In vitro and abiotic synthesis of cellulose 1 have indicated that glucan sheet formation is most likely to be the first stage in crystallization of this allomorph of cellulose. Bonding schemes for the different glucan sheets found in crystals of cellulose I-alpha and I-beta were analyzed energetically with the molecular mechanics program, MM3. Using high and low dielectric constants, favouring van der Waals forces and hydrogen bonding, respectively, van der Waals-associated mini-sheets had lower energies than hydrogen bonded mini-sheets. Furthermore, the first glucan mini-sheet most likely to form is the one with the lowest energy. In the case of cellulose I~, this mini-sheet was the one along the (1 10) plane; in the case of cellulose I-alpha, it was the one along the (010) plane. Incorporating these results into known experimental evidence, we theorize the requirement of at least three sequential steps for native cellulose I crystallization: (1) formation of mini-sheets by van der Waals forces, (2) association of these sheets by hydrogen bonding into mini-crystals, and (3) the convergence of mini-crystals to form the crystalline microfibril.
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