Adsorption Behavior of Arabinogalactan-Proteins (AGPs) from Acacia senegal Gum at a Solid–Liquid Interface

Abstract

Adsorption of five different hyperbranched arabinogalactan-protein (AGP) fractions from Acacia senegal gum was thoroughly studied at the solid–liquid interface using a quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance (SPR), and atomic force microscopy (AFM). The impact of the protein/sugar ratio, molecular weight, and aggregation state on the adsorption capacity was investigated by studying AGP fractions with different structural and biochemical features. Adsorption on a solid surface would be primarily driven by the protein moiety of the AGPs through hydrophobic forces and electrostatic interactions. Increasing ionic strength allows the decrease in electrostatic repulsions and, therefore, the formation of high-coverage films with aggregates on the surface. However, the maximum adsorption capacity was not reached by fractions with a higher protein content but by a fraction that contains an average protein quantity and presents a high content of high-molecular-weight AGPs. The results of this thorough study highlighted that the AGP surface adsorption process would depend not only on the protein moiety and high-molecular-weight AGP content but also on other parameters such as the structural accessibility of proteins, the molecular weight distribution, and the AGP flexibility, allowing structural rearrangements on the surface and spreading to form a viscoelastic film.

In Situ ATR Spectroscopy Study of the Interaction of Acacia senegal Gum with Gold Nanoparticles Films at the Solid–Liquid Interface

“…This isotherm experiment was performed by gradually increasing the solution concentration after reaching equilibrium in the previous condition. As we already observed using QCM-D and SPR techniques, − despite the fact that both gold surfaces and A. senegal gum present a negatively charged surface, A.…”

“…For data analysis, the concentration of 150 ppm was chosen, corresponding to the concentration of gum in solution used in our previous study using QCM-D and SPR. 48,49 Detailed infrared spectra and gum structure are discussed in the last part of the study. Figure 4 displays the semiquantitative kinetics as a function of pH of A. senegal gum adsorption , which is attributed to changes in the amide II area and in the carbohydrate area.…”

“…The biochemical composition and structural parameters of A. senegal gum used in this study were presented in previous works. , Stock dispersions of A. senegal gum ( C = 10 g/L) were prepared within the same week before each experiment at room temperature and filtered with a 0.2 μm membrane (GHP, Life Science, Merck, France) using fresh purified Milli-Q water with a resistivity of 18.2 MΩ·cm.…”

“…Previous studies with classical methods (QCM-D and SPR) highlighted the high adsorption capacity of A. senegal gum on hydrophobic and negatively charged surfaces, such as gold surface, by forming very viscoelastic and hydrated layers. − These studies have shown the higher impact of hydrophobic effects, over electrostatic forces, on the adsorption process between the protein moieties and the surface through conformational changes. In addition, the swelling capacity would depend on the hydrophobic effects and hydrophilic forces between polysaccharide blocks and water molecules .…”

“…In addition, the swelling capacity would depend on the hydrophobic effects and hydrophilic forces between polysaccharide blocks and water molecules . The adsorption mechanism seems to be mostly driven by both the protein moiety and its molecular weight distribution . In addition, it was demonstrated that by increasing the interfacial packing (e.g., concentration) of amphiphilic species present in Acacia gum, an irreversible interfacial aggregation took place .…”