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

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 a-L-Araf, with small amounts of T-GlcpA, T-Arap, T-Rhap and T-Galp. About 14.2% of the total sugar residues were -> 6)-beta-D-Galp-(1 -> branched at 3rd and 4th positions. The linear portion of the arabinogalactanwas composed of -> 4)-GlcpA(1 ->, -> 6)-Galp(1 -> and -> 2)-L-Araf-(1 -> linkages. Based on the results from methylation analysis, 1D (H-1, C-13) and 2D (COSY, TOCSY, HMQC and HMBC) NMR spectroscopy, the structure of F80 was proposed as follows: [GRAPHICS] R is one of the following groups: -> 3)-b-D-Galp-(1 ->, -> 5)-beta-D-Araf-(1 ->, -> 2,3-Manp1 ->, T-alpha-L-Araf 1 ->, T-GlcpA1 -> and T-L-Arap 1 ->. Galactose has the -beta-D configuration, while the arabinose and rhamnose are in the a-L form. (C) 2010 Published by Elsevier Ltd.