Structural data on the intra-crystalline swelling of β-chitin

The intra-crystalline swelling of the highly crystalline beta-chitin from Tevnia jerichonana was investigated by X-ray crystallography and Fourier transform infrared (FTIR) spectroscopy, using hydrogenated and deuterated hydrochloric acids as swelling agents. Three levels of swelling were identified that could be defined as inter- and intra-sheet swelling. A moderate and reversible swelling in water and methanol gave crystalline beta-chitin cystallosolvates, namely dihydrate and methanolate, respectively. In these, an inter-sheet swelling was observed, corresponding to an expansion of only the b parameter of the unit cell of beta-chitin. Under these swelling conditions, the use of deuterated reagents had no effect on the amide N-H ... O=C hydrogen bonds that hold the structure of beta-chitin together, but only induced a partial and reversible deuteration of the chitin hydroxymethyl groups. A more severe swelling - but still reversible - occurred with 6 N HCl or DCl, which converted the crystals of beta-chitin into a paracrystalline gel-like product resulting from inter-sheet + intra-sheet swelling. With this acid strength, the deuteration pattern indicated that a fraction of the amide hydrogen bonds was broken and became susceptible to an irreversible deuteration. A very severe and irreversible swelling occurred with 8 N HCl or DCl. In that case, the inter- and intra-sheet swelling was extensive to the point where all memory of the parallel-chain P-chitin was lost. In addition, this swelling was accompanied by a drastic and rapid depolymerization. The treatment with 8 N HCl led invariably to crystalline alpha-chitin when the samples were neutralized. (C) 2000 Elsevier Science B.V. All rights reserved.