Structural studies of an acidic polysaccharide of Mesona blumes gum

Mesona blumes gum can be isolated into a neutral polysaccharide (NMBG) and an acidic polysaccharide (AMBG). The yield of AMBG can be up to 90% (w/w). AMBG with a molecular weight of 6566 Da is composed of galactose, glucose, mannose, xylose, arabinose, rhamnose and galacturonic acid in the molar ratio 2.66:1:0.37:2.29:12.5:5.99:23.5. Structural features of the purified AMBG were investigated by a combination of chemical and instrumental analysis, such as periodate oxidation, Smith degradation, methylation, and C-13 and H-1 NMR. It was found that AMBG possessed a (1 -> 4)-alpha-galacturonan backbone with some insertions of (1 -> 2)-alpha-L-Rhap residues. The branches of arabinogalactan, arabinan, galactan and xylan were all attached to the backbone via O-4 of alpha-L-Rhap residues. In addition, some a-L-Rhap residues on the backbone terminated with alpha-L-Araf and some O-6 in galacturonic acid residues were acetylated or methyl esterified. The molecular structure of AMBG at different concentrations was observed by atomic force microscopy (AFM). It was found that AMBG showed spherical lumps at 1 mu gmL(-1) but an irregular worm-like shape at 10 mu gmL(-1), which indicated that the viscosity of AMBG might be caused by its notable molecular aggregation. (C) 2007 Society of Chemical Industry.