STRUCTURAL DIFFERENCES BETWEEN CHITIN POLYMORPHS AND THEIR PRECIPITATES FROM SOLUTIONS - EVIDENCE FROM CP-MAS C-13-NMR, FT-IR AND FT-RAMAN SPECTROSCOPY

Structural differences between alpha-chitin from shrimp (Crangon crangon) and beta-chitin from squid (Loligo), as well as between their precipitated products from N,N-dimethylacetamide-LiCl solutions, are indicated by the CP-MAS C-13-NMR, FT-IR, FT-Raman spectra and the X-ray diffractograms. The C-13-NMR spectra in the solid state of alpha-chitin consisting of eight major resonances suggest a high degree of structural homogeneity. In the spectrum of beta-chitin, the C-3 and C-5 signals merged in a single resonance. The bands contour of deconvoluted and curve-fit FT-IR and FT-Raman spectra shows a more detailed structure of alpha-chitin in the region of O - H, N - H and C = O stretching regions; in particular the Amide I band is split, whereas a single broad band dominates in the corresponding beta-form. Precipitation treatments induce a general disorder in alpha-chitin, while the metastable structure of beta-chitin tends toward a more ordered architecture. All the present spectroscopic data of chitin samples are consistent with the corresponding X-ray diffraction patterns.