Conformational Study of Glycal-Type Neuraminidase Inhibitors

The conformational flexibility of two glycal-type neuraminidase inhibitors has been studied, using several molecular modeling techniques. In agreement with the experimental data available, an intramolecular hydrogen bond, representing a key structural feature that controls the conformer distribution in solution, has been identified. The contribution of each substituent to the overall equilibrium was evaluated using simplified derivatives. Additionally, four methods for estimating NMR coupling constants from dihedral angles were evaluated and the Haasnoot method was found to be appropriate for this class of sugars. These results should allow a better understanding of the structural parameters governing physico-chemical properties of glycal-like compounds. Supplemental materials are available for this article. Go to the publisher's online edition of Journal of Carbohydrate Chemistry to view the free supplemental file.     

Graph 3-coloring with a hybrid self-adaptive evolutionary algorithm

This paper proposes a hybrid self-adaptive evolutionary algorithm for graph coloring that is hybridized with the following novel elements: heuristic genotype-phenotype mapping, a swap local search heuristic, and a neutral survivor selection operator. This algorithm was compared with the evolutionary algorithm with the SAW method of Eiben et al., the Tabucol algorithm of Hertz and de Werra, and the hybrid evolutionary algorithm of Galinier and Hao. The performance of these algorithms were tested on a test suite consisting of randomly generated 3-colorable graphs of various structural features, such as graph size, type, edge density, and variability in sizes of color classes. Furthermore, the test graphs were generated including the phase transition where the graphs are hard to color. The purpose of the extensive experimental work was threefold: to investigate the behavior of the tested algorithms in the phase transition, to identify what impact hybridization with the DSatur traditional heuristic has on the evolutionary algorithm, and to show how graph structural features influence the performance of the graph-coloring algorithms. The results indicate that the performance of the hybrid self-adaptive evolutionary algorithm is comparable with, or better than, the performance of the hybrid evolutionary algorithm which is one of the best graph-coloring algorithms today. Moreover, the fact that all the considered algorithms performed poorly on flat graphs confirms that graphs of this type are really the hardest to color.     

Computer-based technique for generating layer sequences of polytypes

The computer operation is used to find all the possible layer sequences of poly-types. Identical arrangements are separated using the corresponding values of calculated structure factor.     

Synthesis of new thiazole-2, -4, and -5-yl-(amino)methylphosphonates and phosphinates: unprecedented cleavage of thiazole-2 derivatives under acidic conditions

An efficient and reliable synthesis of new thiazole-(amino)methylphosphonic, phosphinic acids, and phosphine oxides is reported. The synthetic protocol is based on nucleophilic addition of phosphorous species to thiazole derived imines. Unexpectedly, it was discovered that heating of thiazole-2-yl-(amino)methylphosphonates and phosphinates with aqueous HCl leads to their decomposition resulting in a rupture of the C-P bond, rejecting of the phosphorus containing fragment and formation of the corresponding secondary N-(thiazole-2-yl-methyl)-alkylamines. Two alternative mechanisms for this cleavage are postulated.     

Sard's theorem for mappings in Hölder and Sobolev spaces

We prove various generalizations of classical Sard's theorem to mappings f:M ^m →N ⁿ between manifolds in Hölder and Sobolev classes. It turns out that if f ∈ C ^k,λ (M ^m ,N ⁿ ), then—for arbitrary k and λ—one can obtain estimates of the Hausdorff measure of the set of critical points in a typical level set f ^?1(y). The classical theorem of Sard holds true for f ∈ C ^k with sufficiently large k, i.e., k>max(m?n,0); our estimates contain Sard's theorem (and improvements due to Dubovitskii and Bates) as special cases. For Sobolev mappings between manifolds, we describe the structure of f ^?1(y).     

Baryon asymmetry of the proton sea at low-x

We predict a nonvanishing baryon asymmetry of the proton sea at low x. It is expected to be about 7% and nearly x-independent at x < 0.5 × 10^-3. The asymmetry arises from the baryon-antibaryon component of the Pomeron, rather than from the valence quarks of the proton, which are wide believed carriers of baryon number. Experimental study of x-distribution of the baryon asymmetry of the proton sea can be performed in ep or γp interactions at HERA, where x ～ 10^-5 are reachable, smaller than at any of existing or planned proton colliders.     

Catalysis of hydrosilylation. XV. A poly(phosphino-organosiloxanyl)silicate-supported rhodium(I) catalyst for gas-phase hydrosilylation of acetylene

Wilkinson's complex [RhCl(PPh₃)₃], anchored to phosphino-organosiloxane macromolecules grafted onto chrysotile asbestos, appeared to be an unusually effective and selective catalyst for the synthesis of vinylsilanes via gas-phase hydrosilylation of acetylene, particularly by methyldichlorosilane in a continuous-flow microreactor as well as in a laboratory reactor. Catalytic and kinetic parameters show an advantage for the catalyst based on the chrysotile-polyorganosiloxane support (catalyst CHR–Rh) over the phosphinated silica (catalyst A).     

Experimental and computational studies of the thermal decomposition of halon 1211

Thermal pyrolysis of halon 1211 (CBrClF₂), diluted in nitrogen, in a tubular alumina reactor, has been studied over the temperature range of 773–1073 K at residence times from 0.3 to 2 s. At temperatures below 973 K, the major products were CCl₂F₂, CBr₂F₂, C₂Cl₂F₄, C₂BrClF₄, C₂F₄, and C₂Br₂F₄. Further increasing temperature resulted in the formation of CBrF₃, CClF₃, and many other species whose formation necessitated the rupture of C? F bonds. Coke formation was also observed on the surface of the reactor at high temperatures. A kinetic reaction scheme involving 16 species and 25 reaction steps was developed and applied to model the thermal pyrolysis of halon 1211 over the temperature range of 773–973 K. Sensitivity analysis suggests that the reaction CBrClF₂ + CClF₂→CCl₂F₂ + CBrF₂ constitutes the major pathway for the decomposition of halon 1211 under the conditions investigated. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 134–146, 2005