Structure conversions of cellulose IIII crystal models in solution state: a molecular dynamics study

This paper re-examines our previous molecular dynamics (MD) study on cellulose IIII crystal models with finite dimensions solvated in explicit water molecules. Eight crystal models, differing in a constituent lattice plane and dimensions, were studied. One calculation allowed for O-H and C-H bond stretching, and had a small time step of 0.5 fs. The other calculation adopted non-scaling factors of the 1-4 non-bonded interactions. As in our previous study, in the former MD calculations, six of the eight crystal models exhibited structure conversion with cooperative chain slippages generated by a progressive fiber bend. This converted the initial non-staggered chain packing of cellulose IIII into a near one-quarter staggering and gave the crystal model a triclinic-like configuration. In contrast, in the non-1-4 scaling MD calculations, all of the eight crystal models retained the initial cellulose IIII crystal structure. Another series of non-1-4 scaling MD calculations were performed for the four crystal models containing chains with a degree of polymerization (DP) of 40 at 370 K, which simulated hot water treatment to convert cellulose IIII to I beta. Some of the hydroxymethyl groups irreversibly rotated from gt into tg conformation. This accompanied exchange of the intrasheet hydrogen bonding scheme along the (1 -1 0) lattice plane from O2-O6 to O3-O6. The original corrugated (1 -1 0) chain sheet was partly converted into a cellulose I-like flat chain sheet.