New studies on gum ghatti ( Anogeissus latifolia) part II. Structure characterization of an arabinogalactan from the gum by 1D, 2D NMR spectroscopy and methylation analysis

Abstract

The structure of a water soluble arabinogalactan isolated from gum ghatti (Gatifolia SD) was elucidated by methylation analysis and 2D NMR spectroscopy. The arabinogalactan fraction (F80) was obtained by a sequential ethanol precipitation of gum ghatti (Gatifolia SD). Methylation and GC–MS analysis indicated that F80 was a highly branched polysaccharide; the terminal sugar residues were about 40.8% of the total sugars. The majority of the terminal units were α- l-Ara f, with small amounts of T-Glc pA, T-Ara p, T-Rha p and T-Gal p. About 14.2% of the total sugar residues were →6)-β- d-Gal p-(1→ branched at 3rd and 4th positions. The linear portion of the arabinogalactan was composed of →4)-Glc pA(1→, →6)-Gal p(1→ and →2)- l-Ara f-(1→ linkages. Based on the results from methylation analysis, 1D ( 1H, 13C) and 2D (COSY, TOCSY, HMQC and HMBC) NMR spectroscopy, the structure of F80 was proposed as follows: [Display omitted] R is one of the following groups: →3)-β- d-Gal p-(1→, →5)-β- d-Ara f-(1→, →2,3-Man p1→, T-α- l-Ara f 1→, T-Glc pA1→ and T- l-Ara p 1→. Galactose has the -β-D configuration, while the arabinose and rhamnose are in the α- l form.