Diffraction from nonperiodic models of cellulose crystals

Powder and fiber diffraction patterns were calculated for model cellulose crystallites with chains 20 glucose units long. Model sizes ranged from four chains to 169 chains, based on cellulose Iβ coordinates. They were subjected to various combinations of energy minimization and molecular dynamics (MD) in water. Disorder induced by MD and one or two layers of water had small effects on the relative intensities, except that together they reduced the low-angle scattering that was otherwise severe enough to shift the 1 \( \bar {1} \) 0 peak. Other shifts in the calculated peaks occurred because the empirical force field used for MD and minimization caused the models to have small discrepancies with the experimental intermolecular distances. Twisting and other disorder induced by minimization or MD increased the breadth of peaks by about 0.2–0.3° 2-θ. Patterns were compared with experimental results. In particular, the calculated fiber patterns revealed a potential for a larger number of experimental diffraction spots to be found for cellulose from some higher plants when crystallites are well-oriented. Either that, or further understanding of those structures is needed. One major use for patterns calculated from models is testing of various proposals for microfibril organization.