human
***task***
the task is to summarize an input biomedical literature in six sentences

***input***
the input is a biomedical literature

***output***
the output is the summary of an input biomedical literature in six sentences

***documentation***

***examples***

input: it has been just over two decades since the role of nitric oxide ( no ) as an essential biological signaling agent was recognized and which led to its selection as science s molecule of the year in 1992 . no is now recognized for its importance in blood pressure regulation , as a neurotransmitter , as well as a ( toxic ) defense mechanism in neutrophils and macrophages . iron ( ii ) porphyrinates are one of the critical biological targets of no , and extensive work has characterized the iron nitrosyl unit in a range of synthetic porphyrin model complexes [ fe ( porph ) ( no ) ] . however , information from early crystallographic studies was limited due to severe problems with disorder of the no ligand . it was only with the report of two ordered crystalline polymorphs of the five - coordinate ( nitrosyl ) iron ( ii ) porphyrinate derivative [ fe ( oep ) ( no ) ] that hitherto unobserved structural trends became apparent , trends that have been since observed in a number of other iron ( ii ) nitrosyl porphyrinates . in these structures , the fen5 coordination group shows substantial deviation from an ideal symmetric coordination , with a significant off - axis tilt of the fen ( no ) bond vector and an apparently correlated asymmetry in the equatorial fe np bond distances . this equatorial asymmetry pattern is displayed wherein the two fe np bonds closest to the tilted fen ( no ) axial vector are effectively identical but significantly shorter than the other two fe np bonds , which are also effectively equal to each other . the tilting / asymmetry pattern has also been observed in a cobalt system , although the deviations from ideal axial symmetry are smaller . further studies focused on preparing a series of additional five - coordinate ( nitrosyl ) iron ( ii ) porphyrinate derivatives that vary in the nature and the position of the - pyrrolic - and meso - substituents . characterization of derivatives of a - oxochlorin and the asymmetrically substituted 2,3,12,13 - tetrabromo -5,10,15,20-tetraphenylporphyrin revealed the same tilt / asymmetry pattern observed in the [ fe ( oep ) ( no ) ] derivatives . to date there is only one structure reported for a naturally occurring porphyrin derivative , that of [ fe ( dpix - dme ) ( no ) ] . this structure displayed the same off - axis tilting as previously observed in the synthetic porphyrins , with the tilt direction lying in the direction of the two pyrrole rings bearing the propionate groups . however , despite these similarities , physical characterization of these naturally occurring porphyrin derivatives showed some significant differences from the synthetic porphyrins . initial investigation of the vibrational spectra of nitrosyl iron derivatives utilizing nuclear resonance vibrational spectroscopy ( all taken as powder spectra ) showed that the in - plane components of the iron vibrational spectra were surprisingly sensitive to the nature of the peripheral groups . even changing only two substituents in the trio of naturally occurring porphyrins ( protoporphyrin ix , deuteroporphyrin ix , and mesoporphyrin ix , shown in scheme 1 ) had observable effects on the in - plane vibrational spectra . to further investigate the differences between the naturally occurring and synthetic porphyrins , we report the synthesis of [ fe ( porph ) ( no ) ] derivatives based on the trio of naturally occurring porphyrins . these derivatives have been characterized by uv vis , ir , and epr spectroscopy . in addition , the three derivatives have been characterized by mssbauer spectroscopy in zero and applied magnetic field . after significant effort , single crystals of [ fe ( ppix - dme ) ( no ) ] have been obtained , and the x - ray structure has been determined . all reactions and manipulations of the iron ( ii ) porphyrin derivatives were carried out under argon using a double - manifold vacuum line , schlenkware , and cannula techniques . dichloromethane , chloroform , methanol , and hexanes were distilled under argon over cah2 , magnesium turnings , and sodium / benzophenone , respectively . the free bases , protoporphyrin ix dimethyl ester , deuteroporphyrin ix dimethyl ester , and mesoporphyrin ix dimethyl ester , were purchased from frontier scientific . nitric oxide ( mittler specialty gases ) was purified by fractional distillation through a trap containing 4a molecular sieves bathed in a dry ice / ethanol slurry . insertion of iron into h2 ( ppix - dme ) , h2 ( dpix - dme ) , or h2 ( mpix - dme ) was accomplished by the reaction of the free base and iron ( ii ) chloride in dmf . the desired five - coordinate [ fe ( porph ) ( no ) ] derivative was obtained by reductive nitrosylation . chloroporphyrinatoiron ( iii ) compounds were reductively nitrosylated under argon , and all solvents used were degassed by sparging with argon for 5 min . liquidliquid diffusion crystallization was done with chloroform solutions and with methanol as the counter solvent in 40 cm 7 mm glass tubes , sealed with rubber septa . crystals of [ fe ( ppix - dme ) ( no ) ] and [ fe ( dpix - dme ) ( no ) ] were thus obtained . the obtained crystals of [ fe ( dpix - dme ) ( no ) ] are a new polymorphic form . unfortunately , the crystal structure determination of the new dpix - dme derivative was only partly successful ; the successful ppix - dme determination is reported herein . all three complexes were characterized by uv vis , ir , epr , and mssbauer spectroscopies . vis spectra were recorded in chloroform solution on a perkin - elmer lambda 19 spectrometer . ir spectra were recorded on a perkin - elmer model 883 as kbr pellets . epr spectra were recorded at 77 k with an x - band varian e-12 spectrometer . mssbauer measurements were performed on a constant acceleration spectrometer at 4.2 k in zero field and at 4.2 k in 4 and 9 t fields using a superconducting magnet system ( knox college ) . single crystal experiments on [ fe ( ppix - dme ) ( no ) ] were carried out on a bruker apex ii system with graphite - monochromated mo k radiation ( = 0.71073 ) . a dark - red crystal with the dimensions 0.440.350.09 mm was used for the structure determination . the crystalline sample was placed in inert oil , mounted on a glass pin , and transferred to the cold gas stream of the diffractometer . crystal data were collected at 100 k. the structure was solved by direct methods and refined against f using shelxl ; subsequent difference fourier syntheses led to the location of all remaining non - hydrogen atoms . for the structure refinement , the asymmetric unit contains one independent porphyrin molecule and one not quite fully occupied chloroform solvate . unfortunately , the structure is somewhat marred by disorder ; most significantly , the oxygen atom of the nitrosyl ligand is found in two distinct locations with occupancies of 0.66 ( 2 ) and 0.34 ( 2 ) . the carbonyl group of one of the propionate ester side arms is also disordered with two positions for the c = o moiety . the chloroform solvate molecule was also found to be disordered with two different orientations of the solvate , centered on the same carbon atom position . complete crystallographic details , atomic coordinates , anisotropic thermal parameters , and hydrogen atom coordinates are given in the supporting information . the synthesis and spectroscopic characterization for three five - coordinate nitrosyl porphyrinates have been successfully carried out . the porphyrinate species are all related to protoporphyrin ix or heme b. protoporphyrin ix has two vinyl groups at the 3 and 7 positions of the porphine nucleus . the two vinyl groups are reduced to ethyl groups in the mesoporphyrin ix species and converted to hydrogen atoms in the deuteroporphyrin derivative . the other six peripheral substituents remain unchanged ( 3,8,12,18 - tetramethyl and 13,17 - dipropionate , cf . the uv vis , ir , and epr data are summarized in table 2 . frozen ch2cl2 glass at 77 k. data taken from ref ( 21 ) the three nitrosyl complexes have been characterized by mssbauer spectroscopy in zero and applied magnetic field ; all samples were found to be consistent with the presence of a single species . the spectra of all three are sharp , slightly asymmetric , quadrupole doublets ; the zero field spectrum of [ fe ( ppix - dme ) ( no ) ] is shown in figure 1 . the zero field mssbauer spectrum of [ fe ( ppix - dme ) ( no ) ] at 4.2 k. the molecular structure of [ fe ( ppix - dme ) ( no ) ] has been determined at 100 k. as noted in the experimental section , there are some modest disorder issues , but the major features of the molecule are clear . a thermal ellipsoid plot of the [ fe ( ppix - dme ) ( no ) ] molecule is given in figure 2 , and a fully labeled diagram with all atom names is given in figure s1 , supporting information . the porphyrin ring appears to be pivoting around a point between n2 and n3 , that leads to the extended ellipsoids in only a part of the ring . a major issue for these nitrosyl derivatives of the naturally occurring porphyrins is whether they have significantly different properties from those based on synthetic porphyrins . the n o stretching frequency of the three are seen to be significantly lower than those of the synthetic species ( table 2 ) . since maxwell and caughey and especially yoshimura had shown that the no stretching frequency in [ fe ( ppix - dme ) ( no ] was sensitive to environment , we compare values for all species measured in as close as possible the same conditions . values for the three new porphyrin species are