Molecular and crystal structure of the regenerated form of (1 → 3)-α- d-mannan

Abstract

The crystal structure of the hydrated form of (1 → 3)-α- d-mannan, obtained by solid-state deacetylation of the partially O-acetylated mannan, was analyzed by combined X-ray diffraction and stereochemical-model refinement techniques. The structure crystallizes in a four-chain, monoclinic unit cell with parameters a = 11.33 Å, b = 18.36 Å, c (fiber repeat) = 8.25 Å, and γ = 101.75°, and the most probable space group is P2 1. In the most probable structure the chain-backbone conformation is a two-fold helix, but with all four O-6 rotational positions nonequivalent. The chains pack with antiparallel polarity and are connected by pairs of intermolecular hydrogen bonds that form an infinite, zig-zag sheet. There are 16 water molecules in the unit cell, generally embedded between the sheets in crystallographic positions, providing additional hydrogen bonding and establishing a three-dimensional hydrogen-bond network in the crystal structure. The reliability of the structure analysis is indicated by the X-ray residual R″ = 0.281, based on 98 hkl reflection intensities.