Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

Synthesis of N'-substituted amidines through the cleavage an oxadiazolone heterocycle by weakly basic nucleophiles. Effect of the nature of the nucleophile and of the nucleophile/substrate molar ratio

The reaction of 1-ethoxycarbonylmethyl-5,5,7,7-tetramethyl-2-oxo-tetrahydroimidazo[1,5-b]oxadiazol-6-oxyl with the weakly basic nucleophiles NaN₃, NaCN, KF, KBr, KCl and NaNO₂ has been studied. It was shown for the first time that, as in the case of NaOH and MeONa, the reaction occurs with opening of the oxadiazolone ring to form exo-N-substituted amidines. It was shown that the weakly basic nucleophiles readily react with substrates which contain a substituent sensitive to attack by such nucleophiles as NaOH or MeONa. The effect of the nature of the nucleophiles on the reaction course for opening of the oxadiazolone ring was also studied. It was found that the reactivity of the nucleophiles in DMSO changes in the series F⁻ > CN⁻ > N₃^– >NO₂⁻ > Cl⁻ > Br⁻ and qualitatively correlates with their basicities in this solvent. Examination of the effect of the ratio of the reagents on the degree of conversion of the starting oxadiazolone has shown that a quantity of nucleophiles less than one equivalent also allowed the cleavage reaction of the oxadiazolone heterocycle to go to completion through just increasing the reaction time. The experimental data obtained lends support to the proposed reaction scheme. Dedicated to Academician B. A. Trofimov in his 70^th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 71–78, January, 2009.     

Optical properties of 3D photonic crystals filled with CdSe/CdS quantum dots

We study 3D globular photonic crystals based on synthetic opals filled with semiconductor core/shell quantum dots CdSe/CdS by measuring the photoluminescence spectra. The spectra were obtained using 369, 384, and 408 nm LED light excitation and involving a pulse YAG laser operating at 365 and 266 nm. The study shows that the photoluminescence spectra of opal filled with CdSe/CdS changes sufficiently in comparison with spectra taken for pure opal and a reference colloidal solution of CdSe/Cds quantum dots in toluene. Such opals may be used to fabricate a narrow-band light sources.     

Raman Scattering in Chiral-Pure and Racemic Phases of Tryptophan and Tyrosine Polycrystals

Optics and Spectroscopy - Raman spectra of tryptophan and tyrosine polycrystals have been analyzed in a wide spectral range by fiber-optic spectroscopy. The Raman spectra have been recorded with a...     

Unbiased estimation of P(Y < X) in the normal case

The considered problem arises in reliability theory.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 136, pp. 5–12, 1984.     

Studies toward the synthesis of 4-(2-R-ethyl)amino-2,2,5,5-tetramethyl-3-imidazoline 1-oxyls. nucleophilic substitution of bromide in the N-Alkyl chain of the 1,2,4-oxadiazol-2-one precursor

A synthetic approach to access the new nitroxides of the amidine type exhibiting pH-dependent EPR spectra through substitution of a halide in the exo-N-halogenoalkyl chain of 1-(2-bromoethyl)-6-oxyl-5,5,7,7-tetramethyltetrahydroimidazo[1,5-b][1,2,4]oxadiazol-2-one is reported. In this approach, an oxycarbonyl moiety of the oxadiazolone heterocycle plays the role of a "protecting group" for the amidine functionality. A nucleophilic cleavage of the oxadiazolone heterocycle under mild nonbasic conditions, applicable to substrates bearing substituents vulnerable to attack by strong basic nucleophiles, is elaborated. The approach allows for the new amidine nitroxides bearing various functional groups (e.g., such as CN, N3, NH2, COOEt) to be synthesized. A series of nitroxides obtained through the Staudinger/intermolecular aza-Wittig reaction of the azido derivative is also described. The nitroxides synthesized here were found to have pH-dependent two-component EPR spectra indicative of a slow, on the EPR time scale, R* left arrow over right arrow R*H+ chemical exchange, and pKa values ranging from 2.8 to 12.5 units. The guanidine derivatives synthesized in this work show the highest pKa values (pKa = 10.2 and 12.5, respectively) ever reported for the nitroxide pH-probes of a "basic type".