Structural differences between chitin polymorphs and their precipitates from solutions—evidence from CP-MAS 13C-NMR, FT-IR and FT-Raman spectroscopy

Abstract

Structural differences between α-chitin from shrimp ( Crangon crangon) and β-chitin from squid (Loligo), as well as between their precipitated products from N,N-dimethylacetamide-LiCl solutions, are indicated by the CP-MAS 13C-NMR, FT-IR, FT-Raman spectra and the X-ray diffractograms. The 13C-NMR spectra in the solid state of α-chitin consisting of eight major resonances suggest a high degree of structural homogeneity. In the spectrum of β-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 α-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 β-form. Precipitation treatments induce a general disorder in α-chitin, while the metastable structure of β-chitin tends toward a more ordered architecture. All the present spectroscopic data of chitin samples are consistent with the corresponding X-ray diffraction patterns.