AN EXTENSION OF THE GROMOS FORCE-FIELD FOR CARBOHYDRATES, RESULTING IN IMPROVEMENT OF THE CRYSTAL-STRUCTURE DETERMINATION OF ALPHA-D-GALACTOSE

For carbohydrates the GROMOS force field has been extended to a more realistic all-atom model, with the use of parameters from the force field proposed by Ha, Giammona, Field & Brady [Carbohydr. Res. (1988), 180, 207-221]. This extended and modified GROMOS force field has been used to simulate the crystal structures of seven monosaccharides. The results, compared with the experimental data, are satisfactory, and an overall improvement over those obtained with the comparable Ha force field. The experimentally determined positions of the H atoms should be used with caution in this comparison, and the thermal parameters can only be used as indicators for the preservation of the symmetry during the simulation. The simulations gave rise to suspicion about the hydroxyl H-atom positions in two of the sugars. These two structures were redetermined by X-ray diffraction at low temperature. For beta-D-glucose essentially the same structure was found as in the original publication. In the case of alpha-D-galactose, the new structure confirmed the results from the simulation, in contrast to earlier experimental determinations.