The molecular origins of twist in cellulose I-beta

The observation of twisted microfibrils in cellulose Iβ both in imaging and in molecular simulations has been reported and studied for years. This article reports a computational modeling study of cellulose Iβ twist showing its strong dependence on fibril diameter and no dependence on fibril length. We report that an important contribution to the twist in the model, empirically and analytically, is the hydrogen bonding that spans the glycosidic linkage, and that the characteristics of the chiral centers involved in the trans-glycosidic-linkage hydrogen bonding determine the directions if those interactions and cause observed right-handed twist. Other crystalline forms of cellulose show evidence of twisting at the microfibril scale, but less than Iβ. The minimal twist in other forms of cellulose was shown previously to be due to inter-layer hydrogen bonds; this study shows it is also partially due to the primary alcohol not occurring in the TG orientation in those forms. Thus, only cellulose I has the primary alcohol in TG orientation, which leads to formation of the twist-causing hydrogen bonds.