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

C. Sanchez ^*, C. Schmitt , E. Kolodziejczyk , A. Lapp , C. Gaillard  and D. Renard 

^* Laboratoire de Science et Génie Alimentaires, ENSAIA-INPL, F-54505 Vandoeuvre-lès-Nancy cedex 5, France; Department of Food Science, Nestlé Research Center, CH-1000 Lausanne 26, Switzerland; Laboratoire Louis Brillouin, CEA Saclay, 91191 Gif-sur-Yvette cedex, France; and Unité de Recherches sur les Biopolymères, Interactions, Assemblages, INRA Centre de Nantes BP 71627, 44316 Nantes cedex 3, France

Correspondence: Address reprint requests to C. Sanchez, Tel.: 33-3-83-59-57-88; Fax: 33-3-83-59-58-04; E-mail: Christian.Sanchez{at}ensaia.inpl-nancy.fr.

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 x 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.