Conformational studies on β-D-(1→3)-linked xylan

Abstract

The "nonbonded" interaction-energy is computed for a pair of D-xylose residues joined through a β -(1→ 3) linkages, and also for the helical xylan chain, as a function of (^Φ , Ψ )-the dihedral angles of rotation of the pyranose residues about the inter-residue glycosidic bonds C-1-O and O-C-3'. The conformational map reveals that rotation of the pyranose residues in the polymer chain is highly restricted. The two- and three-strand models proposed for a β -D-(1→ 3)-linked xylan from X-ray studies have been examined. The individual strands for a double-helical model have lower energies than those in the triple-helical model. However the triple-strand models have the minimum energy if the interstrand interactions are considered. Even though the right- and left-handed, triple-helical conformations have about equal energy (-7.0 kcal.mole^-1 per residue), the right-handed triple helix is favoured over the left because of the additional possibility in the former of weak hydrogen bonds between strands through water molecules.