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

Abstract

Properties of proteins at interfaces are important to a many processes as well as in soft matter materials such as aqueous foam. Particularly, the protein interfacial behavior is strongly linked to different factors like the solution pH or the presence of electrolytes. Here the nature of the electrolyte ions can significantly modify the interfacial properties of proteins. Therefore, molecular level studies on interfacial structures and charging states are needed. 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 airwater 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 electrophoretic 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

La3+ and Y3+ interactions with the carboxylic acid moiety at the liquid/vapour interface: identification of binding complexes, charge reversal, and detection limits.

“…This effect has been previously observed in several systems undergoing charge inversion, such as when adsorbing the positively charged CTAB surfactant on a negatively charged silica/water interface, [72] and at the TiO2/water [73] and CaF2/water [74,75] interfaces when varying the solution pH, among many other examples. [28,34] The charge reversion or overcompensation is typically driven by specific interactions of multivalent counterions with surfaces that display a relatively high charge density. [76][77][78] For the case of yttrium (and lanthanum), this strong interaction is demonstrated by the formation of binding complexes with the carboxylate headgroup, as revealed by VSFS.…”

“…Nevertheless, trivalent ions are known to play a central role in proteins and colloidal reactivity, being held responsible for crystallization phenomena, phase separation and surface charge reversion. [28][29][30] For example, the adsorption of Fe 3+ and La 3+ on bacterial and bovine serum albumin proteins was revealed to be strongly pH dependent at micromolar concentrations, [31,32] and caused a charge reversion of lipid surfaces starting from sub-micromolar LaCl3 concentrations. [33,34] The complexation of trivalent ions La 3+ and Fe 3+ to the specific carboxylic acid of a fatty acid monolayer has been probed by X-ray reflectivity, X-ray fluorescence, and more recently VSFS.…”

β-Lactoglobulin Adsorption Layers at the Water/Air Surface: 4. Impact on the Stability of Foam Films and Foams

“…In this respect, multi-scale approaches have been used, where macroscopic foaming properties are compared with the microscopic (nanoscopic) properties of lamellae (foam films) and interfaces [18][19][20][21][22][23][24][25][26][27] as foam's building blocks. Regarding interfaces, on the focus are the traditional adsorption dynamics studies, as well as rheological [9][10][11][12][13][14], electrical [27,29,35,[47][48][49][50][51], and structural [30,50,51] interfacial characteristics-it is emphasized the crucial role of the rate of adsorption, surface viscoelasticity (surface elasticity is the resistance force to mechanical disturbances), interface charging state and interfacial molecular organization, respectively, to foamability, foam stability and foam rheology. Pronounced viscoelastic characteristics of interfaces have been recognized to enhance foaming properties [10,11,14,28,31].…”

“…The complexity and high sensitivity of globular proteins to environmental conditions make it difficult to describe their impact on the mechanisms of foam (de)stabilization [8,9,11,12,14,26]. Moreover, protein aggregation (reversible or irreversible) can have complex (enhancing/inhibiting) influence on interfacial and foaming properties as compared to those of the respective native protein [8,19,[39][40][41][42][43][44]49,55]. The behavior of proteins at interfaces and in corresponding foam films and foams is governed by the interplay of the protein concentration and the solvent conditions (salt type and concentration, pH), and the unique effect of pH is of special interest.…”

Multivalent ions and biomolecules: Attempting a comprehensive perspective

“…Furthermore, we wish to mention several other systems because of their model character. As an example, the adsorption of β-lactoglobulin to the air-water interface was studied by Richert et al [435] in the presence of Y 3 + and Nd 3 + cations using a multi-method approach including sum frequency generation (SFG) spectroscopy. Binding of the cations resulted in a reduction of the net protein charge and subsequent aggregation.…”