title: Structure of arabinogalactan-protein from Acacia gum: from porous ellipsoids to supramolecular architectures.
abstract: The structure of the arabinogalactan-protein (AGP) fraction of the gum exudate of Acacia senegal (gum Arabic) isolated from hydrophobic interaction chromatography was investigated using HPSEC-MALLS, small angle neutron scattering and TEM observations. Literature reported that the AGP structure of gum Arabic adopts a very compact conformation in solution due to the attachment of short arabinoside side chains and much larger blocks of carbohydrate to the polypeptidic backbone. The present study revealed that AGP in solution had a weight average molecular weight Mw of 1.86×10(6) g mol(-1) and a radius of gyration Rg of 30 nm. In addition, two exponent values were identified in the Rg, [η], Rh and ρ vs. Mw relationships highlighting two types of conformations depending on the molecular weight range considered: a low molar mass population with long-chain branching and a compact conformation and a high molar mass population with short-chain branching and an elongated conformation. AGP would behave in solution as a branched or hyper-branched polymer with conformations ranging from globular to elongated shape depending on the size of the carbohydrate branches. Small angle scattering form factor revealed an elongated average conformation corresponding to a triaxial ellipsoid while inverse Fourier transform of the scattering form factor gave a maximum dimension for AGP of 64 nm. Transmission electron microscopy highlighted the existence of two types of flat objects with thicknesses below 3-5 nm, single particles with a more or less anisotropic spheroidal shape and aggregated structures with a more elongated shape. A remarkable feature of all particle morphologies was the presence of an outer structure combined to an inner more or less porous network of interspersed chains or interacting structural blocks, as previously found for the arabinogalactan (AG) main molecular fraction of Acacia gum. However, clear differences were observed in the density and morphology of the inner porous network, probably highlighting differences in the degree of branching. The existence of assembled AG as part of the AGP family was confirmed using TEM micrographs at high resolution. Fused AGP dimers, trimers, tetramers and multimers were also identified. These molecular assemblies questioned about the nature of interactions involved.
authors.affiliation: INRA, UR1268 Biopolymères Interactions Assemblages, F-44300 Nantes, France.
authors.forename: D
authors.initials: D
authors.lastname: Renard
authors.affiliation: UMR CNRS 6290 Equipe TAF «Translation and Folding», Université de Rennes 1, Campus Beaulieu, F-35042 Rennes Cedex, France.
authors.forename: C
authors.initials: C
authors.lastname: Garnier
authors.affiliation: Laboratoire Léon Brillouin, CEA Saclay, Bâtiment 563, F-91191 Gif-sur-Yvette, France.
authors.forename: A
authors.initials: A
authors.lastname: Lapp
authors.affiliation: Department of Food Science and Technology, Nestlé Research Center, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland.
authors.forename: C
authors.initials: C
authors.lastname: Schmitt
authors.affiliation: UMR1208 Ingénierie des Agropolymères et Technologies Emergentes, INRA-Montpellier SupAgro-CIRAD-Université Montpellier 2, Place Pierre Viala, F-34060 Montpellier, France.
authors.forename: C
authors.initials: C
authors.lastname: Sanchez
mesh_terms: D006170:Gum Arabic / Q000737:chemistry* / Q000302:isolation & purification*; D046911:Macromolecular Substances / Q000737:chemistry / Q000302:isolation & purification; D015394:Molecular Structure; D009088:Mucoproteins / Q000737:chemistry* / Q000302:isolation & purification*; D010936:Plant Extracts / Q000737:chemistry / Q000302:isolation & purification; D010940:Plant Proteins / Q000737:chemistry / Q000302:isolation & purification; D016062:Porosity
chemical_list: D046911:Macromolecular Substances; D009088:Mucoproteins; D010936:Plant Extracts; D010940:Plant Proteins; C006619:arabinogalactan proteins; D006170:Gum Arabic