Conformational analysis of xylan chains.

The present study provides a theoretical description of the different levels of structural organization that characterize the xylan polysaccharide in its native and hydrophobic lauroyl esterified forms. The goal of this study was to ascertain the role played by the hydroxyl or lauroyl side groups on the conformational flexibility of the xylan chain backbone. The results reported provide a detailed description of the low-energy conformers of the dimer segments, a complete characterization of the helical structures, an insight into the disordered state of the polysaccharide chains and an estimation of the cohesion of the amorphous solids. Esterification of xylan hydroxyl groups by lauric acid has a large effect on the conformational properties of the glycosidic bonds linking two repeat units. Both the location and the relative energies of the low energy areas of the potential energy surfaces strongly differ: extended and coiled conformations are preferred for the native and hydrophobic forms, respectively. Consequently, the predicted unperturbed polymer chain extension strongly depends on the structure, predicted Lp of the native xylan of 35 A compares favourably well with the experimental ones, this characteristic dramatically decreases to 9A for the hydrophobically modified chain. Curiously, only extended 2(1) and left-handed 3(1) helical structures are calculated stable for both polymers. The estimated cohesive parameters of amorphous bulks reveal that inter-chain interactions are stronger for the xylan chain than that for modified one, the former being stabilized by hydrogen bonds whereas hydrophobic interactions play a determinant role for the latter.