|Stevens, Edwin - DEPT CHEMISTRY/UNO|
Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 26, 2005
Publication Date: April 11, 2005
Citation: Ford, Z.M., Stevens, E.D., Johnson, G.P., French, A.D. 2005. Determining the Crystal Structure of Cellulose IIII by Modeling. Carbohydrate Research. 340(5):827-833. Interpretive Summary: Cotton is almost pure cellulose, a molecule that is subjected to many physical and chemical treatments to improve its usefulness. However, much is as yet unknown regarding the structures of materials that are made out of cellulose, and this inhibits the ability to apply knowledge-based modifications that could allow expanded use of this important agricultural material. One well known treatment of cellulose is "ammonia mercerization" which alters the crystal structure of native cellulose to make the form known as cellulose III. The present work was carried out to learn just what the ammonia treatment had done to the cellulose, using computational molecular modeling. It was done in parallel with experimental work, and the two techniques yielded remarkably similar results. This information is of interest to scientists involved in experimental and theoretical determinations of carbohydrate structure and those carrying out chemical modification of cellulose.
Technical Abstract: Recently, a one-chain unit cell for Cellulose IIII having P21 symmetry and a single glucose in the asymmetric unit was proposed based on a high-resolution diffraction pattern. The work contradicted the two-chain structure that had been put forth nearly 30 years ago, but it did not provide new three-dimensional coordinates. Our goal was to solve the structure by modeling and compare the resulting structure with the previously determined two-chain unit cell. Combinations of three rotamers of the O2, O3, and O6 hydroxyls produced 27 'up' and 27 'down' starting structures. Clusters ('minicrystals') of 13 cellotetraose chains capped by methyls for each of the 54 starting structures were optimized with MM3(96). Hydroxyl groups on 16 of these 54 structures reoriented to give very similar hydrogen bonding schemes in the interiors, along with the lowest energies. Hydrogen bonds include the usual intramolecular O-3H'O-5' linkage, with O-6' also accepting from O-3H. Inter-chain hydrogen bonds form an infinite, cooperative O-6H'O-2H'O-6 network. Direct comparison with the two-chain unit cell was inappropriate because the alternate chains in the two-chain cell are shifted along the z-axis. To get comparable energy values, models were built with both cellotetraose and cellohexaose chains. The difference in their energies represents the energy for the central layer of cellobiose units. The one-chain cell models had much lower energy. The eight best 'up' one-chain models agree reasonably well with the structure newly determined by experiment.