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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 Iα, Iβ,
II, and III<sub>I</sub> 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 III<sub>I</sub> (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