X-ray Structure of Anhydrous β-Chitin at 1 Å Resolution

The crystal structure of β-chitin, a homopolymer of N-acetylglucosamine, was determined using highly resolved X-ray fiber diffraction data obtained from uniaxially oriented specimen of diatom spines. The labile N-acetyl groups and hydroxymethyl groups were directly located in the Fourier omit maps constructed using the glycosyl backbone as phasing model. Using the 216 independent intensities extending to a resolution of 1 Å, we could refine the structure without any restraints on bond angles to obtain a reasonable model with high accuracy. The carbonyl oxygen of acetamide group formed intermolecular hydrogen bonding between the primary hydroxyl and amine groups forming a two-dimensional hydrogen bonding network in a plane perpendicular to the pyranosyl plane. The hydrogen bonding pattern explains why the conformation of the acetamide groups is slightly rotated from standard conformation.