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.
Highlights
Acacia gum (AG, E414), called gum arabic, is a plant exudate used since the Stone Age in various applications, such as food, and the cosmetics and pharmaceutical industries, because of its stabilization, emulsification, encapsulation, and adhesion properties [1,2,3]
The present study focuses on the adsorption of Acacia gums (A. senegal and A. seyal) at the solid–liquid interfaces of gold surfaces, as characterized with a quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR)
The adsorption process is generally driven by hydrophobic interactions between the polypeptide backbone and the gold surface, while the swelling of the adsorbed film is mainly dependent on the hydrophilic interactions between polysaccharide blocks and water
Summary
Acacia gum (AG, E414), called gum arabic, is a plant exudate used since the Stone Age in various applications, such as food, and the cosmetics and pharmaceutical industries, because of its stabilization, emulsification, encapsulation, and adhesion properties [1,2,3]. The structure, chemical compositions, and physico-chemical properties of Acacia gums have been recently reviewed [1]. These hyperbranched arabinogalactan-proteins (AGPs) display high added-value functionalities, their ability to adsorb at solid–liquid (adhesion, adsorption) and liquid–liquid interfaces (emulsion stabilization). The third fraction (1.3% of total gum), referred to as glycoproteins (HIC-F3), contains 14.4% protein and has a molecular mass of 16 × 105 g mol−1 [7]. These values may vary depending on the gum origin, age, storage conditions, etc. These values may vary depending on the gum origin, age, storage conditions, etc. [8]
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