cell cycle, and thus likely to be affected by cell growth conditions. Fourth, the chemical reagents used to induce gpmv formation can modify membrane protein and lipid structure. Finally, there is at least a partial loss of lipid asymmetry when gpmv form and bud off of cells ,60.Recent work has clarified the relationship between gpmv domain formation and asymmetry . StudiesThe observation that lipid asymmetry affects ordered domain formation may provide new insights into the origin of detergent resistant membranes from cells. Detergent resistant membranes can be isolated from living cells, and are rich in sphingolipids, cholesterol and proteins with a high affinity for ordered domains . For thiStudies in asymmetric vesicles that resemble plasma membranes in terms of their lipid asymmetry are providing a more realistic picture of the conditions in which ordered domains (rafts) form or fail to form. It is clear that the precise lipid composition of an asymmetric membrane is important to define under what conditions rafts are likely to form. Under some conditions the loss of asymmetry may be the trigger for raft formation in vivo. Further studies of both artificial and natural asymmetric membranes should help further clarify the rules for raft formation in vitro and in vivo.