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

TL;DR

This study investigates how trivalent Y$^{3+}$ and Nd$^{3+}$ ions influence the charge, structure, and stability of $eta$-lactoglobulin adsorption layers at air-water interfaces, revealing charge reversal and reentrant condensation effects.

Contribution

It provides new insights into specific ion effects on protein interfacial charge reversal, structure, and foam stability, highlighting the role of trivalent cations in these processes.

Findings

Charge reversal occurs at very low ion concentrations (~30-40 μM).

Reentrant condensation is observed around the zero net charge point.

Foam stability correlates with interfacial charge state, showing a minimum at neutral charge.

Abstract

In this work, we addressed the effects of Y$^{3+}$ and Nd$^{3+}$ cations on the adsorption of the whey protein $\beta$-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) $\mu$M of Y$^{3+}$ or Nd$^{3+}$ for a BLG concentration of 15 $\mu$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$\mu$M and >50$\mu$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$\mu$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.