CONFORMATIONAL-ANALYSIS AND MOLECULAR-DYNAMICS SIMULATION OF CELLOBIOSE AND LARGER CELLOOLIGOMERS

The conformational behavior of cellobiose (D-glc-beta(1 --> 4)-D-glc), cellotetraose, and cellooctaose was studied by a combination of energy minimization and molecular dynamics simulations in vacuo at 400 K. These di- and oligosaccharide models have considerable flexibility and exhibit a variety of different motions in glycosidic and exocyclic torsions. The glycosidic phi, psi torsions moved frequently between two local minima on the cellobiose energy surface in the region of known crystal structures. Transitions of the hydroxymethyl side chain were observed between gt, gg, and tg conformations accompanied by changes in intramolecular hydrogen bonding patterns. A reasonable fit to the experimental optical rotation and nuclear magnetic resonance vicinal coupling data of cellobiose in solution required a distribution of its conformations. The oligomers, although generally extended, assumed a more coiled or twisted shape than is observed in the crystalline state of cellulose and exhibited considerable backbone motion due to local ring rotations about the glycosidic bonds. Long-lived transitions to structures having torsion angles 180-degrees from the major minima (ring flips) introduced kinks and bends into the tetramer and octamer. While the glucose rings of the structures remained primarily in the C-4(1) conformation, twist and boat structures were also observed in the tetramer and octamer structures. Reducing the simulation temperature to 300 K eliminated some of the transitions seen at 400 K.