ANGULAR-DEPENDENCE OF THE C-6 CHEMICAL-SHIFT AND THE CONFORMATION OF THE HYDROXYMETHYL GROUP IN CARBOHYDRATES

Semiempirical quantum-mechanical nuclear shielding calculations (FTP I NDO method) have been performed on a series of 16 hexopyranoses for co nformational studies of hydroxymethyl groups in carbohydrates. The cal culated shielding for the C-6 and C-5 atoms shows an angular dependenc e of up to delta = 8 which is in agreement with that found experimenta lly. The dependence of the C-6 chemical shift on the dihedral angle om ega is presented by a trigonometric function of the form delta(C) = A sin omega+B sin 2 omega+C sin 3 omega+D cos omega+E cos 2 omega+F cos 3 omega+G with different A-G constants for both series of hexopyranose s with different configuration at their C-4 atoms. This angular depend ence may be applied for prediction of conformations about the C-5-C-6 bond in noncrystalline materials. It is also demonstrated that this de pendence together with the angular dependence for C-5 atom, can be use ful in estimating the conformational preferences of the hydroxymethyl group in carbohydrates in solution having the configurations at C-4 as in D-glucose and D-galactose.