HIGH-RESOLUTION SOLID-STATE C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY OF CHITIN

High-resolution13C cross-polarization magic-angle spinning (CPMAS) NMR spectra have been acquired from a variety of solid chitin samples. Spectra of highly crystalline β-chitin contain peaks that have remarkably narrow line widths. Single lines can be assigned to all the chemically distinct carbon atoms of the chitin repeat, except for the carbonyl and the C2, which give rise to asymmetric doublets. These doublets arise from the failure of magic-angle spinning (MAS) to remove the dipolar interactions occurring between these carbon atoms and the directly bonded quadrupolar14N nucleus. The single lines, assigned to the remaining carbon atoms, are consistent with a proposed crystal structure of the β polymorph. Spectra from at least three different forms of β-chitin have been identified. The differences between these three forms are a consequence of the differing states of hydration of the β polymorph. Spectra of several highly crystalline forms of β-chitin serve as reference spectra when interpreting the more complex spectra acquired from less ordered samples. The spectra of α-chitin, presented here, contain broad lines and show distinct peak asymmetries. This makes it difficult to interpret spectra, to measure accurate values of chemical shifts, and to make useful comparisons between spectra of the α- and β-chitin polymorphs.13C relaxation rates for the C6 carbon are large relative to the other chemically distinct carbon atoms in chitin of low crystallinity. This difference in relaxation rate is particularly marked in α-chitin. © 1990, American Chemical Society. All rights reserved.