Enantio- and Diastereoselective Formal Hetero-Diels-Alder Reactions of Trifluoromethylated Enones Catalyzed by Chiral Primary Amines.

Enantioselective formal hetero-Diels-Alder reactions of trifluoromethylated enones and 2-amino-1,3-butadienes generated in situ from aliphatic acyclic enones and chiral primary amines are reported. The corresponding tetrahydropyran-4-ones are formed in up to 94 % yield and with up to 94 % ee. The reaction was carried out through a stepwise mechanism, including initial aminocatalytic aldol condensation of 2-amino-1,3-butadiene to the trifluoromethylated carbonyl group followed by an intramolecular oxa-Michael addition.

Stimulation of N--glycoside transfer in deoxythymidine glycol: mechanism of the initial step in base excision repair.

Thymine glycol (Tg), a toxic oxidative DNA lesion, is preferentially removed by endonuclease III (Endo III). To investigate the glycosylase activity of Endo III, the N--glycoside transfer mechanism in deoxythymidine glycol (dTg) is examined in this theoretical study based on the BHandHLYP/6-311++G(d,p) level of theory. Two controversial mechanisms were characterized, i.

Quantum mechanics study and Monte Carlo simulation on the hydrolytic deamination of 5-methylcytosine glycol.

The efficient formation of 5-methylcytosine glycol (mCg) and its facile deamination to thymine glycol (Tg) may account for the prevalent C → T transition mutation found at methylated CpG site (mCpG) in human p53 gene, a hallmark for many types of human tumors. In this work, the hydrolytic deamination of mCg was investigated at the MP2 and B3LYP levels of theory using the 6-311G(d,p) basis set. In the gas phase, three pathways were explored, paths A-C, and it indicates that the direct deamination of mCg with H(2)O by either pathway is unlikely because of the high activation free energies involved in the rate-determining steps, the formation of the tetrahedral intermediate for paths A and B as well as the formation of the Tg tautomer for path C.

Theoretical investigations on the thermal decomposition mechanism of 5-hydroxy-6-hydroperoxy-5,6-dihydrothymidine in water.

The main primary product of DNA oxidation by free radicals is 5-hydroxy-6- hydroperoxy-5,6-dihydrothymidine (5-OH-6-OOH-DHT), whose further degradation can yield the other mutagenic products and amplify the spectrum of DNA damage. In this study, to illustrate the thermal stability of 5-OH-6-OOH-DHT in DNA, the decomposition mechanism of 5-OH-6-OOH-DHT was identified based on the cis-(5R,6S) diastereomer. Optimized structures for all of the stationary points in the gas phase were investigated at the B3LYP/6-31+G(d,p) level of theory.

Molecular dynamics and density functional theory studies of substrate binding and catalysis of human brain aspartoacylase.

The active-site dynamics of human brain aspartoacylase (hASPA) complexed with its substrate (N-acetyl-L-aspartate) has been studied using a hybrid quantum mechanical/molecular mechanical (QM/MM) approach based on the self-consistent charge-density functional tight-binding (SCC-DFTB) model. The Michaelis complex, which is constructed from a recent X-ray structure of the human brain aspartoacylase with a stable tetrahedral intermediate analogue, is reproduced in 1ns molecular dynamics simulations at 300K. The simulation shows that the substrate is tightly held in the active site by a hydrogen bond network and the putative nucleophilic water molecule is reasonably close to the nucleophilic center.

Fast determination of the release degree of compound sulfamethoxazole tablets by using circulation release system combined with ultraviolet spectral net analyte signal method.

Based on ultraviolet spectral net analyte signals, this research has studied the determination of the two effective components, sulfamethoxazole (SMZ) and trimethoprim (TMP), contained in compound sulfamethoxazole tablets in acid environment. The linear ranges of SMZ and TMP were 0.48-7.

Theoretical studies on the thermodynamics and kinetics of the N-glycosidic bond cleavage in deoxythymidine glycol.

The thermodynamic and kinetic properties for the nonenzymatic N-glycosidic bond cleavage in cis-(5R,6S)-5,6-dihydroxy-5,6-dihydrodeoxythymidine (deoxythymidine glycol, dTg) were studied by computational techniques. Optimized structures for all of the stationary points in the gas phase were investigated using the BHandHLYP/6-311++G(d,p) and B3LYP/6-311++G(d,p) methods. Single-point energies were determined employing the ab initio MP2 method in conjunction with the 6-311++G(d,p) basis set.

Chemometric classification of traditional Chinese medicines by their geographical origins using near-infrared reflectance spectra.

Some raw materials that have different places of production for the plant sources of the drugs Astragalus membranaceus and ginseng have been studied, based on their near-infrared reflectance spectra. The experimentally recorded spectra represent heavily ill-posed and highly correlative data sets. Three related methods, i.