Conformational differences between O- and C-glycosides: the alpha-O-man-(1-->1)-beta-Gal/alpha-C-Man-(1-->1)-beta-Gal case--a decisive demonstration of the importance of the exo-anomeric effect on the conformation of glycosides

The conformational behavior of alpha-O-Man-(1-->1)-beta-Gal (1) and its C-analogue (2) has been studied using J/NOE NMR data, molecular mechanics, molecular dynamics, and ab initio calculations. The population distribution around the glycosidic linkages of 1 and 2 is rather different, especially for the alpha-Man linkage. A lower limit for the exo-anomeric effect in water has been experimentally determined.     

Conversion of quassinoids for enhancement of inhibitory effect against Epstein-Barr virus early antigen activation. Introduction of lipophilic side chain and esterification of diosphenol

Introduction of a senecioyl group into shinjulactones B and C, and esterification of the diosphenol moiety in brusatol and brucein A enhanced inhibitory effect against Epstein-Barr virus early antigen activation.     

Quantitation of caffeine by capillary zone electrophoresis with end-column amperometric detection at a carbon microdisk array electrode

Capillary zone electrophoresis is employed for the determination of caffeine using end-column amperometric detection with a carbon fiber microdisk array electrode at a constant potential of 1.45 V versus a saturated calomel electrode. The optimum conditions of separation and detection are 0.1 52mM NaH2PO4-0.648mM Na2HPO4 for the buffer solution, 20 kV for the separation voltage, 5 kV for the injection voltage, and 10s for the injection time. The limit of detection is 2.9 x 10(-4)mM or 1.2 fmol (signal-to-noise ratio = 2). The relative standard deviation is 0.68% for the migration time and 2.3% for the electrophoretic peak current. The method is applied to determining caffeine in human serum and a cola drink.     

Chemoenzymatic synthesis of isogalactofagomine

A new chemoenzymatic synthesis of optically pure isogalactofagomine 2 starting from achiral starting materials is presented. Dimethyl 4-hydroxypyridine-3,5-dicarboxylate (7) was synthesized and converted to the corresponding saturated piperidine 8. Then the key step of the synthesis was carried out: Lipase M catalyzed hydrolysis of the prochiral diester 8 to cause formation of an asymmetric monoacid with at least 98% enantiomeric excess. Reduction of the acid, saponification of the remaining ester, and radical iododecarboxylation gave an iodide that after substitution with silver trifluoroacetate and hydrolysis gave 2.     

Determination of barbituric acid and 2-thiobarbituric acid with end-column electrochemical detection by capillary electrophoresis

Capillary electrophoresis (CE) with end-column electrochemical detection (EC) of barbituric acid (BA) and 2-thiobarbituric acid (TA) has been described. Under optimum condition, BA and TA were separated satisfactorily, and a response of high sensitivity and stability was obtained at a detection potential of 1.25 V versus Ag/AgCl. Optimized end-column detection provides detection limit as low as 0.5 and 0.1 μM for BA and TA, respectively. The calibration graph was linear over three orders of magnitude. The relative standard deviations (n=10) of peak currents and migration times obtained for both BA and TA were 3.4, 3.7, and 1.7, 1.2%, respectively. The proposed method has been applied to analyze water sample with satisfactory results.     

Prediction of programmed-temperature retention values of naphthas by artificial neural networks

It is proposed for the first time a method of prediction of the programmed-temperature retention times of components of naphthas in capillary gas chromatography using artificial neural networks. People are used to predict the programmed-temperature retention time using many formulas such as the integral formula, which requires that four parameters must be determined by calculation or experiments. However the results obtained by the formula are not so good to meet the demand of industry. In order to predict retention time accurately and conveniently, artificial neural networks using five-fold cross-validation and leave-20%-out methods have been applied. Only two parameters: density and isothermal retention index were used as input vectors. The average RMS error for predicted values of five different networks was 0.18, whereas the RMS error of predictions by the integral formula was 0.69. Obviously, the predictions by neural networks were much better than predictions by the formula, and neural networks need fewer parameters than the formula. So neural networks can successfully and conveniently solve the problem of predictions of programmed-temperature retention times, and provide useful data for analysis of naphthas in petrochemical industry.     

NH-S hydrogen bonding in zinc enzyme model complex with S2N2 binding set studied by normal coordinate analysis of vibrational spectra

In order to provide theoretical evidence for the existence and effect of NH-S hydrogen bonding in the zinc enzyme model complex [Zn(S-2-C6H4NHCOC6H5)2(1-MeIm)2] (1-MeIm = 1-methylimidazole) in addition to the spectroscopic and crystallographic investigations, normal coordinate analysis (NCA) was carried out using a modified Urey-Bradley force field. The vibrational frequencies of the complex with and without the NH-S hydrogen bonding as well as the corresponding internal coordinates were obtained. The good agreement found between the calculated and observed frequencies in the presence of the NH-S hydrogen bond, supports the reliability of the analysis. The stretching force constant of the NH-S hydrogen bond obtained from the calculation is 0.18 mdyne per A. The calculation shows that the N-H bond is weakened by formation of the NH-S hydrogen bond. The results are indicative of the existence of the NH-S hydrogen bond in the complex.     

Digital light-directed synthesis. A microarray platform that permits rapid reaction optimization on a combinatorial basis

Solution reactions using photogenerated reagents (Gao, X.; Yu, P.; LeProust, E.; Sonigo, L.; Pellois, J. P.; Zhang, H. J. Am. Chem. Soc. 1998, 120, 12698) are a potentially powerful means for combinatorial parallel synthesis of addressable molecular microarrays. In this report, we demonstrate that this chemistry permits combinatorial screening of reaction conditions on a microarray platform. Using this method of optimization and our reaction apparatus, efficient photogenerated acids and reaction conditions suitable for removal of the acid labile protection group on 5'-O of nucleotides are identified in a short period of time. The chemistry platform demonstrated opens new avenues for rapid, simultaneous investigation of multiple reactions using different reagents and reaction parameters directly on a solid support (e.g., a glass plate). The combinatorial screening method described may be extended to include general organic reactions employing photogenerated and conventional reagents as well as a microarray reaction device. This should be especially valuable for efficient synthesis of addressable organic compound libraries.