98032634
Progress in the Development of an Alternative Approach to Macromolecular Crystallization
We are developing an alternate strategy for the crystallization of macromolecules that does not, like current methods, depend on the optimization of traditional variables such as pH and precipitant concentration, but is based on the hypothesis that many conventional small molecules might establish stabilizing, intermolecular, noncovalent cross-links in crystals, and thereby promote lattice formation. Earlier experiments provided encouraging results that suggested further research was warranted (Larson, S. B.; Day, J. S.; Cudney, R.; McPherson, A. A novel strategy for the crystallization of proteins: X-ray diffraction validation. Acta Crystallogr., D: Biol. Crystallogr. 2007, 63, 310−318. McPherson, A.; Cudney, B. Searching for silver bullets: an alternative strategy for crystallizing macromolecules. J. Struct. Biol. 2006, 156, 387−406). Here we report additional, large-scale crystallization screening experiments that lend further support, though they suggest that additional mechanisms may play a positive ...
28242232
The RNA of turnip yellow mosaic virus exhibits icosahedral order.
Difference electron density maps, based on structure factor amplitudes and experimental phases from crystals of wild-type turnip yellow mosaic virus and those of empty capsids prepared by freeze-thawing, show a large portion of the encapsidated RNA to have an icosahedral distribution. Four unique segments of base-paired, double-helical RNA, one to two turns in length, lie between 33-A and 101-A radius and are organized about either 2-fold or 5-fold icosahedral axes. In addition, single-stranded loops of RNA invade the pentameric and hexameric capsomeres where they contact the interior capsid surface. The remaining RNA, not seen in electron density maps, must serve as connecting links between these secondary structural elements and is likely icosahedrally disordered. The distribution of RNA observed crystallographically appears to be in agreement with models based on biochemical data and secondary structural analyses.
38714824
The structure of tomato aspermy virus by X-ray crystallography.
The three-dimensional structure of tomato aspermy virus (TAV) has been solved by X-ray crystallography and refined to an R factor of 0.218 for 3.4-40 A data (effective resolution of 4A). Molecular replacement, using cucumber mosaic virus (Smith et al., 2000), provided phases for the initial maps used for model building. The coat protein of the 280 A diameter virion has the canonical "Swiss roll" beta-barrel topology with a distinctive amino-terminal alpha-helix directed into the interior of the virus where it interacts with encapsidated RNA. The N-terminal helices are joined to the beta-barrels of protein subunits by extended polypeptides of six amino acids, which serve as flexible hinges allowing movement of the helices in response to local RNA distribution. Segments of three nucleotides of partially disordered RNA interact with the capsid, primarily through arginine residues, at interfaces between A and B subunits. Side chains of cys64 and cys106 form the first disulfide observed in a cucumovirus, including a unique cysteine, 106, in a region otherwise conserved. A positive ion, putatively modeled as a Mg(+)ion, lies on the quasi-threefold axis surrounded by three quasi-symmetric glutamate 175 side chains.
44857183
The Catalina Real-time Transient Survey
The Catalina Real-time Transient Survey (CRTS) currently covers 33,000 deg^2 of the sky in search of transient astrophysical events, with time base-lines ranging from 10 minutes to ~7 years. Data provided by the Catalina Sky Survey provide an unequalled base-line against which > 4,000 unique optical transient events have been discovered and openly published in real-time. Here we highlight some of the discoveries of CRTS.
