Abstract
Plant Acacia gum exudates are composed by glycosylated hydroxyproline-rich proteins, which have a high proportion of heavily branched neutral and charged sugars in the polysaccharide moiety. These hyperbranched arabinogalactan-proteins (AGP) display a complexity arising from its composition, architecture, and conformation, but also from its polydispersity and capacity to form supramolecular assemblies. Flexibility and hydration partly determined colloidal and interfacial properties of AGPs. In the present article, these parameters were estimated based on measurements of density and sound velocity and the determination of volumetric parameters, e.g., partial specific volume (vs°) and coefficient of partial specific adiabatic compressibility coefficient (βs°). Measurements were done with Acacia senegal, Acacia seyal, and fractions from the former separated according to their hydrophobicity by Hydrophobic Interaction Chromatography, i.e., HIC-F1, HIC-F2, and HIC-F3. Both gums presented close values of vs° and βs°. However, data on fractions suggested a less hydrated and more flexible structure of HIC-F3, in contrast to a less flexible and more hydrated structure of HIC-F2, and especially HIC-F1. The differences between the macromolecular fractions of A. senegal are significantly related to the fraction composition, protein/polysaccharide ratio, and type of amino acids and sugars, with a polysaccharide moiety mainly contributing to the global hydrophilicity and a protein part mainly contributing to the global hydrophobicity. These properties form the basis of hydration ability and flexibility of hyperbranched AGP from Acacia gums.
Highlights
Plant Acacia gum exudates from the trunk and branches of Acacia senegal and Acacia seyal trees are natural viscous fluids that are produced as a protection mechanism of trees [1,2]
Within fractions, hydrophobic interaction chromatography (HIC)-F3 contained the larger amount of minerals (5%), which can be due both to difference in the applied demineralization treatment and to the higher content in aspartic and glutamic amino acid residues (13.5%, 9.6% and 5.7% for HIC-F3, HIC-F2, and HIC-F1, respectively)
Armed with the βM values, we calculated the adiabatic compressibility of hydrating water, and we found that HIC-F2 and HIC-F3 hydration induced a decrease of Ks◦h by 45% and 36%, respectively (Table 5)
Summary
Plant Acacia gum exudates from the trunk and branches of Acacia senegal and Acacia seyal trees are natural viscous fluids that are produced as a protection mechanism of trees [1,2]. Acacia gum exudates contain structurally complex biopolymers and minor associated components, such as minerals, traces of lipids and flavonoids, and enzymes [2,4]. Acacia gum biopolymers are highly glycosylated hydroxyproline-rich, arabinogalactan-peptide, and arabinogalactan-proteins that belong to the glycoprotein superfamily [5,6,7]. Arabinogalactan-proteins (AGPs) have important biological functions since they are implicated in plant growth, development, signaling, and plant–pathogen interactions [8,9]. AGPs are basically composed of a protein core, which is decorated by arabinose and galactose-rich polysaccharide units with varying amounts of rhamnose, fucose and glucuronic acid [10,11]. The highly branched polysaccharidic structure consists of 1,3-linked β-D-galactopyranose monomers with side branches that are linked to the main chain mainly through substitution at O-6 position. In Acacia gums, various populations of hyperbranched AGPs coexist presenting slightly different sugar, amino acid and mineral composition, sugar to amino acid molar ratio, charge density, molar mass, size, shape, and anisotropy [12,13,14,15,16,17,18,19,20,21,22]
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