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

Four β-linked glucobioses selectively ¹³C 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 α-sophoroside (methyl 2-O-β-d-glucopyranosyl-α-d-glucopyranoside), methyl α-laminarabioside (methyl 3-O-β-d-glucopyranosyl-α-d-glucopyranoside), and methyl α-cellobioside (methyl 4-O-β-d-glucopyranosyl-α-d-glucopyranoside) were found to be close to those observed in the solid state (39° < φ_H < 41°, −24° < ψ_H < −36°). 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 α-gentiobioside (methyl 6-O-β-d-glucopyranosyl-α-d-glucopyranoside) shows greater flexibility at the ψ-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 β-d-glucopyranosyl residues in any of the disaccharides appear to be involved in inter-residue hydrogen bonding since ¹J_CH, ¹J_CC, and ²J_CH values sensitive to C2‘−O2‘ rotamer distribution remain close to those observed in methyl β-d-glucopyranoside.