Direct Determination of the Hydrogen Bonding Arrangement in Anhydrous β-Chitin by Neutron Fiber Diffraction

The hydrogen bonding arrangement in anhydrous β-chitin,
a
homopolymer of N-acetylglucosamine, was directly
determined by neutron fiber diffraction. Data were collected from
a sample prepared from the bathophilous tubeworm Lamellibrachia
satsuma in which all labile hydrogen atoms had been
replaced by deuterium. Initial positions of deuterium atoms on hydroxyl
and acetamide groups were directly located in Fourier maps synthesized
using phases calculated from the X-ray structure and amplitudes measured
from the neutron data. The hydrogen bond arrangement in the refined
structure is in general agreement with predictions based on the X-ray
structure: O3 donates a hydrogen bond to the O5 ring oxygen atom of
a neighboring residue in the same chain; N2 and O6 donate hydrogen
bonds to the same carbonyl oxygen O7 of an adjacent chain. The intramolecular
O3···O5 hydrogen bond has the most energetically favorable
geometry with a hydrogen to acceptor distance of 1.77 Å and a
hydrogen bond angle of 171°.