Two-dimensional magic angle spinning NMR investigation of naturally occurring chitins: Precise 1H and 13C resonance assignment of (alpha)- and (beta)-chitin

13C homonuclear through-bond correlations of (alpha)- and (beta)-chitin were determined by using two-dimensional (2D) INADEQUATE spectra of these allomorphs purified from crab shell and squid pen, respectively. The 2D 13C-13C correlation spectra where two directly bonded carbons share a common doublequantum frequency (DQ) enabled us to precisely assign all 13C resonances of the chitin allomorphs for the first time. Following the complete 13C assignment, 1H chemical shifts of protons attached to each carbon nuclei were assigned by 2D frequency-switched Lee-Goldberg (FSLG) 1H-13C heteronuclear correlation (HETCOR) spectra of the chitin allomorphs, recorded with a short mixing time (60 (mu)s) to provide isotropic 1H-13C chemical shift correlations between bonded pairs proton and carbon nuclei. From the 13C and 1H chemical shifts of chitin allomorphs, all 2-deoxy-2-acetamide-D-glucose (N-acetyl-D-glucosamine) monomer units in each allomorph were revealed to be an identical 13C-13C backbone conformation and magnetically equivalent. In addition, it was strongly suggested that there are two different hydrogen-bonding patterns at the hydroxyl groups of (alpha)-chitin by comparing 1H chemical shifts at the C6 site of (alpha)-chitin with those at the same site of (bata)-chitin.