Geminal 2JCCH spin-spin coupling constants as probes of the phi glycosidic torsion angle in oligosaccharides.
Two-bond (13)C-(1)H NMR spin-spin coupling constants ((2)J(CCH)) 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 (2)J(C2,H1) magnitude and sign and the C1-O1 (phi, phi) and C2-O2 (alpha) torsion angles. Related calculations were also conducted for the reverse coupling pathway, (2)J(C1,H2). Computed J-couplings were validated by comparison to experimentally measured couplings. (2)J(CCH) 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 (2)J(C1,H2) and (2)J(C2,H1) in aldopyranosyl rings. The equation for (2)J(C2,H1) has particular value as a potential NMR structure constraint for the C1-O1 torsion angle (phi) comprising the glycosidic linkages of oligosaccharides.
Department of Chemistry and Biochemistry, University of Notre Dame, Indiana 46556-5670, USA.
Thomas E
TE
Klepach
Ian
I
Carmichael
Anthony S
AS
Serianni
D002244:Carbon / Q000737:chemistry; D006027:Glycosides / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D015394:Molecular Structure; D009844:Oligosaccharides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D013816:Thermodynamics
D006027:Glycosides; D009844:Oligosaccharides; D002244:Carbon; D006859:Hydrogen; D010100:Oxygen