Crystal structure of cellulose-iodine complex

Abstract

Different from starch and amylose [(1 → 4)-α-D-glucopyranan], cellulose [(1 → 4)-β-D-glucopyranan] or wood fiber (natural cellulose or cellulose I) and paper (re-generated cellulose or cellulose II) have been believed for long years in many papers and high-school textbooks that they do not form an iodine complex at all. This legend originated from the characteristic conformation of cellulose chain, which is fully extended and had been believed to be impossible to entrap the long iodine ions likely the case of amylose helical chain. However, people often observe such a phenomenon that a filter paper is dyed easily when it is immersed into the iodine aqueous solution. On the basis of the X-ray diffraction measurements of the various kinds of cellulose materials (a piece of wood, bacteria cellulose, paper, cellophane, and so on), the present study has confirmed that cellulose forms the crystalline complex with iodine, as known from the change of X-ray diffraction pattern before and after the complexation. The crystal structure of cellulose-iodine complex has been proposed through the X-ray data analysis and the complex formation mechanism was discussed quantum-chemically by comparing the crystal structure between the original cellulose and its iodine complex. After the completion of the structural analysis of this complex, we happened to notice the paper by Hess et al. (Kolloid Z. (153) 1956, 128) reported about 60 years ago, who measured the X-ray diffraction patterns of cellulose immersed into iodine solution and assigned one reflection of 9.0 Å spacing to the characteristic to the complex. But no detailed analysis was made, and their paper did not get much attention in these 60 years, resulting in the above-mentioned wrong and long legend. The present paper has succeeded for the first time to break through such a common sense and to establish the formation of crystalline cellulose-iodine complex by the detailed X-ray structure analysis.