Microscopic observations of a native cellulose microfibril using atomic force and transmission electron microscopy have often revealed the formation of a right-handed twist. We have reported molecular dynamics (MD) simulations of finite crystals of the native forms in aqueous solution using the AMBER force field coupled with Glycam04 parameters, and the right-handed fiber twists were reproduced in the solvated cellulose Iα and Iβ crystal models with the Iα crystal models showing greater twisting. Just prior to our reports, Matthews et al. also reported the right-handed twist of the cellulose Iβ crystal in a similar MD study. As for the cellulose allomorphs other than the native ones, cellulose IIII can be obtained by treatment of the cellulose Iβ form with liquid ammonia or amines, and is readily converted back into the latter form by hot-water treatment. Our MD study of the cellulose crystal models were then adopted to the cellulose IIII crystal models in hot-water simulation at 370 K, exhibiting partial crystalline transformation of (1 -1 0) chain sheets accompanied by irreversible rotation of the hydroxymethyl groups from gt to tg conformations and hydrogen-bond exchange from the original O3–O6 to cellulose-I-like O2–O6 bonds. The corrugated (1 -1 0) chain sheet was converted to a cellulose-I-like flat chain sheet with a slightly right handed twist. Recently, the structural stabilities of the molecular chain sheets constituting the crystal structures of the cellulose allomorphs Iα, Iβ, II, 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. Obviously, 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 described above.

Crystal deformation and transformation of cellulose allomorphs derived from stability of molecular chain sheets

253rd National Meeting (2017)

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