Assignment of the carbon-13 nuclear magnetic resonance spectrum of aqueous ganglioside GM1 micelles

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Abstract
This article describes the natural-abundance Fourier-transform carbon-13 NMR spectrum, at 67.88 MHz, of aqueous micelles of bovine brain ganglioside GM1 of purity greater than 99%. Assignments are given for every carbon nucleus in the molecule, on the basis of a comprehensive study of the relevant mono-, di-, tri- and polysaccharides, including several containing sialic acid (5-acetamido-3,5-dideoxy-D-glycero-D-galacto-nonulopyranosonic acid), and phospho-, sphingo- and glycosphingolipids. These assignments represent an extension of the 13C NMR data from monosaccharides and lipids to complex oligosaccharides and glycolipids. They also form the basis for interpretation of spectral perturbations induced in GM1 by titration with paramagnetic europium(III). The single sialic acid in GM1 was .alpha.-glycosidically linked in the oligosaccharide from considerations of its unique anomeric chemical shift. The sialic acid carboxyl and glyceryl side chain, along with additional ligands donated by the 2-acetamido-2-deoxy-.beta.-D-galactopyranoside and terminal .beta.-D-galactopyranoside residues in the oligosaccharide portion of GM1, were intimately involved with cation binding. The higher affinity, compared with monomeric sialic acid, of GM1 for cations may result from these additional oligosaccharide groups, which may effectively compete for water ligands in the metal cation coordination sphere.