Reevaluation of rotamer populations for 1,6 linkages: reconciliation with potential energy calculations

Applications of ensemble averaging to the solution conformation of model compounds for N-linked glycans are further investigated. Specifically, the interpretation usually applied to observed values of J5,6', a parameter reflecting the rotameric distribution about C5-C6 bonds (torsion angle omega in 1,6 glycosidic linkages) in 6-O-substituted hexopyranosides, was found to be inconsistent with populations derived from potential energy calculations. However, agreement between observed and calculated, ensemble-averaged values of J5,6' was obtained and the distribution of omega rotamers reinterpreted. Values of J5,6' that were previously interpreted as indicative of equipartition between two rotamers in fact reflect a marked preference for one of them. Additional potential energy terms, previously absent from energy calculations, are introduced and shown to be without effect on interpretations of omega rotamer distributions. From comparisons with both NMR relaxation and scalar coupling constant data, it is concluded that a simple empirical algorithm, HSEA, calculating van der Waals, exo-anomeric, and (as appropriate) hydrogen-bonding terms, is best suited for describing the population distributions in solution for oligosaccharides and N-linked glycans.