Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography

The chemical and physicochemical characteristics of eight authenticated Acacia senegal gum samples from trees of varying age growing in Sudan have been investigated. The samples originated from the Kordofan region in west Sudan which is the traditional source of the gum and from the new plantations established in the El Daley region in east Sudan. The monosaccharide compositions, protein and amino acid contents and optical rotation were found to be similar to values reported in the literature and there were no obvious trends observed with age or source of the tree. The molecular mass distribution was determined by gel permeation chromatography (GPC) using four different detection systems, namely, UV absorbance, refractive index, photon correlation spectroscopy (PCS) and multiangle laser light scattering (MALLS). The weight average molecular masses obtained by GPC agreed well with the values calculated from the intrinsic viscosity and varied for the individual samples between 2x10(5) and 7.9x10(5). The highest values were obtained for samples from trees 15 years old. All samples were found to be polydisperse (Mw/Mn = 1.3-1.8) and the GPC UV elution profiles demonstrated the existence of three principal molecular mass components with molecular masses of 14x10(6), 1.5-3.0x10(5) and 1.8x10(5). The radii of gyration, Rg, and hydrodynamic radii, Rh, obtained by GPC/MALLS and GPC/ PCS of all of the molecular species were found to be small relative to their molecular mass. The plot of log Rg against log molecular mass for each sample was linear with a slope of similar to 0.42 and the ratio of Rg/Rh was consistently about 0.8 + /-0.2 nm. These findings were taken as evidence of a highly branched globular structure consistent with the 'wattle blossom' model proposed for arabinogalactan protein complexes generally. (C) 1998 Elsevier Science Ltd. All rights reserved.