The Acacia Gum Arabinogalactan Fraction Is a Thin Oblate Ellipsoid: A New Model Based on Small-Angle Neutron Scattering and Ab Initio Calculation

Acacia gum is a branched complex polysaccharide whose main chain consists of 1,3-linked β-D-galactopyranosyl units. Acacia gum is defined as a heteropolysaccharide since it contains ∼2% of a polypeptide. The major molecular fraction (F1) accounting for ∼88% of the total acacia gum mass is an arabinogalactan peptide with a weight-average molecular weight of 2.86 × 105 g/mol. The molecular structure of F1 is actually unknown. From small angle neutron scattering experiments in charge screening conditions, F1 appeared to be a dispersion of two-dimensional structures with a radius of gyration of ∼6.5 nm and an inner dense branched structure. Inverse Fourier transform of F1 scattering form factor revealed a disk-like morphology with a diameter of ∼20 nm and a thickness below 2 nm. Ab initio calculations on the pair distance distribution function produced a porous oblate ellipsoid particle with a central intricated “network”. Both transmission electron microscopy and atomic force microscopy confirm the thin disk model and structural dimensions. The model proposed is a breakthrough in the field of arabinogalactan-protein-type macromolecules. In particular, concerning the site of biosynthesis of these macromolecules, the structural dimensions found in this study would be in agreement with a phloem-mediated long-distance transport. In addition, the structure of F1 could also explain the low viscosity of acacia gum solutions, and its ability to self-assemble and to interact with proteins.