Flexibility and Hydration of Amphiphilic Hyperbranched Arabinogalactan-Protein from Plant Exudate: A Volumetric Perspective

Abstract

Plant Acacia gum exudates are composed by glycosylated hydroxyproline-rich proteins, which have a high proportion of heavily branched neutral and charged sugars in the polysaccharide moiety. These hyperbranched arabinogalactan-proteins (AGP) display a complexity arising from its composition, architecture, and conformation, but also from its polydispersity and capacity to form supramolecular assemblies. Flexibility and hydration partly determined colloidal and interfacial properties of AGPs. In the present article, these parameters were estimated based on measurements of density and sound velocity and the determination of volumetric parameters, e.g., partial specific volume (v s •) and coefficient of partial specific adiabatic compressibility coefficient (β s •). Measurements were done with Acacia senegal, Acacia seyal, and fractions from the former separated according to their hydrophobicity by Hydrophobic Interaction Chromatography, i.e., HIC-F1, HIC-F2, and HIC-F3. Both gums presented close values of v s • and β s •. However, data on fractions suggested a less hydrated and more flexible structure of HIC-F3, in contrast to a less flexible and more hydrated structure of HIC-F2, and especially HIC-F1. The differences between the macromolecular fractions of A. senegal are significantly related to the fraction composition, protein/polysaccharide ratio, and type of amino acids and sugars, with a polysaccharide moiety mainly contributing to the global hydrophilicity and a protein part mainly contributing to the global hydrophobicity. These properties form the basis of hydration ability and flexibility of hyperbranched AGP from Acacia gums.

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

“…This result is in agreement with the more compact conformation of gum with the increase of the salt concentration, as previously observed using QCM-D. The effect of the salt concentration was previously observed with a decrease of the macromolecule hydration through partial shielding of the charges of the gum in solution [7].…”

“…1 I bu f f er = C tot * 10 pH−pKa 1+10 pH−pKa with C tot = 0.01 M and pK a = 4.75; 2 κ −1 = 0.304/I 1/2 From the QCM-D results, it was observed that A. seyal films are less stable and display a more compact and less viscoelastic behavior than A. senegal films regardless of the experimental conditions. This behavior may be related to the differences in the flexibility of each gum in solution [6,7], and the present study confirms that the A. senegal film has a more flexible and deformable structure than the more rigid A. seyal film. A. seyal films have close similarities to locust bean gum (LBG) films [41].…”

“…Assuming that the hydration of water contributes to the Acacia gum macromolecule by ca. 52% [7], the mass of the saturated film, corrected for the molecule hydration, should be 131.7 and 183.2 ng cm −2 for A. senegal and A. seyal, respectively. The mass obtained from the QCM-D measurements for the adsorbed gum films is 1.3 to 12.7 times greater than the saturation Γ value calculated according to Equation (8).…”

“…The film density, ρ, is ill-defined in the literature and two opposite views are presented: One view for which the film density is equal to 1 g cm −3 when the adsorbed film is highly hydrated [51] and the other view for which film density values vary depending on the nature of the adsorbate and range typically from 1.009 to 1.77 g cm −3 [37,42]. In the present study, it was considered, in accordance with the refractive index value of a dry film corresponding to 100% of the adsorbed species (see Section 2.2.3), that the film density is assumed to be the inverse of the partial specific volume, ϑ, with ϑ = 0.5870 and 0.5767 cm 3 g −1 for A. senegal and A. seyal, respectively [7]. The Sauerbrey equation is only valid for the adsorbed film considered as homogeneous and rigid on the surface.…”

“…The second fraction (10.4% of total), an arabinogalactan-protein complex (HIC-F2), contains 6.3% protein and has a molecular mass of 15 × 10 5 g mol −1 . 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 × 10 5 g mol −1 [7]. These values may vary depending on the gum origin, age, storage conditions, etc.…”