Revisit of α-Chitin Crystal Structure Using High Resolution X-ray Diffraction Data

High resolution synchrotron X-ray fiber diffraction data recorded from crab tendon chitin have been used to describe the crystal structure of α-chitin. Crystal structures at 100 and 300 K have been solved using restrained crystallographic refinement against diffraction intensities measured from the fiber diffraction patterns. The unit cell contains two polymer chains in a 21 helix conformation and in the antiparallel orientation. The best agreement between predicated and observed X-ray diffraction intensities is obtained for a model that includes two distinctive conformations of C6−O6 hydroxymethl group. Those conformations are different from what is proposed in the generally accepted α-chitin crystal structure (J. Mol. Biol. 1978, 120, 167−181). Based on refined positions of the O6 atoms, a network of hydrogen bonds involving O6 is proposed. This network of hydrogen bonds can explain the main features of the polarized FTIR spectra of α-chitin and sheds some light on the origin of splitting of the amide I band observed on α-chitin IR spectra.