Structure determination of peracetylated glycosphingolipids by one‐ and two‐dimensional ¹H NMR at 360 and 500 MHz

Abstract

The analysis of two-dimensional proton shift-correlated 500 MHz NMR spectra of several peracetylated glycosphingolipids confirmed that peracetylated oligosaccharides are particularly well suited for the identification of the constituent sugar residues and the elucidation of the sites of glycosidic linkage. Although a number of partial sequences could be determined by inter-residue nuclear Overhauser effect or long-range scalar coupling across the glycosidic bonds, the full sequence determination of larger oligosaccharides has several limitations owing to experimental dilficulties and possible ambiguities in assignments due to signal overlap. The following peracetylated glycosphmgolipids were investigated by two-dimensional methods at

Pigments of Fungi. LXV. Tetrahydroanthraquinone and Anthraquinone Gentiobiosides from the Fungus Dermocybe splendida

“…spectrum of the glycoside (16) was recorded at 80°C in (D 6 )dimethyl sulfoxide containing deuterium oxide, doublet resonances were observed at δ 4.21 (J 7.8 Hz) and 4.99 (J 7.6 Hz) that identify two trans-diaxially coupled anomeric protons, consistent with a β-configuration at both of the anomeric centres. 13 Signals from H 1, H 2 and H 4 in the austrocortinin moiety are clearly discernible in the spectrum of the glycoside (16) and correspond closely in their chemical shift with their counterparts in the spectrum of the aglycone (13). 11 Notably, the signal due to H 7 in the spec-trum of the glycoside (16) is somewhat deshielded (∆δ 0.65 ppm) with respect to H 7 in the spectrum of (13), 11 a result which implies, but does not prove, that the disaccharide is linked to the C 8 hydroxy group in (16).…”

“…Thereafter, the large coupling constants (J 7-10 Hz) between all vicinally disposed protons between H 1´ and H 5´ and between H 1´´ and 5´´, and the chemical shifts of H 2 6´ (δ 3.73 and 3.87) and H 2 6´´(δ 4.20 and 4.09) identify unequivocally that both the inner and outer sugars correspond to β-D-glucopyranose and that the linkage is C 6´ → C 1´´. 13 This connectivity is borne out by the chemical shifts of C 6´ (δ 68.5) and C 6´´ (δ 61.8) in the 13 C n.m.r. spectrum of (18) ( Table 3).…”

“…mass spectrum {[M+Na] + 631} and the results of combustion analysis. Incubation of (16) with β-D-glucosidase gave an aglycone that proved to be identical in all respects with austrocortinin (13). 11 Alternatively, exposure of (16) to sulfuric acid in methanol at reflux gave (13) together with D-glucose as the only sugar that could be detected by thin-layer chromatography.…”

Sequencing of peracetylated oligosaccharides by rotating‐frame nuclear Overhauser enhancement spectroscopy

“…The advantages of peracetylated derivatives vs parent native sugars for structural analysis by tH-NMR have been described in several papers originating from our laboratories [1][2][3][4][5][6] and by Gasa et al [7] and Nishida et al [8]. The spectra of the former are usually better resolved, the signals more evenly distributed, and the glycosylation sites directly recognizable by the relative -1 ppm high-field shift for the protons located at these sites, as compared with protons at acetoxylated carbons.…”

“…The spectra of the former are usually better resolved, the signals more evenly distributed, and the glycosylation sites directly recognizable by the relative -1 ppm high-field shift for the protons located at these sites, as compared with protons at acetoxylated carbons. On the other hand, sequencing of peracetylated oligosaccharides by inter-residue NOE turned out to be more difficult, due to the much greater mobility of their molecules in chloroform resulting in an NOE close to zero [3], typical [121). The combined application of these two methods led to an unequivocal elucidation of the structure of 1 and we would suggest this combination as an optimum strategy for peracetylated oligosaccharides.…”