Theoretical Study of the Structural Stability of Molecular Chain Sheet Models of Cellulose Crystal Allomorphs

The structural stabilities of the molecular chain sheets constituting the crystal structures of the cellulose allomorphs l alpha, I beta H, and IIII, were investigated by density functional theory (DFT) optimization of the isolated chain sheet models with finite dimensions. The DFT-optimized chain sheet models of the two native cellulose crystals developed a right-handed twist with a similar amount of twisting. The DFT-optimized cellulose II (010) and (020) models twisted in opposite directions with right- and left-handed chirality, respectively. The cellulose IIII (1-10) model retained the initial flat structure after the DFT-optimization. The structural features of the DFT-optimized chain sheet models were reflected in the structures of the parent crystal models observed in solvated molecular dynamics (MD) simulations. The minor conformations of the hydroxymethyl groups proposed in the real crystal structures were detected in the MD crystal models and the DFT-optimized (010) model of cellulose II. The crystal chain packing and crystal conversions are interpreted in terms of principal chain sheet stacking.