Conformational analysis of xylobiose by DFT quantum mechanics

Xylobiose (beta-d-xylopyranosyl-(1 -> 4)-d-xylopyranose) could be regarded as the shortest and simplest xylan, a member of the hemicellulose family of molecules that interact with cellulose and lignin in plants. Those interactions, important in plant growth as well as in utilization of wood and cereal grains, depend partly on the molecular shapes. In this work, xylan conformations were modeled with xylobiose using the same density functional theory (DFT) quantum mechanics approach used previously for cellobiose, with both vacuum and solvated models. The region of lowest energy for the solvated model (but not for the vacuum model) accommodates the experimentally observed left-handed, threefold helical shape of xylan hydrate as well as most di- and oligosaccharide structures from the Cambridge Structural Database and the Protein Data Bank. We compared the energy surfaces of xylobiose and cellobiose to learn the effect of the CH2OH group of cellobiose. That comparison, and a similar comparison of non-hydroxyl bearing analogs, showed that the C6 group increased the relative energies of structures having psi(C5) values between - 100 degrees and + 40 degrees. The energy map for the solvated, non-hydroxyl bearing xylobiose analog was surprisingly predictive of the observed experimental crystal structures. Comparisons with xylobiose maps from the literature are also presented. Although hydrogen bonding is frequently invoked to explain changes in carbohydrate structure and properties, condensed-phase experimental structures were predicted better by modeling methods with diminished strengths of hydrogen bonds. That suggests they are relatively unimportant in influencing the structure of beta-(1 -> 4)-linked molecules, in agreement with other work in the literature.