Molecular dynamics simulations and diffraction-based analysis of the native cellulose fibre: Structural modelling of the I-alpha and I-beta phases and their interconversion

In this report we describe the building of diffraction-based models of the two phases of cellulose I, which subsequently are subjected to molecular dynamics simulation. The models showed an interesting variety of behaviour, including glycosidic and exocyclic torsional motion and isomerization, hydrogen-bond breakage and formation, individual and collective chain motion, and sheet deformation in the non-hydrogen-bonding direction. The I-alpha phase exhibited a greater dynamic range of behaviour than the I-beta phase, including considerable movement of glycosidic torsions away from initial diffraction-based positions and considerable relative motion of the chains. Based on motions observed in the simulations, we suggest a break-slip model for the I-alpha --> I-beta phase transition, which proposes that the transition is initiated by heating-induced hydroxymethyl and hydroxyl side-group torsional rotations accompanied by hydrogen-bond breakage. Chains of the I-alpha phase are hence freed for rotation and sliding into the more stable I-beta morphology. This model was tested with molecular mechanics refinement of likely intermediate structures. The results suggested that a facile transformation path is available via such a mechanism. Copyright (C) 1996 Published by Elsevier Science Ltd.