Assignment of the Carbon-13 Nuclear Magnetic Resonance Spectra of Gangliosides G_M4, G_M3, G_M2, G_M1, G_D1a, G_D1b, and G_T1b

Complete ¹³C nuclear magnetic resonance assignments are presented for gangliosides in the series G_M4, G_M3, G_M2, G_M1, G_D1a, G_D1b, and G_T1b. 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 four G_M1 resonances from our previous work [Sillerud, L.O., Prestegard, J.H., Yu, R.K., Schafer, D.E., & Konigsberg, W.H. (1978) Biochemistry 17, 2619–2628]. The carboxyl-containing sialic acids in gangliosides have glycosidic linkage resonance shifts only ~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 G_M2 and the more complex gangliosides, leading to inequivalence of the sialic acids in G_D1a, G_D1b, and G_T1b.