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  • Analytical Chemistry and Spectroscopy  (2)
  • 1
    ISSN: 0749-1581
    Keywords: N-Acetylneuraminic acid ; Configurational assignment ; 1H and 13C NMR spectra ; Geminal C,H coupling constants ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High field 1H and 13C NMR spectra were recorded of N-acetylneuraminic acid, its methyl ester and its per-O-acetylated methyl ester. The sign and the magnitudes of the geminal coupling constants between the H-3 protons and the C-2 quaternary anomeric carbon were determined by heteronuclear 2D-experiments. The C-2,H-3ax coupling in the α anomers (-8 Hz) was significantly more negative than that in the β anomers (-3 to -4 Hz). Therefore, the determination of 2J(C-2,H-3ax) offers a unique criterion for anomeric assignment in sialic acid ketosides. The geminal coupling constants are discussed in terms of the number and orientation of the oxygen substituents. Due to their 3-deoxy function adjacent to the quaternary anomeric centre, sialic acids give novel contributions to the correlation of carbon-proton coupling constants with molecular geometries in carbohydrates.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0749-1581
    Keywords: Humic substances ; Quantification ; 13C NMR ; DEPT ; SEMUT-90 ; J-resolved ; Volume integration ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Partial structures of a reference groundwater humic substance were quantified by J-resolved two-dimensional 13C NMR spectroscopy applied in its phase-sensitive variant. The results agree with those obtained by the one-dimensional techniques DEPT (distortionless enhancement by polarization transfer) and SEMUT-90 (subspectral editing using a multiple quantum trap), thus demonstrating the accuracy of both methods. However, the two-dimensional technique is recommended, because quantification is based on only one spectrum. Quantification provides the ratio of different substructures (carbons belonging to ketones, carboxylic acids, different aromatics, carbohydrates, paraffinic C, CH, CH2 and CH3 groups) and, in combination with elemental analysis, the partition of hydrogen between C and O (83% bonded to C and 17% to O). Also, insight is obtained into the biogenesis of the humic acids. Aromatic moieties probably originate mainly from lignin rather than from other aromatic compounds. CH—O moieties mainly belong to carbons in ethers and alcohols and only to a small extent to carbons in carbohydrates, as shown by comparing NMR results with gas chromatographic analysis of the carbohydrate part performed by other workers. The ratio of paraffinic CH:CH2:CH3 is about 1:1:1; the high proportion of paraffinic methyl groups cannot yet be explained.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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