|French, Alfred - Al|
Submitted to: Biomacromolecules
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/5/2008
Publication Date: 10/15/2008
Citation: Nishiyama, Y., Johnson, G.P., French, A.D., Forsyth, T., Langans, P. 2008. Neutron Crystallography, Molecular Dynamics, and Quantum Mechanics Studies of the Nature of Hydrogen Bonding in Cellulose I beta. Biomacromolecules. 9:3133-3140. Interpretive Summary: The three-dimensional structure of cellulose, the main molecule of cotton fibers and plant cell walls in general, was mostly determined several years ago by advanced X-ray and neutron diffraction experiments. One of the remaining questions, critical to understanding chemical modification of cellulose, including textiles, changes in behavior in the presence of varying amounts of water, and the action of enzymes, is the exact hydrogen bonding system. These weak forces, promoted extensively by Linus Pauling, make a big difference in the structures of materials. New experiments were carried out at room temperature and very low temperature with neutron diffraction to learn whether there were changes in the network of hydrogen bonds. Theoretical experiments were also carried out with both electronic structure theory (quantum mechanics) and empirical force field molecular dynamics. The results are mostly of interest to scientists who study cellulose the chemistry and biochemistry of substances that contain cellulose. History of the paper. This work was led by Paul Langan, a contractor for the U.S. Department of Energy. Nishiyama and Forsyth collaborated on the experiments, and Johnson carried out the calculations. The collaboration was initiated during a visit by Johnson and me to Los Alamos last September, and we began the quantum mechanics calculations at that time. As results became available, we forwarded them to Langan. In June, Langan sent a draft of a manuscript that had places to fill in regarding the calculations, and I wrote the requested sections and prepared illustrations and tabulations of the work. I also revisited the molecular dynamics studies that we had done and presented some time ago at the International Carbohydrate Symposium and the Beltwide Cotton Conferences but had not published in a refereed journal. After several exchanges and cycles of editorial revision, Langan submitted the paper. I note that there was considerable comment on the written version from Nishiyama as well, also a world expert on cellulose structure. Also, that both Langan and Nishiyama have close colleagues who are computational chemists with substantial interests in cellulose, but we were asked to participate.
Technical Abstract: In the crystal structure of cellulose Ibeta, disordered hydrogen (H) bonding can be represented by the average of two mutually exclusive H bonding schemes that have been designated A and B. An unanswered question is whether A and B interconvert dynamically, or whether they are static but present in different regions of the microfibril (giving temporally or a spatially averaged structures, respectively). We have used neutron crystallographic techniques to determine the occupancies of A and B at 295K and 15K, quantum mechanical calculations to compare the energies of A and B, and molecular dynamics calculations to look at the stability of A. Microfibrils are found to have most chains arranged in a crystalline Iß structure with H bonding scheme A. Smaller regions of static disorder exist, perhaps at defects within or between crystalline domains, in which the H bonding is complex but with certain features that are found in B.