|French, Alfred - Al|
Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2004
Publication Date: 9/1/2004
Citation: French, A.D., Johnson, G.P. Advanced conformational energy surfaces for cellobiose. Cellulose. 2004. v. 11(3-4). p. 449-462. Interpretive Summary: This paper presents the results of three different computer modeling studies of a fragment of the cellulose molecule that is the major constituent of cotton. Such studies are carried out for several reasons to aide in the understanding of biosynthesis and the performance properties of the cotton fiber. The problem has been that such studies do not give very similar results when carried out with different computer software in different laboratories. Therefore, previous work has been in error. The work in this paper shows that the observable shapes of small molecules related to cellulose can be predicted well by the advanced energy calculations used in this paper. These advanced methods included electronic structure theory (quantum mechanics). The work shows that hydrogen bonding does not determine the molecular shape because models that lack the capacity of making hydrogen bonds predict the observed shapes better than the ones that have the ability to make those bonds. Finally, we achieved very similar results with three independently derived models. This is a long sought goal. We were then able to comment on controversies involving strain of the cellulose molecule in the crystalline regions and folding of the cellulose chain molecules. This information is of interest to scientists studying cellulose structure and chemistry, as well as those with a more theoretical interest who want to know "why certain structures are found, and others not found."
Technical Abstract: Energy surfaces imply which molecular shapes are more likely and which are less likely. Therefore, they are valuable for assisting the understanding of the structures involved in the biosynthesis and many practical post harvest exploitations of cellulose. Although usually calculated with molecular mechanics, energy surfaces for the cellobiose fragment of cellulose can now be based on quantum mechanics. This paper presents three sets of energy surfaces. One set is for three analogs of cellobiose that lack hydroxyl groups, calculated based on geometries minimized at the HF/6-31G(d) or B3LYP/6-31G(d) levels. Larger basis sets were also used without further atomic movement. Predictions of experimental crystal structures of cellobiose and related compounds by these gas-phase analogs improved as the analog became sterically more like cellobiose. The second set of maps, for cellobiose itself, gave low HF energies for some experimental structures that had higher energies on the other sets of maps, but they were generally less predictive of the small molecule crystal structure shapes. Finally, AMBER*, a trial version of MM4, and a hybrid B3LYP/6-311++G(d,p)::MM3(96) method all gave similar results, increasing the promise of modeling. These analyses suggest that folding conformations depend on hydrogen bonding to overcome intrinsic strain, and that there may be intrinsic strain for two-fold shapes in crystalline cellulose, with a maximum of about 1.0 kcal for each mole of cellobiose.