The crystal and molecular structures of cellulose I and II |
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Authors: | LMJ Kroon-Batenburg J Kroon |
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Institution: | (1) Department of Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands |
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Abstract: | The paper describes molecular dynamics (MD) simulations on the crystal structures of the Iβ 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 Iβ 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 Iβ structure. In contrast to what is expected, in view of the irreversible transition of the cellulose
I to II form, the energies of the Iβ form is found to be lower than that of II by 1 kcal mol-1 per cellobiose.
This revised version was published online in November 2006 with corrections to the Cover Date. |
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Keywords: | molecular dynamics crystal structure cellulose I and II |
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