Geminal 2JCCH spin-spin coupling constants as probes of the phi glycosidic torsion angle in oligosaccharides

Two-bond 13C−1H NMR spin−spin coupling constants (2JCCH) between C2 and H1 of aldopyranosyl rings depend not only on the relative orientation of electronegative substituents on the C1−C2 fragment but also on the C−O torsions involving the same carbons. The latter dependencies were elucidated theoretically using density functional theory and appropriate model pyranosyl rings representing the four relative configurations at C1 and C2, and a 2-deoxy derivative, to probe the relationship between 2JC2,H1 magnitude and sign and the C1−O1 (phi, φ) and C2−O2 (α) torsion angles. Related calculations were also conducted for the reverse coupling pathway, 2JC1,H2. Computed J-couplings were validated by comparison to experimentally measured couplings. 2JCCH displays a primary dependence on the C−O torsion involving the carbon bearing the coupled proton and a secondary dependence on the C−O torsion involving the coupled carbon. These dependencies appear to be caused mainly by the effects of oxygen lone pairs on the C−H and C−C bond lengths along the C−C−H coupling pathway. New parameterized equations are proposed to interpret 2JC1,H2 and 2JC2,H1 in aldopyranosyl rings. The equation for 2JC2,H1 has particular value as a potential NMR structure constraint for the C1−O1 torsion angle (φ) comprising the glycosidic linkages of oligosaccharides.