CRYSTALLINE CELLULOSE I-ALPHA AND I-BETA STUDIED BY MOLECULAR-DYNAMICS SIMULATION

A recent structure determination of native cellulose has shown that it is composed of two different crystal structures, a two-chain monoclin ic phase and a single-chain triclinic phase. In this article a molecul ar dynamics study of the two allomorphs is presented, and a general pi cture of structure and energetics is provided. Consistent with experim ental data, the monoclinic phase is more stable than the triclinic pha se by -8.7 kJ mol(-1) cellobiose(-1). In the monoclinic phase a small angle is observed between glucose residues that belong to alternate (2 00) planes. The glucose residues in every second plane are parallel to the (200) plane, and form more favorable intermolecular hydrogen bond s. In the triclinic phase the glucose residues are not parallel to the (200) plane. The C-13 NMR shifts for C-6 are fully accounted for by t he distribution of the C-6 dihedral angle. The nonbonded environment i s important to the splitting for C-1. The fine structure of IR spectra in the OH-stretching region can be qualitatively correlated with the number of different hydrogen bands observed. Results indicate that cha ins in one set of alternating (200)-planes in the monoclinic phase res emble the triclinic phase.