Isolation and characterization of an acidic polysaccharide from Mesona Blumes gum

A neutral polysaccharide (NMBG) and an acidic polysaccharide (AMBG) were fractionated from Mesona Blumes gum (MBG) with yields of 0.5% and 90% (w/w), respectively. AMBG was composed of Gal, Glc, Man, Xyl, Ara, Rha and GalA with a molar ratio of 2.66:1:0.37:2.29:12.5:5.99:23.5, and NMBG consisted of Gal, Glc, Man, Xyl, Ara, Rha with a molar ratio of 9.9:15.3:4.31:1.48:11.6:1. But there was no GalA in NMBG. Both AMBG and NMBG were homogeneous with Mw of 6566 and 5277, respectively. AMBG could form a viscous solution but NMBG could not. Structure features of the purified AMBG were investigated by a combination of chemical and instrumental analyses, such as periodate oxidation, Smith degradation, GC–MS, 13C and 1H NMR. It was found that AMBG possessed a α-(1 → 4)-galacturonan backbone with some insertions of α-1,2-Rhap residues. The branches of arabinogalactan, arabinan, galactan and xylan could be all attached to the backbone via O-4 of Rhap residues. In addition, some Rhap residues on the backbone terminated with α-l-Araf and some O-6 in galacturonic acid residues could be acetylated and some O-6 in GalAp residues could be methyl estered. The molecular structure of AMBG at different concentrations was observed with atomic force microscopy (AFM). AMBG showed a spherical lump at 1 μg/mL, but an irregular shape like worm at 10 μg/mL, which indicated that the viscous property of AMBG might be caused by its strong tendency to aggregate.