seen to be lower than those for either the tpp or oep derivatives . given the ( no ) sensitivity to environment shown by [ fe ( ppix - dme ) ( no ) ] , we thought that these lower no stretching frequencies might be reflected in interactions between the no and the polar propionate side chains . a careful examination of the crystalline packing environments of [ fe ( ppix - dme ) ( no ) ] ( reported here ) and [ fe ( mpix - dme ) ( no ) ] show no such interactions . we conclude that the peripheral groups of the naturally occurring porphyrins have a real effect on the no stretching frequency . gas phase spectra of the no complex of protoporphyrin ix itself , although protonated on one vinyl group , shows ( no ) of 1720 cm . the ( no ) of the iron ( iii ) species also shows a higher value of than those reported in kbr pellet or in solution . although the data are limited , an effect stemming from the peripheral group substituents on the mssbauer parameters is also observed . values of the mssbauer parameters for all three derivatives are given in table 3 . there are only limited mssbauer data available for other nitrosyls and one previous study in applied magnetic field ; these data are also given in table 3 . the mssbauer parameters for the nitrosyl derivatives show slightly greater quadrupole splitting and isomer shift values in the natural porphyrins compared to ttp and oep . the increased isomer shift results from greater population of the valence orbitals which , through the resulting screening of the nuclear charge , causes a reduced s - electron density at the nucleus . an increased valence electron population must occur among orbitals that make a positive contribution to the electric field gradient : dxy , dx y , px , py in order to result in a corresponding increase in the ( positive ) quadrupole splitting . the spectroscopic data are thus consistent with a modest difference in the electronic structures of the natural heme derivatives relative to those of the synthetic hemes . lang and marshall reported mssbauer spectra for a number of liganded hemoglobin derivatives including a nitrosyl derivative ; their observed no spectrum showed a broad quadrupole doublet , and the investigators did not give values for the isomer shift and quadrupole splitting constant . the structural parameters of a number of five - coordinate nitrosyl derivatives are provided in table 4 . previous structure determinations of heme nitrosyls have revealed a number of distinct structural features . the resolution of these subtle features requires high - quality single crystals and the extensive and accurate data provided by modern area detector diffractometers . prominent among these features are an off - axis tilt of the fe no bond and a correlated asymmetry in the equatorial fe np bonds . these features seemed to be accentuated in the structure of [ fe ( dpix - dme ) ( no ) ] relative to the synthetic porphyrins . the crystallization and structure determination of the protoporphyrin derivative was undertaken to see if indeed these differences were as large in that derivative as well . unfortunately the structure of [ fe ( ppix - dme ) ( no ) ] is disordered , especially in the axial no ligand that obscures and limits the statistical significance of any comparisons of equatorial asymmetry and off - axis no tilt . the structural parameters found for [ fe ( ppix - dme ) ( no ) ] can be compared with those of [ fe ( dpix - dme ) ( no ) ] and other derivatives found in table 4 . despite the disorder that would obscure the off - axis tilt and equatorial asymmetry , the geometrical parameters of [ fe ( ppix - dme ) ( no ) ] are comparable to all other nitrosyls . this includes most parameters of the feno group other than the ( unresolvable ) off - axis tilt , which clearly must be greater than 2.5 and likely much larger . interestingly , the orientation of the major feno group ( figure 2 ) is between the pyrrole groups bearing the propionate side chains , the same orientation as that found for [ fe ( dpix - dme ) ( no ) ] ( with a single no orientation ) . this orientation of the no , with its projection onto the porphyrin plane approximately midway between a pair of equatorial fe np bonds , is the most common orientation . triclinic form , 33 k. the porphyrin core conformation in [ fe ( ppix - dme ) ( no ) ] is shown in the formal diagram of figure 3 and is seen to be ruffled . although the core conformation of the deuteroporphyrin analogue is also ruffled , it is less so . there does not appear to be a consistent pattern of ring conformations in the nitrosyls listed in table 4 . formal diagram of [ fe ( ppix - dme ) ( no ] showing the displacements of each atom from the 24 - atom mean plane in units of 0.01 . one interesting solid - state packing feature is the formation of pairwise interacting molecules as shown in figure 4 . the pairs of molecules are related by