Residual H-2 quadrupolar couplings in weakly aligned carbohydrates

We report a novel two-dimensional NMR pulse scheme for the H-1-detected obs ervation of H-2 in isotopically C-13, H-2-enriched carbohydrates, This sche me is used for the indirect observation of residual quadrupolar couplings i n C-13, H-2-enriched methyl-beta -D-glucopyranoside weakly aligned in a dil ute lyotropic liquid-crystalline medium comprising 20% (w/v) dihexanoyl-pho sphatidylcholine/dimyristoyl-phosphatidylcholine(1:3 mol/mol) in D2O, The o bserved residual quadrupolar couplings are substantially larger than residu al dipolar one-bond C-13-H-1 couplings under the same experimental conditio ns. These quadrupolar couplings are thus a useful alternative to dipolar co uplings for the structural analysis of small molecules that align very weak ly in dilute liquid-crystalline media. Moreover, since the quadrupolar coup ling constant is very uniform throughout endocyclic deuterons of the carboh ydrate, these data suggest that adoption of a single average value of this parameter in H-2 relaxation studies on the glycan moieties of glycoproteins and glycopeptides is a valid assumption, (C) 2001 Academic Press.