Multiplication Factor for Subcritical Multiplying System and the Efficiency of Source Neutrons

Physics of Atomic Nuclei - In this paper, the analysis of the relation between the multiplication factor of neutrons of the multiplying system measured in critical experiments and the system...     

Thermal rearrangement of 2,5-diaryl-3,3,4,4-tetracyanopyrrolidines

It was shown that briefly boiling solutions of 2,5-diaryl-3,3,4,4-tetracyanopyrrolidines in DMF caused rearrangement into 5-aryl-2-(N-arylidenamino)-3,4-dicyanopyrroles, which are methylated with diazomethane both at the NH bond and at the carbon atom of the azomethine fragment.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 42–46, January, 1995.     

Application of Horner–Emmons Olefination to the Crown-Ether Derivatives

An effective synthetic approach to the preparation of a new crown-ether vinylogs involving the Horner–Emmons olefination of carbonyl precursors with the use of C₂- and C₅-phosphonates was proposed. The effects of the conjugation chain length and the nature of the terminal polar functions in the phosphonate reagent on the yield and process stereoselectivity were discussed.     

Chemistry of 1,1,2,2-tetracyanoethane

Conclusions 1-(N,N-Dimethylhydrazino)-4-methyl-2,2,3,3-tetracyanocyclopentane is isomerized reverslbly in solution, with inversion of the configuration of the tetrahedral carbon atom in position 1.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1075–1079, May, 1989.     

Synthesis of 5-amino-3a,4-dicyano-2,3,3a,6a-tetrahydrofuro[2,3-b]pyrroles

Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, p. 282, February, 1992.     

Determination of Silicon Impurities in High-Purity Arsenic Using Atomic Emission Spectrometry with Matrix Distillation from a Tipped Electrode

A procedure for analyzing high-purity arsenic by atomic emission spectrometry (AES) was proposed that provides the preconcentration of silicon and other nonvolatile impurities in a crater of a graphite electrode using matrix distillation from 2-g sample portions as As₂O₃. The procedure is characterized by a low correction for the blank experiment. The detection limit for silicon was 4 × 10^–7 wt %.     

NMR and X-ray crystallography, complementary tools in structural proteomics of small proteins

NMR spectroscopy and X-ray crystallography, the two primary experimental methods for protein structure determination at high resolution, have different advantages and disadvantages in terms of sample preparation and data collection and analysis. It is therefore of interest to assess their complementarity when applied to small proteins. Structural genomics/proteomics projects provide an ideal opportunity to make such comparisons as they generate data in a systematic manner for large enough numbers of proteins to allow firm conclusions to be drawn. Here we report a comparison for 263 unique proteins screened by both NMR spectroscopy and X-ray crystallography in our structural proteomics pipeline. Only 21 targets (8%) were deemed amenable to both methods based on an initial 2D 15N-HSQC NMR spectrum and optimized crystallization trials. However, the use of both methods in the pipeline increased the total number of targets amenable to structure determination to 107, with 43 amenable to NMR only and 43 amenable to X-ray crystallographic methods only. We did not observe a correlation between 15N-HSQC spectral quality and the success of the same protein in crystallization screens. Similar results were found for an independent set of 159 proteins as reported in the accompanying paper by Snyder et al. Thus, we conclude that both methods are highly complementary, and in order to increase the number of proteins suited for structure determination, we suggest that both methods be used in parallel in screening of all small proteins for structure determination.     

Covalent chemistry and conformational dynamics of topologically chiral amide-based molecular knots

The readily available in gram quantities tris(allyloxy)knot of the amide-type 5 (knotane) can be completely and partially deprotected with nBu(3)SnH in the presence of a palladium catalyst resulting in hydroxyknotanes 7-9. These, in turn, react with diethylchlorophosphate giving rise to knotanes equipped with between one and three phosphoryl groups. Sulfonylation of bis(allyloxy)monohydroxyknotane 8 with p-toluenesulfonyl chloride and, following removal of one or two allyl groups from the intermediate monosulfonate 13, give rise to sulfonyloxy-allyloxy-hydroxy- and sulfonyloxy-dihydroxy-knotanes 15 and 14, respectively. This provides a convenient method for the preparation of knotanes with any substitution pattern. All new knotanes have been isolated in preparative amounts and as highly pure substances with an exception of allyloxy-dihydroxyknotane 9. This compound could only be obtained as a mixture with the corresponding monohydroxy-derivative 8. The structures of all synthesized compounds were established by means of FAB and MALDI TOF mass spectrometry, (1)H and (31)P NMR spectroscopy. The triphosphorylated knotane 10 exhibits high solubility in alcohols, allowing its complete enantiomeric resolution with a commercially available chiral HPLC column. (1)H,(1)H DQF-COSY correlation spectroscopy along with H/D exchange experiments and ab initio calculations provided the first detailed (1)H NMR signal assignments of knotanes in [D(6)]DMSO solution. The combination of variable temperature (1)H and (31)P NMR spectroscopy and molecular modeling has been applied to study the conformational behavior of the new knotanes in different solvents. It has been shown that in DMSO solution at room temperature knotanes exist in a relatively rigid nonsymmetrical conformation similar to that found in the solid state while faster conformational exchange leading to the average D(3) symmetrical structure was detected in a number of other solvents.