Advanced conformational energy surfaces for cellobiose

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 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 2-fold shapes in crystalline cellulose, with a maximum of about 1.0 kcal for each mole of cellobiose.