Long-range carbon-proton coupling constants: application to conformational studies of oligosaccharides.

3JC,H values have been measured using selective 2D heteronuclear J-resolved n.m.r. spectroscopy for the COCH fragment in various carbohydrates. Measurements on model compounds have been used to characterise a Karplus-type relationship between 3JCOCH and dihedral angles in sugar. The 3JC,H values have also been measured for C-2'-O-2'-C-1-H-1 of sucrose and the sucrose residues in raffinose, stachyose, and melezitose. These values are similar to each other for solutions in D2O and (CD3)2SO and are little affected by the change in solvent, but differ from those predicted from the crystal conformations. The method has been used to correct some assignments in the published 13C-n.m.r. spectrum of melezitose.