The features of the spatial structure of the gramicidine A-cesium complex

AbstractEarlier obtained two-dimensional 1H-NMR spectroscopy data were used to analyze the spatial structure and conformational mobility of the double right ↑↓ ππLD7.2 helix of the complex formed by gramicidine A and Cs+ ions in an organic solvent (a chloroform-methanol mixture). Analysis of the experimental data permitted the determination of a set of conformations for each of the high-mobility residue side chains in the solution. The energy refinement of the most probable conformation of the double right ↑↓ ππLD7.2 helix was made and conformational rearrangements of the tryptophan residue side chain were studied in detail

Operational experience of GNSS receivers with Chip Scale Atomic Clocks for baseline measurements

Currently, one of the topical issues of improving GLONASS system is modernization of its uniformity measurement equipment, including RF measurement equipment and electronic length measurement equipment. To this end, at the Spatial Reference Proving Ground of theSiberian State University of Geosystems and Technologies (SSUGT), the authors of this article carried out a successful experiment to measure a short GNSS baseline by receivers equipped with Chip Scale Atomic Clocks (CSACs) with instability of 10−11 showed that the mean deviation between the slant distance (D) measured using GNSS receivers connected to CSACs and their certified value varied in the range of 0.1–2.5 mm, with the average value of 0.9 mm. The mean deviation obtained using GNSS geodetic receivers not connected to CSAC and their certified value made up 9.4 mm. The obtained experimental results suggest that substitution of quartz frequency generators with temperature compensation used in geodetic GNSS receivers for Chip Scale Atomic Clocks in any metrological or verification kit increases accuracy and reliability of short baselines measurements results, which highly perspective in view of development of techniques for creating reference baselines with a reproduction error of unit length of about 1 mm per 1 km. The above-mentioned experiment opens up new horizons for the use of Chip Scale Atomic Clocks in such fields of science as metrological support of geodetic equipment, geodesy, etc

Formation of Ge-Sn nanodots on Si(100) surfaces by molecular beam epitaxy

The surface morphology of Ge0.96Sn0.04/Si(100) heterostructures grown at temperatures from 250 to 450°C by atomic force microscopy (AFM) and scanning tunnel microscopy (STM) ex situ has been studied. The statistical data for the density of Ge0.96Sn0.04 nanodots (ND) depending on their lateral size have been obtained. Maximum density of ND (6 × 1011 cm-2) with the average lateral size of 7 nm can be obtained at 250°C. Relying on the reflection of high energy electron diffraction, AFM, and STM, it is concluded that molecular beam growth of Ge1-xSnx heterostructures with the small concentrations of Sn in the range of substrate temperatures from 250 to 450°C follows the Stranski-Krastanow mechanism. Based on the technique of recording diffractometry of high energy electrons during the process of epitaxy, the wetting layer thickness of Ge0.96Sn0.04 films is found to depend on the temperature of the substrate

The difference between ADP-beryllium fluoride and ADP-aluminium fluoride complexes of the spin-labeled myosin subfragment 1

AbstractElectron paramagnetic resonance (EPR) spectroscopy was used for investigation of the structure of spin-labeled myosin subfragment 1 (S1) containing ADP and phosphate analogues, such as orthovanadate, aluminium fluoride (AlF4), and beryllium fluoride (BeFx). It has been shown that the local conformational changes in the region of Cys-707, induced by formation of the S1-ADP-BeFx complex, differ from those of S1 containing ADP-AlF4 or other phosphate analogues but are similar to the changes which occur in the presence of ADP or ATPγS. It is suggested that S1-ADP-AlF4 and S1-ADP-BeFx complexes represent structural analogues of different transition states of the ATPase cycle, namely the intermediate states S1**-ADP-Pi and S1*-ATP, respectively

Stereoselective Double Lewis Acid/Organo-Catalyzed Dimerization of Donor–Acceptor Cyclopropanes into Substituted 2-Oxabicyclo[3.3.0]octanes

A new approach for the dimerization of donor–acceptor cyclopropanes (2-arylcyclopropane-1,1-dicarboxylates) under double-catalysis conditions by treatment with 20 mol % of GaCl₃ and dimethyl 3,5-dimethyl-1-pyrazoline-3,5-dicarboxylate as a specific organocatalyst has been found. Under these conditions, the starting compounds are regio- and stereospecifically converted into polysubstituted 2-oxabicyclo[3.3.0]­octanes. Two new rings, one C–O bond, and two C–C bonds are formed in this process, and four stereocenters are thus created. The reaction mechanism was thoroughly studied by NMR spectroscopy, and a number of intermediates were detected

Complexes of Donor–Acceptor Cyclopropanes with Tin, Titanium, and Gallium Chlorides  Mechanism Studies

Hitherto unknown complexes of dimethyl cyclopropane-1,1-dicarboxylate with Lewis acids (Sn, Ti, and Ga chlorides) have been isolated and characterized. Their structures, which were previously assumed from theoretical considerations, have been confirmed, and the activating effect of Lewis acids on the cyclopropane ring has been shown experimentally. A single crystal of the complex with SnCl₄ has been obtained, and an X-ray diffraction study has been performed. Hitherto unknown complexes of donor–acceptor cyclopropanes with GaCl₃ belonging to a new type and having a 1,2-dipole (ylide) structure have been obtained and characterized. 1D and 2D NMR spectroscopy on ¹H, ¹³C, ³⁵Cl, ⁷¹Ga, and ¹¹⁹Sn nuclei were used to study the structures of the complexes. Furthermore, DOSY diffusion NMR spectroscopy was used to determine the diffusion coefficient in solution and the molecular masses of the complexes. The data obtained enrich the picture of transformations of donor–acceptor cyclopropanes widely used in organic synthesis. The practical use of the gallium phenylcyclopropanedicarboxylate complex in a synthesis of polysubstituted cyclic structures in one synthetic stage has also been demonstrated

