CONFORMATIONAL-ANALYSIS OF CHITOBIOSE AND CHITOSAN

The relaxed potential energy surfaces of chitobiose were calculated ba sed on the MM3-force field by optimizing structures on a 10 degrees gr id spacing of the torsional angles about the glycosidic bonds (Phi,Psi ). The 36 conformations; the four combinations of the hydroxymethyl gr oup orientations coupled with the nine of the secondary group ones-wer e assumed for each Phi,Psi conformation. The four conformations, each differing in the hydroxymethyl group orientations, were considered for the whole Phi,Psi space, and all the 36 conformations, for the restri cted space of low energy. While the resulting energy map and the struc tures of the energy minima were similar to those proposed for cellobio se in many respects, more restricted energy profile was suggested for the relaxed map of chitobiose where differences in the energy level be tween the global minimum and the local minima were within 5.4 kcal/mol , compared with the equivalent value of 3.6 kcal/mol for cellobiose. F urther depression of the global minimum occurred when the acidic resid ue was used. The Monte Carlo samples of the chitosan chain were genera ted based on the relaxed map to predict the unperturbed coil dimension in solution. The chitosan chains showed Gaussian behavior at x = 500 (x, degree of polymerization) and gave the characteristic ratio C-x, o f about 70, which was much larger than the experimental values observe d for the chitosan and cellulosic chains. (C) 1994 John Wiley & Sons, Inc.