COSMO-DFTr study of cellulose fragments: Structural features, relative energy, and hydration energies

Cellulosic fragments of size DP-3–8 and DP-10 have been studied using a reduced basis set and density functional (B3LYP). The reduced basis set lowered computer demands so that energy optimization of large cellulosic fragments was possible. 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) allowed calculation of hydration energy contributions per residue. Different conformations are examined including clockwise and counterclockwise hydroxyl groups, the three major hydroxymethyl conformers, gg, gt, and tg and sequentially mixed hydroxymethyl groups i.e. (gg–gt) or (gt–tg). Hydration values are provided as a function of fragment length. Of interest is the overall three-dimensional structure showing twisted states that differ with different hydroxymethyl conformations. A comparison with cellulosic structures is made that 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.