Main features of the modernized small-angle neutron scattering spectrometer (YuMO) at IBR-2M pulsed reactor are described. New installations for sample environment of the spectrometer are highlighted. The modernized SANS instrument (YuMO) is equipped with a new type of position sensitive detector as well as two detector system which provide a unique dynamic range (Q max /Q min ratio is about 90).Sample environment is extended with a magnetic system (magnetic field about 2.5 Tesla), automated high pressure setup which allows simultaneous SANS and volumetric high pressure studies and light illumination system. In particular, these developments led to considerable improvements of resolution of the instrument (about 1%) and opened the possibility to study anisotropic materials and perform efficient high pressure studies.

The mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS) in bilayers of multilamellar vesicles were studied by method of densitometry. In the range of DOPS molar fraction 0-100%, specific volumes of mixtures of lipids coincide with theoretical values in the case of ideal mixing of lipids. The coefficient of thermal volume expansivity was evaluated for different DOPS molar fractions; it has values in the range (71.1-73.6) × 10(-5) K(-1). Molecular volumes for pure DOPC and DOPS were evaluated for temperature range 15-45 °C. At 30 °C, molecular volumes are 1304 Å(3) and 1254 Å(3) for DOPC and DOPS, respectively. The estimated volume of head group of DOPS at 30 °C is 275 Å(3). Time-dependent density scans revealed that the dispersion of DOPC vesicle sedimentation during measurements induces an observed increasing density of dispersion in agreement with recently published observations. The presence of charged DOPS in vesicles prevents them from sedimentation and values of density are stable over a prolonged time.

Despite remarkable progress, mainly due to the development of LCP and ‘bicelle’ crystallization, lack of structural information remains a bottleneck in membrane protein (MP) research. A major reason is the absence of complete understanding of the mechanism of crystallization. Here we present small-angle scattering studies of the evolution of the “bicelle” crystallization matrix in the course of MP crystal growth. Initially, the matrix corresponds to liquid-like bicelle state. However, after adding the precipitant, the crystallization matrix transforms to jelly-like state. The data suggest that this final phase is composed of interconnected ribbon-like bilayers, where crystals grow. A small amount of multilamellar phase appears, and its volume increases concomitantly with the volume of growing crystals. We suggest that the lamellar phase surrounds the crystals and is critical for crystal growth, which is also common for LCP crystallization. The study discloses mechanisms of “bicelle” MP crystallization and will support rational design of crystallization.