High-resolution electron microscopy on cellulose II and α-chitin single crystals

Single-crystalline cellulose II and alpha-chitin regenerated from low-molecular weight solutions using phosphoric acids as a solvent were investigated by electron diffraction together with high-resolution imaging. Cellulose II of two different degrees of polymerization (DP) were regenerated by precipitation either in water (DP = 15) or in ethanol (DP = 7), and the latter revealed better crystalline perfection. In both cases, the structures are in good accord with the previously published 2-chain monoclinic model. Although the lattice fringes of base planes of the crystal were identified by Fourier transform of negatives, the precise localization of cellulose molecules was not visualized due to local azimuthal fluctuation of the crystal. alpha-Chitin was regenerated by precipitation only in ethanol. The crystals are flat ribbons with a smooth surface and, in structure, fit well a previous antiparallel-chain model because their a*b* electron diffraction patterns agree well with the proposed P2(1)2(1)2(1) symmetry. Two-dimensional lattice images were obtained with a resolution up to 0.38 nm. Molecular packing in the unit cell was demonstrated clearly with the help of image processing, and corroborated by kinematical calculation of high-resolution images of alpha-chitin. 0969-0239 (C) 1998 Blackie Academic & Professional.