2JCOC Spin−Spin Coupling Constants Across Glycosidic Linkages Exhibit a Valence Bond-Angle Dependence

Abstract

The recent intense interest in the biological functions and biomedical applications of oligosaccharides has stimulated renewed efforts to improve experimental tools for the determination of their conformational properties. NMR spectroscopy provides the most detailed structural information in solution derived mainly from {sup 1}H-{sup 1}H NOEs and more recently from {sup 3}J{sub COCH}values across their constituent O-glycosidic linkages. However, in many cases, these linkages are not conformationally rigid, and identifying those that exhibit flexibility requires the use of multiple NMR parameters which provide the needed redundancy to make such determinations with confidence. In ongoing efforts to develop trans-O-glycosidic {sup 2}J{sub COC} and {sup 3}J{sub COCC} values as conformational constraints, it was shown recently that {sup 2}J{sub COC} across an O-glycosidic linkage depends mainly on the {phi} torsion angle and considerably less so on {psi}, using an experimentally derived projection resultant rule. This rule predicts that more negative projection resultants translate into more negative {sup 2}J{sub CCC} and, by analogy, into more negative {sup 2}J{sub COC}. The authors show that the COC bond angle also influences {sup 2}J{sub COC} magnitude, with increasing angle producing more negative coupling.