119305527
Limiting alternative theories of gravity using gravitational wave observations across the spectrum
The advent of gravitational wave astronomy provides new proving grounds for testing theories of gravity. Recent work has reinvigorated the study of bimetric theories of gravity and massive gravity theories. One of the most interesting predictions of these theories, as well as some string theories, is the subluminal speed of propagating gravitational waves. Multi-messenger astronomy provides a unique opportunity to put limits on the difference (either positive or negative) between the propagation speed of electromagnetic and gravitational waves from these sources. This paper considers limits from multi-messenger cases across the planned measurable spectrum: first, the limits from isolated pulsars based on the current best limits from LIGO on gravitational wave emission, second, the limits from ultra-compact binaries that will be visible to a low-frequency space-based gravitational wave observatory like LISA, and third, limits from super massive black hole binaries using pulsar timing arrays. The required phase comparison between the electromagnetic signal and the gravitational wave signal is derived and, assuming a null result in that comparison, the current bounds on emission are used to place limits on alternative theories that exhibit propagation delays. Observations of the pulsars in the most sensitive range of LIGO could put an upper limit on the graviton mass as low as $10^{-38}\frac{eV}{c^{2}}$ and an upper limit on the fractional difference between the gravitational wave and electromagnetic wave speeds as low as $10^{-9}$. This paper shows results from the initial LIGO limit catalog of known pulsars. The bounds are stronger for binaries. A LISA-like mission bounds $m_{g}<10^{-40}\frac{eV}{c^{2}}$ and $\delta<10^{-12}$. A PTA source gives even better bounds of $m_{g}<10^{-45}\frac{eV}{c^{2}}$ and $\delta<10^{-14}$.
16574163
Can Gravitational Waves Be Detected in Quasar Microlensing
Studies of the lensed quasar ${\rm Q}0957+561 {\rm A,B}$ have shown evidence for microlensing in the brightness history of the quasar images. It had been suggested that a frequency offset between the brightness fluctuations in each of the two images might possibly be caused by gravitational radiation generated by a massive black hole binary at the center of the lensing galaxy. This paper demonstrates that the fluctuations produced by such a source of gravitational waves will be too small to account for the observed frequency offsets.
55385537
Spurious acceleration noise in spaceborne gravitational wave interferometers
An important source of noise in the Laser Interferometer Space Antenna (LISA) is residual acceleration on the proof masses at the heart of the interferometer system. Two proof masses are carried by each sciencecraft in the LISA constellation, oriented along each of two laser links that are maintained between the distant partners in the constellation. Any change in the local mass distribution will create spurious forces on the individual proof masses which will have to be understood as part of the data analysis reduction. This paper considers the general case of accelerations on the individual proof masses in three dimensions for perturbing masses passing by a LISA sciencecraft with arbitrary velocity vectors and impact parameters. Encounters of this kind are impulsive, occurring over short time scales and appearing in the data record as bursts. The formalism is then applied in a few sample cases, including a meteor fly-by and a thruster maneuver.
14423906
Sensitivity curves for spaceborne gravitational wave interferometers
To determine whether particular sources of gravitational radiation will be detectable by a specific gravitational wave detector, it is necessary to know the sensitivity limits of the instrument. These instrumental sensitivities are often depicted ~after averaging over source position and polarization! by graphing the minimal values of the gravitational wave amplitude detectable by the instrument versus the frequency of the gravitational wave. This paper describes in detail how to compute such a sensitivity curve given a set of specifications for a spaceborne laser interferometer gravitational wave observatory. Minor errors in the prior literature are corrected, and the first ~mostly! analytic calculation of the gravitational wave transfer function is presented. Example sensitivity curve calculations are presented for the proposed LISA interferometer. PACS number~s!: 04.80.Nn, 95.55.Ym Advances in modern technology have ushered in an era of large laser interferometers designed to be used in the detection of gravitational radiation, both on the ground and in space. Such projects include the Laser Interferometric Gravitational Wave Observatory ~LIGO! and VIRGO @1,2# ground-based interferometers, and the proposed Laser Interferometer Space Antenna ~LISA! and OMEGA @3,4# spacebased interferometers. As these detectors come on-line, a new branch of astronomy will be created and a radically new view of the Universe is expected to be revealed. With the era of gravitational wave astronomy on the horizon, much effort has been devoted to the problem of categorizing sources of gravitational radiation, and extensive studies are underway to determine what sources will be visible to the various detectors. Typically, the sensitivity of detectors to sources of gravitational radiation has been illustrated using graphs which compare source strengths ~dimensionless strain! to instrument noise as functions of the gravitational wave frequency. Many different types of plots have appeared in the literature, ranging from single plots of spectral density to separate am