Adsorption of arabinogalactan-proteins from Acacia gums (senegal and seyal) and its molecular fractions onto latex particles
Résumé
Adsorption of Acacia gums (GA) (A. senegal and A. seyal) and A. senegal molecular fractions, made of
arabinogalactan-proteins (AGPs) with different molecular weights, onto latex particles, with different sizes and
surface charges, was studied using molecular (fluorescence and nanoIR spectroscopies, microscopies including
Atomic Force Microscopy) and mesoscopic (Dynamic Light Scattering, zeta potential, Asymmetrical flow field-
flow fractionation) methods. Adsorption of GA onto latex particles revealed a complex mechanism of adsorp-
tion where the highest adsorption was observed when A. senegal and latex particles were negatively charged. The
invert was observed for A. seyal where the maximum of adsorption was observed when gum and latex were
oppositely charged. The duality of positive and negative charges on GA macromolecules together with confor-
mational changes certainly explain the complex mechanism of adsorption. A mechanism where either the protein
moiety either the sugars blocks adsorb is to be preferred. This mechanism seems to produce differences of
behaviour due to viscoelastic properties of adsorbed layers, and differences of hydration due to differences of
polarity of AGPs. The consequence of electrostatics in the adsorption process is a water release from the adsorbed
layers and important gain in entropy. A fair agreement was found between protein content and adsorbed layer
thickness of GA, with the formation of partial coverage, monolayer and multilayer films depending on GA
concentration. Structural changes induced by GA adsorption onto latex particles was probed using fluorescence
and nanoIR spectroscopies without giving clear evidence of conformational changes induced after GA adsorption.
The results of this study highlighted that GA surface adsorption process depend not only on the protein moiety
and electrostatic interactions but also on other parameters related to AGP hydration status. The protein structural
accessibility, the molecular weight distribution, the AGPs intrinsic viscoelastic properties allowing structural
rearrangements on the surface and spreading in order to form a viscoelastic film onto latex particles should also
play a pivotal role in the adsorption process.