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

摘要

C-13 homonuclear through-bond correlations of alpha- and beta-chitin were determined by using two-dimensional (2D) INADEQUATE spectra of these allomorphs purified front crab shell and squid pen, respectively. The 2D C-13-C-13 correlation spectra where two directly bonded carbons share a common double-quantum frequency (DQ) enabled its to precisely assign all C-13 resonances of the chitin allomorphs for the first time. Following the complete C-13 assignment, H-1 chemical shifts of protons attached to each carbon nuclei were assigned by 2D frequency-switched Lee-Goldberg (FSLG) H-1-C-13 heteronuclear correlation (HETCOR) spectra of the chitin allomorphs, recorded with a short mixing time (60 mus) to provide isotropic H-1-C-13 chemical shaft correlations between bonded pairs proton and carbon nuclei. From the C-13 and H-1 chemical shafts 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 C-13-C-13 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 H-1 chemical shifts at the C6 site of alpha-chitin with those at the same site of beta-chitin. (C) 2004 Wiley Periodicals, Inc.