Physicochemical characterization of α-chitin, β-chitin, and γ-chitin separated from natural resources

We isolated alpha-chitin, beta-chitin, and gamma-chitin from natural resources by a chemical method to investigate the crystalline structure of chitin. Its characteristics were identified with Fourier transform infrared (FTIR) and solid-state cross-polarization/magic-angle-spinning (CP-MAS) C-13 NMR spectrophotometers. The average molecular weights of alpha-chitin, beta-chitin, and gamma-chitin, calculated with the relative viscosity, were about 701, 612, and 524 kDa, respectively. In the FTIR spectra, alpha-chitin, beta-chitin, and gamma-chitin showed a doublet, a singlet, and a semidoublet at the amide I band, respectively. The solid-state CP-MAS C-13 NMR spectra revealed that alpha-chitin was sharply resolved around 73 and 75 ppm and that beta-chitin had a singlet around 74 ppm. For gamma-chitin, two signals appeared around 73 and 75 ppm. From the X-ray diffraction results, alpha-chitin was observed to have four crystalline reflections at 9.6, 19.6, 21.1, and 23.7 by the crystalline structure. Also, beta-chitin was observed to have two crystalline reflections at 9.1 and 20.3 by the crystalline structure. gamma-Chitin, having an antiparallel and parallel structure, was similar in its X-ray diffraction patterns to alpha-chitin. The exothermic peaks of alpha-chitin, beta-chitin, and gamma-chitin appeared at 3 30, 230, and 3 10, respectively. The thermal decomposition activation energies of alpha-chitin, beta-chitin, and gamma-chitin, calculated by thermogravimetric analysis, were 60.56, 58.16, and 59.26 kJ mol(-1), respectively. With the Arrhenius law, In beta was plotted against the reciprocal of the maximum decomposition temperature as a straight line; there was a large slope for large activation energies and a small slope for small activation energies. alpha-Chitin with high activation energies was very temperature-sensitive; beta-Chitin with low activation energies was relatively temperature-insensitive. (C) 2004 Wiley Periodicals, Inc.