inversion symmetry , and thus the two porphyrin rings are precisely parallel . both top - down and side - on views of the two systems are shown along with some metrics of the interactions . the lateral shift for [ fe ( ppix - dme ) ( no ) ] is 3.27 , which puts it into the group i class according to the classification scheme of scheidt and lee . the ring overlap in the deuteroporphyrin derivative is tighter with a lateral shift of 2.30 , putting this complex in the class s group . the lateral shifts of these pairwise interactions are , however , much larger than those observed in two different crystalline forms of the [ fe ( oep ) ( no ) ] cation . in those two derivatives , the very strong pairwise interaction leads to lateral shifts of the two rings of 1.32 and 1.43 . interestingly , despite the disordered no group , the porphyrin peripheral groups are completely ordered ; there is no interchange of the methyl and vinyl groups . diagram illustrating the solid - state interactions for ( a ) [ fe ( dpix - dme ) ( no ) ] and ( b ) [ fe ( ppix - dme ) ( no ) ] . the spectroscopic properties ( uv vis , ir , epr , and mssbauer ) of the nitrosyliron ( ii ) derivatives of proto - , deutero - and mesoporphyrin dimethyl esters are reported . these are similar to those of synthetic porphyrin derivatives but are consistent with small differences in electronic structure . these are the presumed result of the asymmetric peripheral substituents compared to the synthetic porphyrins . the molecular structure of [ fe ( ppix - dme ) ( no ) ] has been determined and compared with that previously reported for the deuteroporphyrin analogue . unfortunately , the determination of the magnitude of equatorial fe np asymmetry and the off - axis tilt could not be made owing to the no axial ligand disorder .
output:

agent
the synthesis and spectroscopic characterization of three five - coordinate nitrosyliron ( ii ) complexes , [ fe ( porph ) ( no ) ] , are reported . these three nitrosyl derivatives , where porph represents protoporphyrin ix dimethyl ester , mesoporphyrin ix dimethyl ester , or deuteroporphyrin ix dimethyl ester , display notable differences in their properties relative to the symmetrical synthetic porphyrins such as oep and tpp . the n o stretching frequencies are in the range of 16511660 cm1 , frequencies that are lower than those of synthetic porphyrin derivatives . mssbauer spectra obtained in both zero and applied magnetic field show that the quadrupole splitting values are slightly larger than those of known synthetic porphyrins . the electronic structures of these naturally occurring porphyrin derivatives are thus seen to be consistently different from those of the synthetic derivatives , the presumed consequence of the asymmetric peripheral substituent pattern . the molecular structure of [ fe ( ppix - dme ) ( no ) ] has been determined by x - ray crystallography . although disorder of the axial nitrosyl ligand limits the structural quality , this derivative appears to show the same subtle structural features as previously characterized five - coordinate nitrosyls .
***task***
the task is to summarize an input biomedical literature in six sentences

***input***
the input is a biomedical literature

***output***
the output is the summary of an input biomedical literature in six sentences

***documentation***

***examples***

input: it has been just over two decades since the role of nitric oxide ( no ) as an essential biological signaling agent was recognized and which led to its selection as science s molecule of the year in 1992 . no is now recognized for its importance in blood pressure regulation , as a neurotransmitter , as well as a ( toxic ) defense mechanism in neutrophils and macrophages . iron ( ii ) porphyrinates are one of the critical biological targets of no , and extensive work has characterized the iron nitrosyl unit in a range of synthetic porphyrin model complexes [ fe ( porph ) ( no ) ] . however , information from early crystallographic studies was limited due to severe problems with disorder of the no ligand . it was only with the report of two ordered crystalline polymorphs of the five - coordinate ( nitrosyl ) iron ( ii ) porphyrinate derivative [ fe ( oep ) ( no ) ] that hitherto unobserved structural trends became apparent , trends that have been since observed in a number of other iron ( ii ) nitrosyl porphyrinates . in these structures , the fen5 coordination group shows substantial deviation from an ideal symmetric coordination , with a significant off - axis tilt of the fen ( no ) bond vector and an apparently correlated asymmetry in the equatorial fe np bond distances . this equatorial asymmetry pattern is displayed wherein the two fe np bonds closest to the tilted fen ( no ) axial vector are effectively identical but significantly shorter than the other two fe np bonds , which are also effectively equal to each