Cellulose III _I crystal structure and hydrogen bonding by synchrotron X-ray and neutron fiber diffraction

The crystal and molecular structure, together with the hydrogen-bonding system in ammonia-mercerized cellulose III _I, has been determined using synchrotron X-ray and neutron fiber diffraction data. The structure has a one-chain monoclinic unit cell with an asymmetric unit that contains only one glucosyl residue and with the hydroxymethyl group in the gt conformation. The hydrogen-bonding system is well-defined with no evidence of disorder. A bifurcated hydrogen bond links a donating secondary alcohol 03 atom to a ring 05 atom (major) and a primary alcohol 06 atom (minor) of an adjacent residue in the same chain. Two hydrogen bonds are present between neighboring chains, perpendicular to the chain direction. A detailed comparison of the crystal structure and hydrogen-bonding system reported here for cellulose III _I and those reported previously for the other cellulose polymorphs is given. The conformation of the chain in cellulose III _I has features similar to that of the center chain in the highly stable cellulose II allomorph. However, unlike in cellulose II, the chains are parallel, as in cellulose I _α and cellulose I _β.