Structural analysis of a pectic polysaccharide from the leaves of Diospyros kaki

Abstract

A pectic polysaccharide DL-2A with a molar mass of 8.5 × 10 5, was obtained from the boiling water extract of Diospyros kaki leaves. It had [ α] 20 D −21.8° ( c 0.22, H 2O) and consisted of rhamnose, arabinose, galactose, xylose and galacturonic acid units in the molar ratio of 0.4:3.4:2.4:1.0:0.8, along with traces of glucuronic acid. About 16.7% of galacturonic acid existed as the methyl ester. A combination of linkage analyses, periodate oxidation, partial acid hydrolysis, selective lithium-degraded reaction, ESIMS, 1H- and 13C- NMR spectral analyses revealed its structural features. It was found that DL-2A possessed an α-(1→4)-galacturonan backbone with some insertions of α-1,2-Rha p residues. The side-chains of arabino-3,6-galactan were attached to the backbone via O-4 of Rha p residues and O-3 of GalA p residues, while 4-linked xylose residues (forming short linear chains) were directly linked to O-4 of rhamnose residues, not as part of the xylogalacturonan. These novel structural features enlarge the knowledge on the fine structure of pectic substances in the plant kingdom.