other . the tilting / asymmetry pattern has also been observed in a cobalt system , although the deviations from ideal axial symmetry are smaller . further studies focused on preparing a series of additional five - coordinate ( nitrosyl ) iron ( ii ) porphyrinate derivatives that vary in the nature and the position of the - pyrrolic - and meso - substituents . characterization of derivatives of a - oxochlorin and the asymmetrically substituted 2,3,12,13 - tetrabromo -5,10,15,20-tetraphenylporphyrin revealed the same tilt / asymmetry pattern observed in the [ fe ( oep ) ( no ) ] derivatives . to date there is only one structure reported for a naturally occurring porphyrin derivative , that of [ fe ( dpix - dme ) ( no ) ] . this structure displayed the same off - axis tilting as previously observed in the synthetic porphyrins , with the tilt direction lying in the direction of the two pyrrole rings bearing the propionate groups . however , despite these similarities , physical characterization of these naturally occurring porphyrin derivatives showed some significant differences from the synthetic porphyrins . initial investigation of the vibrational spectra of nitrosyl iron derivatives utilizing nuclear resonance vibrational spectroscopy ( all taken as powder spectra ) showed that the in - plane components of the iron vibrational spectra were surprisingly sensitive to the nature of the peripheral groups . even changing only two substituents in the trio of naturally occurring porphyrins ( protoporphyrin ix , deuteroporphyrin ix , and mesoporphyrin ix , shown in scheme 1 ) had observable effects on the in - plane vibrational spectra . to further investigate the differences between the naturally occurring and synthetic porphyrins , we report the synthesis of [ fe ( porph ) ( no ) ] derivatives based on the trio of naturally occurring porphyrins . these derivatives have been characterized by uv vis , ir , and epr spectroscopy . in addition , the three derivatives have been characterized by mssbauer spectroscopy in zero and applied magnetic field . after significant effort , single crystals of [ fe ( ppix - dme ) ( no ) ] have been obtained , and the x - ray structure has been determined . all reactions and manipulations of the iron ( ii ) porphyrin derivatives were carried out under argon using a double - manifold vacuum line , schlenkware , and cannula techniques . dichloromethane , chloroform , methanol , and hexanes were distilled under argon over cah2 , magnesium turnings , and sodium / benzophenone , respectively . the free bases , protoporphyrin ix dimethyl ester , deuteroporphyrin ix dimethyl ester , and mesoporphyrin ix dimethyl ester , were purchased from frontier scientific . nitric oxide ( mittler specialty gases ) was purified by fractional distillation through a trap containing 4a molecular sieves bathed in a dry ice / ethanol slurry . insertion of iron into h2 ( ppix - dme ) , h2 ( dpix - dme ) , or h2 ( mpix - dme ) was accomplished by the reaction of the free base and iron ( ii ) chloride in dmf . the desired five - coordinate [ fe ( porph ) ( no ) ] derivative was obtained by reductive nitrosylation . chloroporphyrinatoiron ( iii ) compounds were reductively nitrosylated under argon , and all solvents used were degassed by sparging with argon for 5 min . liquidliquid diffusion crystallization was done with chloroform solutions and with methanol as the counter solvent in 40 cm 7 mm glass tubes , sealed with rubber septa . crystals of [ fe ( ppix - dme ) ( no ) ] and [ fe ( dpix - dme ) ( no ) ] were thus obtained . the obtained crystals of [ fe ( dpix - dme ) ( no ) ] are a new polymorphic form . unfortunately , the crystal structure determination of the new dpix - dme derivative was only partly successful ; the successful ppix - dme determination is reported herein . all three complexes were characterized by uv vis , ir , epr , and mssbauer spectroscopies . vis spectra were recorded in chloroform solution on a perkin - elmer lambda 19 spectrometer . ir spectra were recorded on a perkin - elmer model 883 as kbr pellets . epr spectra were recorded at 77 k with an x - band varian e-12 spectrometer . mssbauer measurements were performed on a constant acceleration spectrometer at 4.2 k in zero field and at 4.2 k in 4 and 9 t fields using a superconducting magnet system ( knox college ) . single crystal experiments on [ fe ( ppix - dme ) ( no ) ] were carried out on a bruker apex ii system with graphite - monochromated mo k radiation ( = 0.71073 ) . a dark - red crystal with the dimensions 0.440.350.09 mm was used for the structure determination . the crystalline sample was placed in inert oil , mounted on a glass pin , and transferred to the cold gas stream of the diffractometer . crystal data were collected at 100 k. the structure was solved by direct methods and refined against f using shelxl ; subsequent difference fourier syntheses