Adsorption of Hyperbranched Arabinogalactan-Proteins from Plant Exudate at the Solid–Liquid Interface

Abstract

Adsorption of hyperbranched arabinogalactan-proteins (AGPs) from two plant exudates, A. senegal and A. seyal, was thoroughly studied at the solid–liquid interface using quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance (SPR), and atomic force microscopy (AFM). Isotherms of the adsorption reveal that 3.3 fold more AGPs from A. seyal (500 ppm) are needed to cover the gold surface compared to A. senegal (150 ppm). The pH and salt concentration of the environment greatly affected the adsorption behavior of both gums, with the surface density ranging from 0.92 to 3.83 mg m−2 using SPR (i.e., “dry” mass) and from 1.16 to 19.07 mg m−2 using QCM-D (wet mass). Surprisingly, the mass adsorbed was the highest in conditions of strong electrostatic repulsions between the gold substrate and AGPs, i.e., pH 7.0, highlighting the contribution of other interactions involved in the adsorption process. Structural changes of AGPs induced by pH would result in swelling of the polysaccharide blocks and conformational changes of the polypeptide backbone, therefore increasing the protein accessibility and hydrophobic interactions and/or hydrogen bonds with the gold substrate.

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

“…This phenomenon is probably due to the increase in ion hydration with the increase in ionic strength and, therefore, a decrease in the AGP hydration as previously observed. 18 At 10 mM, GA and fractions present a hydrated state around 90% and a decrease around 75−80% at 110 mM. F1, a hydrophilic fraction susceptible to form more hydrogen bonds with water, presents however the less hydrated film with 87.5% at 10 mM and 74.8% at 110 mM.…”

“…The QCM-D principles are described in detail elsewhere. 18,32,33 This method allows the determination of the mass change, the reversibility of the adsorption process, the kinetics, and the process of macromolecule swelling by monitoring two parameters: the change in the resonance frequency (ΔF) and the change in the dissipation energy (ΔD). Adsorption of molecules causes a decrease in the resonance frequency, while the dissipation energy is related to the viscoelastic properties of the adsorbed molecules.…”

“…The temperature of the measuring chamber was stabilized at 25 °C with a flow rate of 200 μL/min. GA or fraction solution at 150 ppm, corresponding to the maximum coverage concentration according to the isotherm performed in our previous study, 18 was left in contact with the substrate until stabilization was reached during the adsorption process. Desorption was investigated by rinsing the system with initial buffer solution.…”

“…The same refractive index n a for GA and fractions was used (n a = 1.48). 18 The dry mass adsorbed Γ SPR (ng/cm 2 ) on the gold surface was calculated according to eq 2:…”

“…2 To the best of our knowledge, only one study focuses on the adsorption properties of GA on 2D solid surfaces. 18 In our previous study, we focused on the adsorption properties of A. senegal and A. seyal gums at three different pH values and salt concentrations. The results highlighted some important differences in the adsorption capacity and behavior of both gums, with the highest adsorption capacity being for A. senegal gum.…”