The crystal and molecular structures of cellulose I and II.

The paper describes molecular dynamics (MD) simulations on the crystal structures of the Ibeta and II phases of cellulose. Structural proposals for each of these were made in the 1970s on the basis of X-ray diffraction data. However, due to the limited resolution of these data some controversies remained and details on hydrogen bonding could not be directly obtained. In contrast to structure factor amplitudes in X-ray diffraction, energies, as obtained from MD simulations, are very sensitive to the positions of the hydroxyl hydrogen atoms. Therefore the latter technique is very suitable for obtaining such structural details. MD simulations of the Ibeta phase clearly shows preference for one of the two possible models in which the chains are packed in a parallel orientation. Only the parallel-down mode (in the definition of Gardner and Blackwell (1974) J Biopolym 13: 1975-2001) presents a stable structure. The hydrogen bonding consists of two intramolecular hydrogen bonds parallel to the glycosidic linkage for both chains, and two intralayer hydrogen bonds. The layers are packed hydrophobically. All hydroxymethyl group are positioned in the tg conformation. For the cellulose II form it was found that, in contrast to what seemed to emerge from the X-ray fibre diffraction data, both independent chains had the gt conformation. This idea already existed because of elastic moduli calculations and 13C-solid state NMR data. Recently, the structure of cellotetraose was determined. There appear to be a striking similarity between the structure obtained from the MD simulations and this cellotetraose structure in terms of packing of the two independent molecules, the hydrogen bonding network and the conformations of the hydroxymethyl group, which were also gt for both molecules. The structure forms a 3D hydrogen bonded network, and the contribution from electrostatics to the packing is more pronounced than in case of the Ibeta structure. In contrast to what is expected, in view of the irreversible transition of the cellulose I to II form, the energies of the Ibeta form is found to be lower than that of II by 1 kcal mol(-1) per cellobiose.