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Abstract: Neutron diffraction has been used to characterize the structure of the solid phase of the completed monolayer of n butane on the MgO(100) surface at low temperature. The monolayer is found to adopt a commensurate structure with lattice constants a=29.47 Å and b=4.21 Å, P2gg symmetry and four molecules in the unit cell. Excellent agreement with the experimental diffraction pattern is realized, using a Lorenztian profile to describe the line shape.

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Abstract: The regulation of striated muscle contraction involves changes in the interactions of troponin and tropomyosin with actin thin filaments. In resting muscle, myosin-binding sites on actin are thought to be blocked by the coiled–coil protein tropomyosin. During muscle activation, Ca2+ binding to troponin alters the tropomyosin position on actin, resulting in cyclic actin–myosin interactions that accompany muscle contraction. Evidence for this steric regulation by troponin–tropomyosin comes from X-ray data [Haselgrove, J.C., 1972. X-ray evidence for a conformational change in the actin-containing filaments of verterbrate striated muscle. Cold Spring Habor Symp. Quant. Biol. 37, 341–352; Huxley, H.E., 1972. Structural changes in actin and myosin-containing filaments during contraction. Cold Spring Habor Symp. Quant. Biol. 37, 361–376; Parry, D.A., Squire, J.M., 1973. Structural role of tropomyosin in muscle regulation: analysis of the X-ray diffraction patterns from relaxed and contracting muscles. J. Mol. Biol. 75, 33–55] and electron microscope (EM) data [Spudich, J.A., Huxley, H.E., Finch, J., 1972. Regulation of skeletal muscle contraction. II. Structural studies of the interaction of the tropomyosin–troponin complex with actin. J. Mol. Biol. 72, 619–632; O’Brien, E.J., Gillis, J.M., Couch, J., 1975. Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments. J. Mol. Biol. 99, 461–475; Lehman, W., Craig, R., Vibert, P., 1994. Ca(2+)-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction. Nature 368, 65–67] each with its own particular strengths and limitations. Here we bring together some of the latest information from EM analysis of single thin filaments from Pirani et al. [Pirani, A., Xu, C., Hatch, V., Craig, R., Tobacman, L.S., Lehman, W. (2005). Single particle analysis of relaxed and activated muscle thin filaments. J. Mol. Biol. 346, 761–772], with synchrotron X-ray data from non-overlapped muscle fibres to refine the models of the striated muscle thin filament. This was done by incorporating current atomic-resolution structures of actin, tropomyosin, troponin and myosin subfragment-1. Fitting these atomic coordinates to EM reconstructions, we present atomic models of the thin filament that are entirely consistent with a steric regulatory mechanism. Furthermore, fitting the atomic models against diffraction data from skinned muscle fibres, stretched to non-overlap to preclude crossbridge binding, produced very similar results, including a large Ca2+-induced shift in tropomyosin azimuthal location but little change in the actin structure or apparent alteration in troponin position.

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Abstract: In condensed matter, light propagation near resonances is described in terms of polaritons, electro-mechanical excitations in which the time-dependent electric field is coupled to the oscillation of charged masses. This description underpins our understanding of the macroscopic optical properties of solids, liquids and plasmas, as well as of their dispersion with frequency. In ferroelectric materials, terahertz radiation propagates by driving infrared-active lattice vibrations, resulting in phonon-polariton waves. Electro-optic sampling with femtosecond optical pulses3, 4, 5 can measure the time-dependent electrical polarization, providing a phase-sensitive analogue to optical Raman scattering6, 7. Here we use femtosecond time-resolved X-ray diffraction8, 9, 10, a phase-sensitive analogue to inelastic X-ray scattering11, 12, 13, to measure the corresponding displacements of ions in ferroelectric lithium tantalate, LiTaO3. Amplitude and phase of all degrees of freedom in a light field are thus directly measured in the time domain. Notably, extension of other X-ray techniques to the femtosecond timescale (for example, magnetic or anomalous scattering) would allow for studies in complex systems, where electric fields couple to multiple degrees of freedom

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Abstract: The commercially available peptide coupling reagent 1-hydroxy-7-azabenzotriazole has been shown to crystallize in two polymorphic forms. The two polymorphs differ in their hydrogen-bonding motif, with form I having an R-2(2)(10) dimer motif and form II having a C(5) chain motif. The previously unreported form II was used as an informal blind test of computational crystal structure prediction for flexible molecules. The crystal structure of form II has been successfully predicted blind from lattice-energy minimization calculations following a series of searches using a large number of rigid conformers. The structure for form II was the third lowest in energy with form I found as the global minimum, with the energy calculated as the sum of the ab initio intramolecular energy penalty for conformational distortion and the intermolecular lattice energy which is calculated from a distributed multipole representation of the charge density. The predicted structure was sufficiently close to the experimental structure that it could be used as a starting model for crystal structure refinement. A subsequent limited polymorph screen failed to yield a third polymorphic form, but demonstrated that alcohol solvents are implicated in the formation of the form I dimer structure.

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Abstract: Measurement of the fill pattern of synchrotron storage rings is important for several reasons, including checking the quality of the injection system and the generation of special fill patterns for time-resolved experiments. For the new national X-ray facility based on the Diamond 3 GeV electron storage ring, it is planned to measure the beam quality in a short time of several minutes by using a time-correlated single photon counting method. The method has very good time resolution (50 ps) and high dynamic range (more than six orders of magnitude). In this paper, we describe our experimental setup, a series of tests to evaluate the system limitations using short pulse lasers on laboratory test benches, the existing SRS national X-ray facility, and the third-generation synchrotron radiation facility SLS. Taking all these results together, we are able to predict the system performance for Diamond.