X-ray diffraction studies on the structure of alpha-chitin.

The structure for alpha-chitin originally proposed by Carlström is generally considered to be the true structure. However, it fails to account for several remaining problems of chitin structure which include a complete explanation of the infrared spectrum and also the different properties of alpha- and beta-chitins. We have reexamined the structure by X-ray diffraction using automatic rigid subunit least-squares refinement and also the difference Fourier method. The R-value was reduced from 0.31 to 0.22 by a number of small modifications to the structure proposed by Carlström. Consideration of symmetry suggests that two distinct types of statistical modifications could be present in the structure, both of which would allow complete intersheet hydrogen bonding between O6H groups within the general framework of Carlström structure. The X-ray results give some support to one of these modifications. However, it is predictable that both would affect the X-ray diffraction only marginally so that a clear proof lies beyond the present sensitivity of the method. The proposed extra hydrogen bonding in alpha-chitin provide the first clear explanation for the different properties of these kinds of chitin.