Young's modulus calculations for cellulose Iβ by MM3 and quantum mechanics

Quantum mechanics (QM) and molecular mechanics (MM) calculations were performed to elucidate Young's moduli for a series of cellulose I-beta models. Computations using the second generation empirical force field MM3 with a disaccharide cellulose model, 1,4'-O-dimethyl-beta-cellobioside (DMCB), and an analogue, 2,3,6,2',3',6'-hexadeoxy-1,4'-O-dimethyl-beta-cellobioside (DODMCB), that cannot make hydrogen bonds reveal a considerable contribution of intramolecular hydrogen bonding to the molecular stiffness of cellulose I-beta; the moduli for DMCB and DODMCB being 85.2 and 37.6 GPa, respectively. QM calculations confirm this contribution with modulus values of 99.7 GPa for DMCB and 33.0 GPa for DODMCB. However, modulus values for DMCB were considerably lower than values previously reported for cellulose I-beta. MM calculations with extended cellulose chains (10-40 glucose units) resulted in modulus values, 126.0-147.5 GPa, more akin to the values reported for cellulose I-beta. Comparison of the cellodecaose model, 1,4'-O-dimethyl-beta-cellodecaoside (DMCD), modulus with that of its hydrogen bonding-deficient analogue, 2,3,6,2',3',6'-hexadeoxy-1,4'-O-dimethyl-beta-cellodecaoside (DODMCD), corroborates the observed stiffness conferred by intramolecular hydrogen bonds; the moduli for DMCD and DODMCD being 126.0 and 63.3 GPa, respectively. Additional MM3 determinations revealed that modulus values were not strongly affected by intermolecular hydrogen bonding, with multiple strand models providing values similar to the single strand models; 87.5 GPa for a 7-strand DMCB model and 129.5 GPa for a 7 strand DMCD model.