Derivation of 13C chemical shift surfaces for the anomeric carbons of oligosaccharides and glycopeptides using ab initio methodology.

The dependence between the anomeric carbon chemical shift and the glycosidic bond (phi, psi) dihedral angles in oligosaccharide and glycopeptide model compounds was studied by Gauge-Including Atomic Orbital (GIAO) ab initio calculations. Complete chemical shift surfaces versus phi and psi for D-Glcp-D-Glcp disaccharides with (1-->1), (1-->2), (1-->3), and (1-->4) linkages in both alpha- and beta-configurations were computed using a 3-21G basis set, and scaled to reference results from calculations at the 6-311G** level of theory. Similar surfaces were obtained for GlcNAcThr and GlcNAcSer model glycopeptides in alpha- and beta-configurations, using in this case different conformations for the peptide moiety. The results obtained for both families of model compounds are discussed. We also present the determination of empirical formulas of the form 13Cdelta = f (phi,psi) obtained by fitting the raw ab initio data to trigonometric series expansions suitable for use in molecular mechanics and dynamics simulations. Our investigations are consistent with experimental observations and earlier calculations performed on smaller glycosidic bond models, and show the applicability of chemical shift surfaces in the study of the conformational behavior of oligosaccharides and glycopeptides.