13C MAS NMR and 1H-29Si and 1H-13C heteronuclear correlation study of model xylooligosaccharides

Abstract

Anomer structures of crystalline D-xylopyranose, all positional isomers of β-D-xylopyranosyl-D-xylopyranose and 2,4-di-β-D-xylopyranosyl-D-xylopyranose were determined by 13C MAS NMR spectroscopy. The saccharides were pertrimethylsilylated by different methods yielding different ratios of pertrimethylsilylated anomers. The NMR spectra (1H, 13C, and 29Si) were assigned by two-dimensional chemical shift correlations. In the anomeric mixtures complete overlap in parts of proton spectra is frequent. In such a case, samples with different anomer ratios considerably facilitate assignment of both 13C and 29Si NMR lines. In analytical applications, however, that method of trimethylsilylation must be chosen which does not allow anomerization prior to silylation. 29Si NMR spectra furnish correct number of OH groups present in the parent compound prior to silylation. Assigned silicon chemical shifts can be used for determination of the sites of glycosidation in oligosaccharides. Glycosidic carbon atoms are considerably shielded after pertrimethylsilylation.