Tuesday, May 3, 2011
We use molecular simulation to elucidate the structural behavior of small hydrated cellulose Iβ microfibrils heated to 227°C (500 K) with two carbohydrate force fields. In contrast to the characteristic two-dimensional hydrogen-bonded layer sheets present in the cellulose Iβ crystal structure we show that at high temperature a three-dimensional hydrogen bond network forms, made possible by hydroxymethyl groups changing conformation from trans-gauche (TG) to gauche-gauche (GG) in every second layer corresponding to ‘center’ chains in cellulose Iβ and from TG to gauche-trans (GT) in the ‘origin’ layer. The presence of a regular three-dimensional hydrogen bond network between neighboring sheets eliminates the possibility of twist, whereas two-dimensional hydrogen bonding allows for microfibril twist to occur. Structural features of this high temperature phase as determined by molecular simulation may explain several experimental observations for which no detailed structural basis has been offered. This includes a proposed mechanism of the irreversible transformation of cellulose Iα to Iβ by thermal annealing, an explanation for the observed temperature and crystal size dependence for the extent of hydrogen/deuterium exchange, and diffraction patterns of cellulose at high temperature.