led to the location of all remaining non - hydrogen atoms . for the structure refinement , the asymmetric unit contains one independent porphyrin molecule and one not quite fully occupied chloroform solvate . unfortunately , the structure is somewhat marred by disorder ; most significantly , the oxygen atom of the nitrosyl ligand is found in two distinct locations with occupancies of 0.66 ( 2 ) and 0.34 ( 2 ) . the carbonyl group of one of the propionate ester side arms is also disordered with two positions for the c = o moiety . the chloroform solvate molecule was also found to be disordered with two different orientations of the solvate , centered on the same carbon atom position . complete crystallographic details , atomic coordinates , anisotropic thermal parameters , and hydrogen atom coordinates are given in the supporting information . the synthesis and spectroscopic characterization for three five - coordinate nitrosyl porphyrinates have been successfully carried out . the porphyrinate species are all related to protoporphyrin ix or heme b. protoporphyrin ix has two vinyl groups at the 3 and 7 positions of the porphine nucleus . the two vinyl groups are reduced to ethyl groups in the mesoporphyrin ix species and converted to hydrogen atoms in the deuteroporphyrin derivative . the other six peripheral substituents remain unchanged ( 3,8,12,18 - tetramethyl and 13,17 - dipropionate , cf . the uv vis , ir , and epr data are summarized in table 2 . frozen ch2cl2 glass at 77 k. data taken from ref ( 21 ) the three nitrosyl complexes have been characterized by mssbauer spectroscopy in zero and applied magnetic field ; all samples were found to be consistent with the presence of a single species . the spectra of all three are sharp , slightly asymmetric , quadrupole doublets ; the zero field spectrum of [ fe ( ppix - dme ) ( no ) ] is shown in figure 1 . the zero field mssbauer spectrum of [ fe ( ppix - dme ) ( no ) ] at 4.2 k. the molecular structure of [ fe ( ppix - dme ) ( no ) ] has been determined at 100 k. as noted in the experimental section , there are some modest disorder issues , but the major features of the molecule are clear . a thermal ellipsoid plot of the [ fe ( ppix - dme ) ( no ) ] molecule is given in figure 2 , and a fully labeled diagram with all atom names is given in figure s1 , supporting information . the porphyrin ring appears to be pivoting around a point between n2 and n3 , that leads to the extended ellipsoids in only a part of the ring . a major issue for these nitrosyl derivatives of the naturally occurring porphyrins is whether they have significantly different properties from those based on synthetic porphyrins . the n o stretching frequency of the three are seen to be significantly lower than those of the synthetic species ( table 2 ) . since maxwell and caughey and especially yoshimura had shown that the no stretching frequency in [ fe ( ppix - dme ) ( no ] was sensitive to environment , we compare values for all species measured in as close as possible the same conditions . values for the three new porphyrin species are seen to be lower than those for either the tpp or oep derivatives . given the ( no ) sensitivity to environment shown by [ fe ( ppix - dme ) ( no ) ] , we thought that these lower no stretching frequencies might be reflected in interactions between the no and the polar propionate side chains . a careful examination of the crystalline packing environments of [ fe ( ppix - dme ) ( no ) ] ( reported here ) and [ fe ( mpix - dme ) ( no ) ] show no such interactions . we conclude that the peripheral groups of the naturally occurring porphyrins have a real effect on the no stretching frequency . gas phase spectra of the no complex of protoporphyrin ix itself , although protonated on one vinyl group , shows ( no ) of 1720 cm . the ( no ) of the iron ( iii ) species also shows a higher value of than those reported in kbr pellet or in solution . although the data are limited , an effect stemming from the peripheral group substituents on the mssbauer parameters is also observed . values of the mssbauer parameters for all three derivatives are given in table 3 . there are only limited mssbauer data available for other nitrosyls and one previous study in applied magnetic field ; these data are also given in table 3 . the mssbauer parameters for the nitrosyl derivatives show slightly greater quadrupole splitting and isomer shift values in the natural porphyrins compared to ttp and oep . the increased isomer shift results from greater population of the valence orbitals which , through the resulting screening of the nuclear charge , causes a reduced s - electron density at the nucleus . an increased valence electron population must occur among orbitals that make a positive contribution to the electric field gradient : dxy , dx y , px , py in order to result in a corresponding increase in the ( positive ) quadrupole splitting . the spectroscopic data are thus consistent with a modest difference in the electronic structures of the natural heme derivatives relative to those of the synthetic hemes . lang and marshall reported mssbauer spectra for a number of liganded hemoglobin derivatives including a nitrosyl derivative ; their observed no spectrum showed a broad quadrupole doublet , and the investigators did not give values for the isomer shift and quadrupole splitting constant . the structural parameters of a number of five - coordinate nitrosyl derivatives are provided in table 4 . previous structure determinations of heme nitrosyls have revealed a number of distinct structural features . the resolution of these subtle features requires high - quality single crystals and the extensive and accurate data provided by modern area detector diffractometers . prominent among these features are an off - axis tilt of the fe no bond and a correlated asymmetry in the equatorial fe np bonds . these features seemed to be accentuated in the structure of [ fe ( dpix - dme ) ( no ) ] relative to the synthetic porphyrins . the crystallization and structure determination of the protoporphyrin derivative was undertaken to see if indeed these differences were as large in that derivative as well . unfortunately the structure of [ fe ( ppix - dme ) ( no ) ] is disordered , especially in the axial no ligand that obscures and limits the statistical significance of any comparisons of equatorial asymmetry and off - axis no tilt . the structural parameters found for [ fe ( ppix - dme ) ( no ) ] can be compared with those of [ fe ( dpix - dme ) ( no ) ] and other derivatives found in table 4 . despite the disorder that would obscure the off - axis tilt and equatorial asymmetry , the geometrical parameters of [ fe ( ppix - dme ) ( no ) ] are comparable to all other nitrosyls . this includes most parameters of the feno group other than the ( unresolvable ) off - axis tilt , which clearly must be greater than 2.5 and likely much larger . interestingly , the orientation of the major feno group ( figure 2 ) is between the pyrrole groups bearing the propionate side chains , the same orientation as that found for [ fe ( dpix - dme ) ( no ) ] ( with a single no orientation ) . this orientation of the no , with its projection onto the porphyrin plane approximately midway between a pair of equatorial fe np bonds , is the most common orientation . triclinic form , 33 k. the porphyrin core conformation in [ fe ( ppix - dme ) ( no ) ] is shown in the formal diagram of figure 3 and is seen to be ruffled . although the core conformation of the deuteroporphyrin analogue is also ruffled , it is less so . there does not appear to be a consistent pattern of ring conformations in the nitrosyls listed in table 4 . formal diagram of [ fe ( ppix - dme ) ( no ] showing the displacements of each atom from the 24 - atom mean plane in units of 0.01 . one interesting solid - state packing feature is the formation of pairwise interacting molecules as shown in figure 4 . the pairs of molecules are related by inversion symmetry , and thus the two porphyrin rings are precisely parallel . both top - down and side - on views of the two systems are shown along with some metrics of the interactions . the lateral shift for [ fe ( ppix - dme ) ( no ) ] is 3.27 , which puts it into the group i class according to the classification scheme of scheidt and lee . the ring overlap in the deuteroporphyrin derivative is tighter with a lateral shift of 2.30 , putting this complex in the class s group . the lateral shifts of these pairwise interactions are , however , much larger than those observed in two different crystalline forms of the [ fe ( oep ) ( no ) ] cation . in those two derivatives , the very strong pairwise interaction leads to lateral shifts of the two rings of 1.32 and 1.43 . interestingly , despite the disordered no group , the porphyrin peripheral groups are completely ordered ; there is no interchange of the methyl and vinyl groups . diagram illustrating the solid - state interactions for ( a ) [ fe ( dpix - dme ) ( no ) ] and ( b ) [ fe ( ppix - dme ) ( no ) ] . the spectroscopic properties ( uv vis , ir , epr , and mssbauer ) of the nitrosyliron ( ii ) derivatives of proto - , deutero - and mesoporphyrin dimethyl esters are reported . these are similar to those of synthetic porphyrin derivatives but are consistent with small differences in electronic structure . these are the presumed result of the asymmetric peripheral substituents compared to the synthetic porphyrins . the molecular structure of [ fe ( ppix - dme ) ( no ) ] has been determined and compared with that previously reported for the deuteroporphyrin analogue . unfortunately , the determination of the magnitude of equatorial fe np asymmetry and the off - axis tilt could not be made owing to the no axial ligand disorder .
output:
