Intrinsic twist in Iβ cellulose microfibrils by tight-binding objective boundary calculations.

Objective boundary conditions are used to simulate at the atomistic scale cellulose Iβ microfibrils. The method enables for the first time a direct calculation of the structural twist from a self-consistent charge density-functional-based tight-binding description of interatomic interactions. Calculations reveal that microfibrils are stabilized under a uniform right-handed twist whose magnitude depends on the area and the shape of the microfibril cross-section. The latter behavior highlights the distinct structural effects imprinted by the complex hydrogen bonding network and the differences in the relative shear strength between the hydrogen and van der Waals interactions: While the intrachain bonding gives a disposition for severe twisting in the glycosidic linkages, the interchain hydrogen and van der Waals bonding contribute to the development of twist at the microfibril level. The interchain hydrogen bonding is much more effective than the van der Waals one in counterbalancing the intrinsic tendency for twist of the microfibril.