Location: Plant Polymer ResearchTitle: COSMO-DFTr study of cellulosic fragments: structural features, relative energy, and hydration energies Author
Submitted to: Computational and Theoretical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2012
Publication Date: 11/13/2012
Citation: Schnupf, U., Momany, F.A. 2012. COSMO-DFTr study of cellulose fragments: Structural features, relative energy, and hydration energies. Computational and Theoretical Chemistry. 999(1):138-151. Interpretive Summary: The study of cellulosic fragments by advanced computational methods is a continuation of efforts to produce urgently needed high quality data for the field of cellulose research especially in the area of biomass conversion. Recent advances in computer technologies and computational programs have lead to the possibility to model large cellulose fragments in their natural environment. Especially important are cellulose fragments in the size range of 3 to 10 glucose residues. Results of this study include structural properties that change with length of fragments, energy increments upon addition of glucose residues, and overall linearity or twist as one proceeds to larger fragments. Furthermore, the interaction of cellulose fragments with water expressed in terms of hydration energy contributions were determined in relation to the number of glucose residues contained within a fragment. The findings suggest that when freed from the cellulose fiber the flat conformation found experimentally is no longer preferred, that is, the fragments become curved or twisted when free in solution. This suggests that the enzymes working on cellulosic fragments may recognize the molecule in the curved form when binding the free cellulosic fragments. This work will enhance the understanding of catalytic mechanism in biomass conversion and will assist scientists in developing new strategies to precondition cellulose fragments and enzyme functions to increase the productivity in enzymatic biomass conversion.
Technical Abstract: The study of cellulosic fragments by DFTr is a continuation of our efforts to produce quality structural data that will be valuable to those working in the field of cellulose structure and enzymatic degradation. Using a reduced basis set and density functional DFTr(B3LYP), the time and computer demands have been reduced such that optimization of large cellulosic fragments with an implicit solvent model, COSMO, is now possible. The fragments examined by optimization methods include syn forms of DP3 to DP8 and DP10. Results include conformational properties that change with length of fragments, energy increments upon addition of residues, and overall curvature or twist as the fragments become larger. The inclusion of solvent as implicit water (COSMO) has proven to be useful for small carbohydrates and is utilized here. Hydration energy contributions are noted as a function of number of residues. Different conformations are examined including clockwise and counterclockwise hydroxyl groups, and the three major hydroxymethyl conformers, gg, gt, and tg. In some examples, sequentially mixed hydroxymethyl groups are included, i.e. (gg-gt) or (gt-tg). Hydration values, provide a means of examining the hydration as a function of fragment length. Of particular interest is the overall three-dimensional structure showing twisted states that differ with different hydroxymethyl conformations. A comparison with cellulosic structures suggests that upon removal of a single chain from the cellulose crystal face the conformation may change from a nearly planar form to a more twisted structure with subsequent change in hydroxymethyl preference. This result has implications for computational studies of enzymatic digestion of cellulose as well as for the aggregation states to form crystals.