Crystallography Reports最新文献

A comparison of absorption spectra revealed activator rare-earth ions at which the effect of impurity and radiation-induced decrease in the optical density at a wavelength of the short-wavelength edge of CaF₂ was observed; this effect is explained by the shift of the interband absorption of the crystal to the spectral vacuum UV region. Experiments showed that a multiple increase in the transparency of CaF₂ in the wavelength range of 190–200 nm occurs when doping crystals with ions of europium and ytterbium—elements having the lowest chemical potentials in the lanthanide series. This feature determines the ability of Eu³⁺ and Yb³⁺ to charge conversion of the oxidation state (+3 → +2). The absorption spectra of these ions were compared with the spectra of Nd³⁺ and Er³⁺ ions, which do not exhibit any changes in charge upon γ irradiation of CaF₂ crystals and have chemical potentials comparable in magnitude with those of other elements of the series, for which this parameter is 2–5 times larger than for Eu and Yb. The spectral features of the absorption of gadolinium ions were additionally investigated in a simple CaF₂ : 2 mol % GdF₃ crystal and a solid solution based on CaF₂ with addition of 10 mol % GdF₃ with the empirical formula Ca_0.9Gd_0.1F_2.1. A peculiarity of Gd is that, in contrast to other rare-earth elements, its ground energy 4f levels in the tri- and divalent states are located, respectively, on the top of the valence band and above the bottom of the conduction band. CaF₂ : 0.1 mol % Eu²⁺ crystals are proposed as an optical medium almost completely transparent in the wavelength range of 190–200 nm. The revealed effect is of interest for designing optical materials for systems of near-Earth and space monitoring for environmental and military purposes, as well as for the production of lenses applied in UV microphotolithography systems.

The TE- and TM-polarized surface plasmon polaritons in a metal film placed between a periodic superlattice and a homogeneous dielectric or between two periodic superlattices have been theoretically investigated. Dispersion equations are derived in the form of the equality to zero of the determinant of a real symmetric matrix composed of the surface impedances of the media and film. Some general properties of the impedances and eigenvalues of the matrices entering the dispersion equations are established.

In order to search for new highly conductive fluoride materials, X-ray diffraction and conductometric properties of alloys (polycrystals) of the compositions 38LaF₃^.57NdF₃^.5MF₂ and 47.5CeF₃^.47.5PrF₃^.5MF₂ (mol %) for M = Ca, Sr, and Ba were studied. The samples were prepared by spontaneous crystallization of homogenized melts in the MF₂−LaF₃−NdF₃ and MF₂−CeF₃−PrF₃ systems in a fluorinating atmosphere. The resulting alloys are solid solutions of the compositions (La_0.4Nd_0.6)_0.95M_0.05F_2.95 and (Ce_0.5Pr_0.5)_0.95M_0.05F_2.95 with a tysonite-type structure (sp. gr. (Pbar {3}c1)). The ion-conductive properties of three-component (R,R')_0.95M_0.05F_2.95 (R, R' = 0.4La + 0.6Nd and 0.5Ce + 0.5Pr) and two-component R_0.95M_0.95F_2.95 (R = La, Ce, Pr, Nd) compositions were carried out. It was found that, in the case of (Ce_0.5Pr_0.5)_0.95Sr_0.05F_2.95 polycrystals, the interior grain conductivity, equal to σ_293 K = 1.3 × 10⁻³ S/cm at room temperature, significantly exceeds the ionic conductivity of bulk Ce_0.95Sr_0.05F_2.95 and Pr_0.95Sr_0.05F_2.95 solid solution crystals, which confirms the influence of the geometric factor on the magnitude of ionic conductivity in the families of crystals with a tysonite-type structure. The study shows the prospects of further comprehensive research of three-component compositions (R,R')_0.95M_0.05F_2.95 to achieve the limit value of room-temperature conductivity.

The first crystal structure of dUTPase from the bacteriophage Vibrio phage phi3 was determined at 1.95 Å resolution. This protein has a homotrimeric quaternary structure characteristic of this type of dUTPases and contains a short loop in the N-terminal region of the polypeptide chain, which is specific for Т5-like bacteriophage dUTPases. It was demonstrated that, despite a low amino acid sequence homology between phage (Т5 and phi3) dUTPases, phi3 dUTPase can effectively replace phage Т5 dUTPase during the lytic cycle.

