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  • 1
    ISSN: 0899-0042
    Keywords: enantiomeric resolution ; site-specific isotope fractionation ; 2H-NMR ; ethanol ; enzymic exchange ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have shown that site-specific natural isotope fractionation of hydrogen studied by NMR (SNIF-NMR) is an important source of information on the mechanistic and environmental effects which govern the photosynthesis of sugars and their fermentation into ethanol. Three isotope ratios associated with the methyl, methylene, and hydroxyl sites of ethanol are determined in achiral media. In this study we show that complementary information about possible stereospecific mechanisms involving the methylenic hydrogens is also rendered accessible by 2H-NMR enantiomeric resolution. The synthesis of mandelate esters enables exchange between the pro-R site of ethanol and water to be investigated. Simultaneous access to the three site-specific isotope ratios of the ethyl group is obtained at isotopic dilutions close to the natural ones. Mediation of the exchange by the enzymic system alcohol dehydrogenase-α-lipoyldehydrogenase and by the yeast Saccharomyces cerevisiae are compared. The progress of the reaction can be followed quantitatively as a function of time and the occurrence of glycolytic metabolism of endogeneous materials by yeast can be substantiated in a one-pot experiment.
    Additional Material: 1 Ill.
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  • 2
    ISSN: 0749-1581
    Keywords: Ethanol ; Deuterium ; SNIF-NMR ; 1H NMR ; 13C NMR ; 31P NMR ; 2D NMR ; Chiral discrimination ; Aromatic ring current ; Molecular structure ; MOPAC ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of a novel chiral discriminating reagent, the ester resulting from the reaction of L-menthylphenylphosphoryl chloride with ethanol, is reported. This reagent, C, is aimed to differentiate the R and S monodeuterated enantiomers of ethanol, which constitutes an important isotopic probe for mechanistic studies and origin inference of natural products. The proportions of the two P-epimeric structures, CA and CB, which compose reagent C are directly accessible by 31P NMR. Analysis of 1D and 2D NMR spectra and theoretical simulations enable the 1H and 13C chemical shifts of CA and CB to be determined. The chemical shift differences exhibited by the methylenic hydrogens of the ethoxy moiety in CA and CB are of the order of 0.3 ppm. These large differences can be mainly attributed to ring current effects. Such effects have been estimated using two different methods on the basis of model conformations of the two epimers, CA and CB, deduced from energy minimization in a quantum mechanical approach. The relative signs of the chemical shift differences δR(C) - δS(C) associated with the pro-R and pro-S positions in the CA and CB isomers cannot be determined from the proton spectrum. The ring current calculations suggest that δR(C) - δS(C) are of opposite signs in the two P-epimers. This interpretation, and consequently the conformational analysis, are supported by the 2H NMR study of a chiral discriminating ester prepared from ethanol selectively enriched in deuterium in the pro-R position by stereospecific exchange reactions involving the enzymatic pair alcohol dehydrogenase-diaphorase. The chiral discriminating reagent, C, was then used for determining the enantiomeric imbalance in ethanol samples obtained from biosynthesis under different conditions, and site-specific fractionation factors were calculated.
    Additional Material: 4 Ill.
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  • 3
    ISSN: 0749-1581
    Keywords: 13C quantitative NMR ; Enantiomeric purity ; Triglycerides ; Lipase transesterification ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 13C NMR spectroscopy can be used for the quantitative analysis of mixtures of glycerides and the enantiomeric purity can be determined by using Eu(hfbe)3-d reagent. This technique was applied to the study of the reaction mixture obtained from the transesterification of tributyrin catalysed by My-lipase (Candida cylindracea). A ‘preference’ of this lipase for the pro-R ester function of tributyrin is demonstrated.
    Additional Material: 3 Ill.
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  • 4
    ISSN: 1434-193X
    Keywords: Transglycosylation ; Glycosidases ; Thermus thermophilus ; Carbohydrates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The aim of this paper is to test the ability of a β-glycosidase from Thermus thermophilus to catalyse transglycosylation reactions in the presence of nitrophenyl glycosides as donors and other monosaccharides as acceptors. Our results show that this enzyme is able to induce such reactions either with nitrophenylgalactosides, -glucosides and -fucosides. With the two former donors, the autocondensation of the donor, which thus acts also as an acceptor, is faster than the transglycosylation with other acceptors. Furthermore, as the regioselectivity of the reactions is mainly of the β-[1→3] type, good yields are obtained for the synthesis of 2-nitrophenyl-β-D-galactopyranosyl-[1→3]-β-D-galactopyranoside and 2-nitrophenyl-β-D-glucopyranosyl-[1→3]-β-D-glucopyranoside. Conversely, in the presence of p-nitrophenylfucoside, the autocondensation is very limited, and with methyl-α-D-galactoside as an acceptor, the regioselectivity is mainly of the β-[1→6]-type resulting in the synthesis of methyl-β-D-fucopyranosyl-[1→6]-α-D-galactopyranoside.
    Additional Material: 7 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 67 (1984), S. 1254-1273 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of all-trans-retinal and 5-(2,6,6-trimethylcyclohexeyl)-3-methyl-2,4-pentadienal with amino acids, amino esters and their salts was studied. The structure of the polyenic imines and iminium salts thus prepared was elucidated with the aid of 1H-and 13C-NMR spectroscopy. The condensation results in an equilibrium between the imine and its zwitterionic form as shown by variable-temperature 13C-NM measurements. Addition of an inorganic salt (LiClO4) favours the zwitterionic form. Comparison of the 13C chemical shifts of these species with those obtained from the protonation of the corresponding imino-ester gave the percentage of the two forms. The species prepared from the amino acids constitute model compounds, and rhodopsin and bacteriorhodopsin are believed to exhibit similar behaviour.
    Additional Material: 7 Ill.
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