Rigid-body refinement and conformation of α-chitin

Abstract

The structure of α-chitin has been refined using the rigid body refinement technique. Polysaccharide chitin composed of repeating chitibiose units has been treated as a rigid body. The refinement has been done using group intensity data. Using the Full Matrix Least Square rigid body refinement procedure the structure has been refined to an R-factor of 40.7%. For the refined structure the agreement between the calculated and the observed intensities is reasonably good. The refinement leads to two possible orientations of the CH₂OH group and in both orientations it is not involved in hydrogen bonding. One of the conformations of CH₂OH group in the refined structure is close to that proposed earlier by Carlström (J. Biophys. Biochem, Cytol., 3 (1957) 669) from X-ray and optical data. The other parts of the chain and the packing are very similar to Carlström's proposition. The structure has been examined from a stereochemical point of view and from hydrogen bond energy considerations. The refined structure is found to be free from all short contracts and also to be well stabilised by one intrachain hydrogen bond of the O-H...O type and one interchain hydrogen bond of the N-H...O type. The strength of the hydrogen bond as compared with the best possible ones are discussed. It is believed that the structure obtained is possibly the limit of refinement that can be reached with visually estimated fibre intensity data.