Comparative studies of the characteristics of high-molecular organic matter (cotton lint), subjected to pyrolytic carbonization under conditions of high-intensity microwave radiation in various gaseous media (N₂, CO₂, Ar), have been carried out. The following methods were applied: determination of the adsorption activity by methylene blue indicator, X-ray fluorescence analysis, transmission electron microscopy with microanalysis, and X-ray phase analysis. Electrodes were fabricated from carbon materials obtained by microwave carbonization using different gases, and symmetric cells were assembled according to the two-electrode scheme. Their electrochemical properties were investigated using cyclic voltammetry and galvanostatic charge–discharge methods. The materials obtained using a CO₂ gaseous medium were found to possess the best characteristics.

Pyridoxal 5'-phosphate (PLP)-dependent enzymes are among the most abundant enzymes present in nearly all organisms, where they perform more than 150 different catalytic functions. Based on the three-dimensional structures, these enzymes are classified into seven (I–VII) different fold types. In these enzymes, the cofactor is bound as a Schiff base with the conserved active-site lysine residue. Recently, we have discovered the protein from the bacterium Variovorax paradoxus (VAPA), which belongs to the fold type IV and has a significant structural similarity to transaminases. It contains an asparagine residue at the position of catalytic lysine in fold type IV transaminases and, consequently, it cannot form a Schiff base with PLP and does not have aminotransferase activity. In this study, a single-point mutant of the VAPA protein with the N174K substitution was obtained and its three-dimensional structure was determined. An analysis of the obtained data showed that the introduced mutation restores the ability of  VAPA to form a Schiff base with the cofactor.

A complex compound based on a divalent cobalt ion as a complexing agent and two different ligands, 1,5-disulfonaphthalene (1,5-naphthalenedisulfonic acid, Na₂NDS) and ethylenediamine, has been synthesized for the first time. The structure of the metal complex was investigated by IR and Raman spectroscopy. It was proved that the compound has a stoichiometric composition [Co(NO₃)⋅(C₂H₄(NH₂)₂)₂](_{2}^{ + })⋅(C₁₀H₆O₆S₂)^2–⋅4H₂O. The complex was subjected to thermal analysis to determine its stability. Its stability, determined by the existence of intramolecular hydrogen bonds, was confirmed. Molecular rearrangements occur in the structure at 142°C, and gradual decomposition of the compound begins; in the range of 305–311°C the material studied undergoes melting, and complete destruction occurs only at 500°C. The crystal structure of the obtained metal complex was studied by X-ray diffraction analysis. A Hirshfeld surface analysis revealed intermolecular interactions between hydrogen and oxygen atoms and between hydrogen and nitrogen atoms, i.e., hydrogen bonds, stabilizing the complex.

Helical particles are of interest because of their similarity to real nanostructures that are formed in processes of self-organization of biopolymers (for example, carrageenans, DNA, etc.). On the other hand, the determination of the structural parameters of such particles from small-angle X-ray scattering data is a complex issue due to the poor conditionality of the inverse problem. This is evident from the practice of using the well-known bead modeling programs. The study considers a modification of the search algorithm in a limited area of space and the behavior of the solutions depending on the parameters of the objective function responsible for the connectivity of the structure, the weight scheme for the scattering intensity curve, and the angular range of the data. The stability of the solutions was statistically assessed using a sequential model search by varying the weights of the penalty terms. The empirical dependences of the optimal values of the search parameters on the parameters of the pair distance distribution curves were determined.

The formation of orthorhombic and monoclinic polymorphic modifications of hydrazine sulfate from an aqueous solution in the temperature range from 5 to 35°C was studied. A curve of the hydrazine sulfate solubility in water in the range of 5–25°C was obtained. It is shown that, with a decrease in the growth temperature, the mass fraction of precipitated monoclinic hydrazine sulfate crystals increases. A possible temperature-induced transition from the orthorhombic to the monoclinic modification was detected when cooling to the boiling temperature of liquid nitrogen.