Analysis of neutron spectra and fluxes obtained with cold and thermal moderators at IBR-2 reactor: Experimental and computer-modeling studies

The results of experimental and computer-modeling investigations of neutron spectra and fluxes obtained with cold and thermal moderators at the IBR-2 reactor (Joint Institute for Nuclear Research (JINR), Dubna) are presented. These studies are for the YuMO small-angle neutron scattering (SANS) spectrometer (IBR-2 beamline 4). The neutron spectra have been measured for two methane cold moderators for the standard configuration of the SANS instrument. The data from both moderators under different conditions of their operation are compared. The ratio of experimentally determined neutron fluxes of cold and thermal moderators is shown at different wavelengths. Monte Carlo simulations have been carried out to determine the spectra for cold-methane and thermal moderators. The results of calculations of the ratio of neutron fluxes of cold and thermal moderators at different wavelengths are demonstrated. In addition, the absorption of neutrons in the air gaps on the way from the moderator to the investigated sample is presented. SANS with the protein apoferritin was done with both cold methane and a thermal moderator and the data were compared. The prospects for the use of a cold moderator for a SANS spectrometer at IBR-2 are discussed. The advantages of using the YuMO spectrometer with a thermal moderator with respect to the tested cold moderator are shown.     

Small-Angle Neutron Scattering at the Pulsed Reactor IBR-2: Current Status and Prospects

Crystallography Reports - Neutron diffraction studies on the small-angle neutron spectrometer YuMO (Joint Institute for Nuclear Research, Dubna), based on the IBR-2 pulsed reactor, have been...     

Erratum to: Small-Angle Neutron Scattering at the Pulsed Reactor IBR-2: Current Status and Prospects

Crystallography Reports - An Erratum to this paper has been published: https://doi.org/10.1134/S1063774522070513     

Small-angle neutron scattering study of the structure of mixed micellar solutions based on heptaethylene glycol monotetradecyl ether and cesium dodecyl sulfate

The micellization in mixed aqueous systems based on a nonionic surfactant, heptaethylene glycol monotetradecyl ether (C₁₄E₇), and an anionic surfactant, cesium dodecyl sulfate, has been investigated by small-angle neutron scattering. Preliminary data on the behavior of the C₁₄E₇ aqueous solutions (with three concentrations, 0.17, 0.5, and 1%) mixed with a small amount of anionic surfactant, cesium dodecyl sulfate, are reported.     

Lyotropic model membrane structures of hydrated DPPC: DSC and small-angle X-ray scattering studies of phase transitions in the presence of membranotropic agents

Multibilayer structures of hydrated phospholipids, often considered as model biological membranes, are, from the physical viewpoint, lyotropic liquid crystalline systems undergoing temperature-induced mesomorphic phase transitions. Effects of silver nitrate and urocanic acid on lyotropic phase states of hydrated L-α-dipalmitoylphosphatidylcholine (DPPC) have been studied by small-angle X-ray scattering and differential scanning calorimetry (DSC). Both methods show increase of the main phase-transition temperature (T_m) of hydrated DPPC upon introduction of AgNO₃ or urocanic acid, decrease of pre-transition temperature (T_p) in the presence of urocanic acid and its increase in the presence of AgNO₃. Thus, urocanic acid widened the ripple-phase temperature region. Silver nitrate caused the appearance of an additional high-temperature peak on DSC thermograms, evidencing phase separation in the system. Both agents caused minor effects on DPPC lipid bilayer repeat distance (D) in gel phase, but resulted in noticeable increase of D in the liquid crystal phase with temperature as compared to undoped DPPC structures.     

Structural Interpretation of Metastable States in Myoglobin–NO

Nitric oxide binding and unbinding from myoglobin (Mb) is central to the function of the protein. By using reactive molecular dynamics (MD) simulations, the dynamics following NO dissociation were characterized in both time and space. Ligand rebinding can be described by two processes on the 10 ps and 100 ps timescale, which agrees with recent optical and X‐ray absorption experiments. Explicitly including the iron out‐of‐plane (Fe‐oop) coordinate is essential for a meaningful interpretation of the data. The proposed existence of an “Fe‐oop/NO‐bound” state is confirmed and assigned to NO at a distance of approximately 3 Å away from the iron atom. However, calculated XANES spectra suggest that it is diffcult to distinguish between NO close to the heme‐Fe and positions further away in the primary site. Another elusive state, with Fe?ON coordination, was not observed experimentally because it is masked by the energetically more favorable but dissociative ⁴A state in this region, which makes the Fe?ON local minimum unobservable in wild‐type Mb. However, suitable active‐site mutations may stabilize this state.     

Duality for nonconvex optimization and its applications

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