Water in Crystalline Fibers of Dihydrate β-Chitin Results in Unexpected Absence of Intramolecular Hydrogen Bonding

The complete crystal structure (including hydrogen) of dihydrate β-chitin, a homopolymer of N-acetylglucosamine hydrate, was determined using high-resolution X-ray and neutron fiber diffraction data collected from bathophilous tubeworm Lamellibrachia satsuma. Two water molecules per N-acetylglucosamine residue are clearly localized in the structure and these participate in most of the hydrogen bonds. The conformation of the labile acetamide groups and hydroxymethyl groups are similar to those found in anhydrous β-chitin, but more relaxed. Unexpectedly, the intrachain O3-H...O5 hydrogen bond typically observed for crystalline β,1-4 glycans is absent, providing important insights into its relative importance and its relationship to solvation.