Structural analysis of polysaccharides in Chinese lacquer by NMR spectroscopy

Structural characterization of the Chinese lacquer polysaccharide of Rhus vernicifera was performed by means of high resolution NMR spectroscopy involving two dimensional NMR measurements such as field gradient DQF-COSY, TOCSY, HMQC, and HMBC experiments. Most of complicated proton and carbon absorptions were assigned. It was identified that the lacquer polysaccharide had a 1,3-linked D-galactopyranosidic main chain with complex side chains consisting of D-galactose, 4-O-methyl-D-glucuronic acid, D-glucuronic acid, L-arabinose, and L-rhamnose. The coupling constants (J(1,2)) of anomeric protons in the DQF-COSY spectrum suggested that the galactopyranose residue in the main chain, and 4-O-methyl-D-glucuronic acid and D-glucuronic acid in the terminal of the side-chains (J(1,2) = 7.2, 9.0 Hz and 8.1 Hz) had a beta-linkage, and the terminal D-galactopyranose, L-arabinose and L-rhamnose (J(1,2) less than or equal to 3.6 Hz) in the side-chains had an alpha-linkage. The proportions of the component sugar units were calculated by the intensity of the anomeric carbon signals and were in good agreement with those by the methylation analysis reported previously. These results indicate that the NMR characterization can be widely used to investigate the relationship between the structure of lacquer polysaccharides and specific biological activities.