Assignment of the carbon-13 nuclear magnetic resonance spectra of gangliosides GM4, GM3, GM2, GM1, GD1a, GD1b, and GT1b

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Abstract
Complete 13C NMR assignments are presented for gangliosides in the series [chicken brain] Gm4, [bovine adrenal medulla] Gm3, [human Tay Sachs brain]GM2, [bovine brain] GM1, GD1b, and GT1b. The gangliosides studied are related by the sequential addition of single saccharide residues. The structural relationships among these molecules were confirmed and subsequently utilized to provide the basis for a detailed investigation of 13C NMR oligomer-monomer shielding differences accompanying increasing oligosaccharide complexity. This gradual increase in complexity was reflected in the 13C NMR spectra and proved to be of significant value in the assignment task, resulting in the reassignment of 4 GM1 resonances from previous work. The carboxyl-containing sialic acids in gangliosides have glycosidic linkage resonance shifts only .apprx. 30% as large as those found for neutral hexopyranosides; thus, care must be used in interpreting the 13C spectra of charged oligosaccharides. Secondary structural effects are also found to produce shifts in the resonances of the sialic acid adjacent to the GAlNAc residue of GM2 and the more complex gangliosides, leading to inequivalence of the siaic acids in GD1a, GD1b and GT1b.