Alternative hydrogen bond models of cellulose II and III_I based on molecular force-fields and density functional theory

Alternative hydrogen-bond structures were found for cellulose II and III_I based on molecular dynamics simulations using four force fields and energy optimization based on density functional theory. All the modeling results were in support to the new hydrogen-bonding network. The revised structures of cellulose II and III_I differ with the fiber diffraction models mainly in the orientation of two hydroxyl groups, namely, OH2 and OH6 forming hydrogen-bond chains perpendicular to the cellulose molecule. In the alternative structures, the sense of hydrogen bond is inversed but little difference can be seen in hydrogen bond geometries. The preference of these alternative hydrogen bond structures comes from the local stabilization of hydroxyl groups with respect to the β carbon. On the other hand when simulated fiber diffraction patterns were compared with experimental ones, the current structure of cellulose II with higher energy and the alternative structure of cellulose III_I with lower energy were in better agreement.