Theoretical Study of Nonradiative Energy Transfer from Exciplex to Perovskites

Russian Physics Journal - The results of theoretical studies of energy transfer from the exciplex of TCTA and Bphen molecules to CaZrO3, SrZrO3, and BaZrO3 perovskites are presented. As a result,...     

Magnetocaloric effect in La(Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>13</Subscript> ferromagnets

The isothermal changes in the magnetic entropy and the lattice entropy and the adiabatic temperature change in La(Fe_0.88Si_0.12)₁₃ and La(Fe_0.86Si_0.14)₁₃ ferromagnets in a magnetic field are calculated. The calculations are performed with a generalized magnetostriction model of a ferromagnet; the calculation results are compared to experimental data. It is shown that the change in the lattice entropy decreases the magnetocaloric effect and makes it possible to explain the experimental data obtained for La(Fe _x Si_{1 − x} )₁₃ (x = 0.86, 0.88) ferromagnets. The temperature dependences of the bulk compression moduli of these ferromagnets are calculated, and these dependences indicate a strong lattice softening in the vicinity of the magnetic phase transition in them. The thermal expansion coefficient and some magnetic properties of the ferromagnet with x = 0.86 are measured to determine the numerical values of the parameters entering into calculation formulas.     

Study of nanostructures on a solar cell surface exposed on the <Emphasis Type="Italic">Mir</Emphasis> orbital station

It is shown that atomic-force microscopy under normal conditions makes it possible to obtain important information on the topography and features of nanostructures formed on the surface of cover glasses of solar cells exposed on the Mir orbital station for more than ten years. It is found that the nanostructures are multiscale; they are present on all vertical visualization scales from ∼1000 to ∼17 nm and on horizontal scales from ∼1000 to ∼100 nm. The nanoindentation study of mechanical properties of the exposed surface layer shows that the exposed surface at nanodepths is characterized by higher plastic deformation, but lower hardness and effective modulus, in comparison with unexposed surface.     

Enhanced in vitro biocompatibility of ultrafine-grained biomedical NiTi alloy with microporous surface

Bulk ultrafine-grained Ni_50.8Ti_49.2 alloy (UFG-NiTi) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study, and to further improve its surface biocompatibility, surface modification techniques including sandblasting, acid etching and alkali treatment were employed to produce either irregularly roughened surface or microporous surface or hierarchical porous surface with bioactivity. The effect of the above surface treatments on the surface roughness, wettability, corrosion behavior, ion release, apatite forming ability and cytocompatibility of UFG-NiTi alloy were systematically investigated with the coarse-grained NiTi alloy as control. The pitting corrosion potential (E_pit) was increased from 393 mV (SCE) to 704 mV (SCE) with sandblasting and further increased to 1539 mV (SCE) with following acid etching in HF/HNO₃ solution. All the above surface treatment increased the apatite forming ability of UFG-NiTi in varying degrees when soaked them in simulated body fluid (SBF). Meanwhile, both sandblasting and acid etching could promote the cytocompatibility for osteoblasts: sandblasting enhanced cell attachment and acid etching increased cell proliferation. The different corrosion behavior, apatite forming ability and cellular response of UFG-NiTi after different surface modifications are attributed to the topography and wettability of the resulting surface oxide layer.     

Contribution of the collective effects to the radiation generated by a narrow stationary γ quanta beam

The space-time distributions of electrons formed at the interaction of γ quanta with a medium are calculated. A model is proposed that describes the radiation of electrons generated by a narrow stationary γ quanta beam with respect to the collective effects.     

Propagation of acoustic waves in a fibrous fluid-filled material

The homogenization method is used for the analysis of acoustic wave propagation in a unidirectional fibrous material for the acoustic control of the fiber arrangement in manufacturing composites. Acoustic equations for a rigid periodic structure filled by a nonviscous fluid are obtained by two-dimensional asymptotic expansions. A regular square and triangular arrangement of the fibers with a round cross-section are considered. The analysis reveals that the velocity of acoustic waves is significantly affected by both the volume content and the fiber arrangement.Moscow State Academy of Chemical Engineering, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 651–655 September–October, 1997.     

The role of the putative catalytic base in the phosphoryl transfer reaction in a protein kinase: first-principles calculations

Protein kinases are important enzymes controlling the majority of cellular signaling events via a transfer of the gamma-phosphate of ATP to a target protein. Even after many years of study, the mechanism of this reaction is still poorly understood. Among many factors that may be responsible for the 1011-fold rate enhancement due to this enzyme, the role of the conserved aspartate (Asp166) has been given special consideration. While the essential presence of Asp166 has been established by mutational studies, its function is still debated. The general base catalyst role assigned to Asp166 on the basis of its position in the active site has been brought into question by the pH dependence of the reaction rate, isotope measurements, and pre-steady-state kinetics. Recent semiempirical calculations have added to the controversy surrounding the role of Asp166 in the catalytic mechanism. No major role for Asp166 has been found in these calculations, which have predicted the reaction process consisting of an early transfer of a substrate proton onto the phosphate group. These conclusions were inconsistent with experimental observations. To address these differences between experimental results and theory with a more reliable computational approach and to provide a theoretical platform for understanding catalysis in this important enzyme family, we have carried out first-principles structural and dynamical calculations of the reaction process in cAPK kinase. To preserve the essential features of the reaction, representations of all of the key conserved residues (82 atoms) were included in the calculation. The structural calculations were performed using the local basis density functional (DFT) approach with both hybrid B3LYP and PBE96 generalized gradient approximations. This kind of calculation has been shown to yield highly accurate structural information for a large number of systems. The optimized reactant state structure is in good agreement with X-ray data. In contrast to semiempirical methods, the lowest energy product state places the substrate proton on Asp166. First-principles molecular dynamics simulations provide additional support for the stability of this product state. The latter also demonstrate that the proton transfer to Asp166 occurs at a point in the reaction where bond cleavage at the PO bridging position is already advanced. This mechanism is further supported by the calculated structure of the transition state in which the substrate hydroxyl group is largely intact. A metaphoshate-like structure is present in the transition state, which is consistent with the X-ray structures of transition state mimics. On the basis of the calculated structure of the transition state, it is estimated to be 85% dissociative. Our analysis also indicates an increase in the hydrogen bond strength between Asp166 and substrate hydroxyl and a small decrease in the bond strength of the latter in the transition state. In summary, our calculations demonstrate the importance of Asp166 in the enzymatic mechanism as a proton acceptor. However, the proton abstraction from the substrate occurs late in the reaction process. Thus, in the catalytic mechanism of cAPK protein kinase, Asp166 plays a role of a "proton trap" that locks the transferred phosphoryl group to the substrate. These results resolve prior inconsistencies between theory and experiment and bring new understanding of the role of Asp166 in the protein kinase catalytic mechanism.