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

Abstract

Acacia gum is a branched complex polysaccharide whose main chain consists of 1,3-linked beta-D-galactopyranosyl units. Acacia gum is defined as a heteropolysaccharide since it contains approximately 2% of a polypeptide. The major molecular fraction (F1) accounting for approximately 88% of the total acacia gum mass is an arabinogalactan peptide with a weight-average molecular weight of 2.86 x 10(5) 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 approximately 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 approximately 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.

A novel analytical ultracentrifugation based approach to the low resolution structure of gum arabic

“…These are depicted in Figure 5. These estimates are somewhat different to previously published results (19), which included a sedimentation coefficient of 0.24S, almost two orders of magnitude lower than what we have found in the present study.…”

Arabinogalactan-Proteins: Key Regulators at the Cell Surface?

“…2,D and E). Gum arabic is generally considered to comprise three fractions: two major ones comprising an AG peptide (88.4%, generally referred to as AG and F1 [for fraction 1]), an AGP fraction (10.4%), and a third minor glycoprotein (GAGP) fraction (1.2%; Sanchez et al, 2008). The twisted hairy rope model was derived from studies of this latter GAGP fraction (Figs.…”

“…In 2008, independent studies of the other (major) gum arabic fractions concluded that the AGP fraction was consistent with the wattle blossom model (Mahendran et al, 2008), whereas the AG peptide F1 was a thin oblate ellipsoid that led to the formulation of a "disc-like model" (Fig. 2C), which was difficult to reconcile with the wattle blossom model (Sanchez et al, 2008). The latter study used a combination of small angle neutron scattering coupled to ab initio calculations and various microscopic techniques (TEM and atomic force microscopy).…”

Gum Arabic: More Than an Edible Emulsifier

“…A main fraction (88-90%) of a polysaccharide of β-(13) galactose, highly branched with units of rhamnose, arabinose and glucuronic acid (which is found in nature like salts of magnesium, potassium and calcium). This fraction is called Arabinogalactan (AG) and contains a low protein content (0.35%) and MW 300 kDa (Renard et al, 2006, Sanchez et al, 2008; ii. A secondary fraction constituting 10% of the total, with a protein content of 11% and a molecular weight of 1400 kDa, corresponding to a complex Arabinogalactan-Protein (AGP) (Goodrum et al, 2000), and finally, iii.…”