Conformational Analysis of β-Glycosidic Linkages in 13C-Labeled Glucobiosides Using Inter-residue Scalar Coupling Constants

Abstract

Four beta-linked glucobioses selectively (13)C labeled at C1' or C2' have been prepared. The inter-residue coupling constants, J(CH), and J(CC), have been determined and related to the solution conformations of the disaccharides using Karplus-type relationships. Relying only on the experimental coupling constants, glycosidic linkage conformation in methyl alpha-sophoroside (methyl 2-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside), methyl alpha-laminarabioside (methyl 3-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside), and methyl alpha-cellobioside (methyl 4-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside) were found to be close to those observed in the solid state (39 degrees < phi(H) < 41 degrees , -24 degrees < psi(H) < -36 degrees ). The laminarabioside and cellobioside were found to have conformations that accommodate an intramolecular hydrogen bond to O5' that is observed in the solid state. In all compounds, the exocyclic hydroxymethyl groups retain a conformation close to that observed in unsubstituted glucose (gt/gg 1:1). Methyl alpha-gentiobioside (methyl 6-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside) shows greater flexibility at the psi-torsion than the other disaccharides, but the population distribution around the C5-C6 bond is essentially unaffected by substitution. None of the O2' hydroxyl groups of the beta-D-glucopyranosyl residues in any of the disaccharides appear to be involved in inter-residue hydrogen bonding since (1)JCH, (1)JCC, and (2)JCH values sensitive to C2'-O2' rotamer distribution remain close to those observed in methyl beta-D-glucopyranoside.