Complexes of Donor–Acceptor Cyclopropanes with Tin, Titanium, and Gallium Chlorides  Mechanism Studies

Hitherto unknown complexes of dimethyl cyclopropane-1,1-dicarboxylate with Lewis acids (Sn, Ti, and Ga chlorides) have been isolated and characterized. Their structures, which were previously assumed from theoretical considerations, have been confirmed, and the activating effect of Lewis acids on the cyclopropane ring has been shown experimentally. A single crystal of the complex with SnCl₄ has been obtained, and an X-ray diffraction study has been performed. Hitherto unknown complexes of donor–acceptor cyclopropanes with GaCl₃ belonging to a new type and having a 1,2-dipole (ylide) structure have been obtained and characterized. 1D and 2D NMR spectroscopy on ¹H, ¹³C, ³⁵Cl, ⁷¹Ga, and ¹¹⁹Sn nuclei were used to study the structures of the complexes. Furthermore, DOSY diffusion NMR spectroscopy was used to determine the diffusion coefficient in solution and the molecular masses of the complexes. The data obtained enrich the picture of transformations of donor–acceptor cyclopropanes widely used in organic synthesis. The practical use of the gallium phenylcyclopropanedicarboxylate complex in a synthesis of polysubstituted cyclic structures in one synthetic stage has also been demonstrated

The nature of the ligand's side chain interacting with the S1'-subsite of metallocarboxypeptidase T (from Thermoactinomyces vulgaris) determines the geometry of the tetrahedral transition complex.

The carboxypeptidase T (CPT) from Thermoactinomyces vulgaris has an active site structure and 3D organization similar to pancreatic carboxypeptidases A and B (CPA and CPB), but differs in broader substrate specificity. The crystal structures of CPT complexes with the transition state analogs N-sulfamoyl-L-leucine and N-sulfamoyl-L-glutamate (SLeu and SGlu) were determined and compared with previously determined structures of CPT complexes with N-sulfamoyl-L-arginine and N-sulfamoyl-L-phenylalanine (SArg and SPhe). The conformations of residues Tyr255 and Glu270, the distances between these residues and the corresponding ligand groups, and the Zn-S gap between the zinc ion and the sulfur atom in the ligand's sulfamoyl group that simulates a distance between the zinc ion and the tetrahedral sp3-hybridized carbon atom of the converted peptide bond, vary depending on the nature of the side chain in the substrate's C-terminus. The increasing affinity of CPT with the transition state analogs in the order SGlu, SArg, SPhe, SLeu correlates well with a decreasing Zn-S gap in these complexes and the increasing efficiency of CPT-catalyzed hydrolysis of the corresponding tripeptide substrates (ZAAL > ZAAF > ZAAR > ZAAE). Thus, the side chain of the ligand that interacts with the primary specificity pocket of CPT, determines the geometry of the transition complex, the relative orientation of the bond to be cleaved by the catalytic groups of the active site and the catalytic properties of the enzyme. In the case of CPB, the relative orientation of the catalytic amino acid residues, as well as the distance between Glu270 and SArg/SPhe, is much less dependent on the nature of the corresponding side chain of the substrate. The influence of the nature of the substrate side chain on the structural organization of the transition state determines catalytic activity and broad substrate specificity of the carboxypeptidase T

Growth of epitaxial SiSn films with high Sn content for IR converters

Growth of SiSn compounds with a Sn content from 10 to 35% is studied. The morphology and surface structure of the SiSn layers are examined and the kinetic diagram of the morphological state of SiSn films is established in the temperature range of 150–450°C. During the growth of SiSn films from 150 to 300°C, oscillations of specular beam were observed. For the first time, periodic multilayer SiSn/Si structures with pseudomorphic monocrystalline SiSn layers with the Sn content from 10 to 25% are grown. The c(8×4) and (5×1) superstructures are identified during the growth of Si on the SiSn layer and the conditions are determined for the formation of the desired Si surface structure by controlling the growth temperature. From the diffraction reflection curves, the lattice parameter, the SiSn composition, and the period in the multilayer periodic structure are defined, which with high precision correspond to the specified values

Growth of epitaxial SiSn films with high Sn content for IR converters

Growth of SiSn compounds with a Sn content from 10 to 35% is studied. The morphology and surface structure of the SiSn layers are examined and the kinetic diagram of the morphological state of SiSn films is established in the temperature range of 150–450°C. During the growth of SiSn films from 150 to 300°C, oscillations of specular beam were observed. For the first time, periodic multilayer SiSn/Si structures with pseudomorphic monocrystalline SiSn layers with the Sn content from 10 to 25% are grown. The c(8×4) and (5×1) superstructures are identified during the growth of Si on the SiSn layer and the conditions are determined for the formation of the desired Si surface structure by controlling the growth temperature. From the diffraction reflection curves, the lattice parameter, the SiSn composition, and the period in the multilayer periodic structure are defined, which with high precision correspond to the specified values