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Specific Ion Effects of Trivalent Cations on the Structure and Charging State of $β$-Lactoglobulin Adsorption Layers

In this work, we addressed the effects of Y$^{3+}$ and Nd$^{3+}$ cations on the adsorption of the whey protein $β$-lactoglobulin (BLG) at air-water interfaces as a function of electrolyte concentration. Both cations caused very similar but dramatic changes at the interface and in the bulk solution. Here, measurements of the electropho-retic mobility and vibrational sum-frequency generation spectroscopy (SFG) were applied and consistently showed a reversal of the BLG net charge at remarkably low ion concentrations of 30 (bulk) and 40 (interface) $μ$M of Y$^{3+}$ or Nd$^{3+}$ for a BLG concentration of 15 $μ$M. SFG spectra of carboxylate stretching vibrations from Asp or Glu residues of interfacial BLG showed significant changes in the carboxylate stretching frequency, which we associate to specific and efficient bind-ing of Y$^{3+}$ or Nd$^{3+}$ ions to the proteins carboxylate groups. Characteristic reentrant condensation for BLG moieties with bound trivalent ions was found in a broad concentration range around the point of zero net charge. The highest colloidal stability of BLG was found for ion concentrations <20$μ$M and >50$μ$M. Investigations on macroscopic foams from BLG solutions, revealed the existence of structure-property relations between the interfacial charging state and the foam stability. In fact, a minimum in foam stability at 20$μ$M ion concentration was found when the interfacial net charge was negligible. Our results provide new information on the charge reversal at the liquid-gas interface of protein/ion dispersions. Therefore, we see our findings as an important step in the clarification of reentrant con-densation effects at interfaces and their relevance to foam stability.