Institute of Physical Chemistry and Center for Soft Nanoscience , Westfälische Wilhelms-Universität Münster Corrensstraße 28/30 , 48149 Münster , Germany.
Langmuir. 2019 Sep 3;35(35):11299-11307. doi: 10.1021/acs.langmuir.9b01803. Epub 2019 Aug 23.
The properties of proteins at interfaces are important to 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 and Nd cations on the adsorption of the whey protein β-lactoglobulin (BLG) at air-water interfaces as the 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 with vibrational sum-frequency generation (SFG) spectroscopy were applied and consistently showed a reversal of the BLG net charge at remarkably low ion concentrations of 30 (bulk) and 40 (interface) μM of Y or Nd for a BLG concentration of 15 μM. SFG spectra of carboxylate stretching vibrations from Asp or Glu residues of interfacial BLG showed significant changes in the resonance frequency, which we associate to specific and efficient binding of Y or Nd 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 μM and >50 μM. 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 μM ion concentration was found when the interfacial net charge was negligible. At this concentration, we propose that the persistent BLG molecules and weakly charged BLG aggregates drive foam stability, while outside the bulk reentrant zone the electrostatic disjoining pressure inside foam lamellae dominates foam stability. 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 condensation effects at interfaces and their relevance to foam stability.
蛋白质在界面处的性质对于许多过程以及水基泡沫等软物质材料都很重要。特别是，蛋白质的界面行为与溶液 pH 值或电解质的存在等不同因素密切相关。在这里，电解质离子的性质可以显著改变蛋白质的界面性质。因此，需要进行界面结构和荷电状态的分子水平研究。在这项工作中，我们研究了 Y 和 Nd 阳离子对乳清蛋白 β-乳球蛋白（BLG）在气-水界面吸附的影响，作为电解质浓度的函数。这两种阳离子在界面和本体溶液中都引起了非常相似但显著的变化。在这里，我们应用电泳迁移率和振动和频产生（SFG）光谱测量，并一致表明，在 BLG 浓度为 15 μM 时，仅在非常低的离子浓度 30（本体）和 40（界面）μM 的 Y 或 Nd 下，BLG 的净电荷发生反转。界面 BLG 中羧酸盐伸缩振动的 SFG 谱显示出共振频率的显著变化，我们将其与 Y 或 Nd 离子与蛋白质羧酸盐基团的特异性和有效结合相关联。在零净电荷点附近的宽浓度范围内，发现带有结合三价离子的 BLG 部分的特征再进入冷凝。在 BLG 浓度<20 μM 和>50 μM 时，BLG 的胶体稳定性最高。从 BLG 溶液中得到的宏观泡沫的研究揭示了界面充电状态和泡沫稳定性之间存在结构-性能关系。事实上，当界面净电荷可忽略不计时，在 20 μM 离子浓度下发现泡沫稳定性最低。在该浓度下，我们提出Persistent BLG 分子和弱带电 BLG 聚集体驱动泡沫稳定性，而在本体再进入区之外，泡沫片层内的静电推斥压力主导泡沫稳定性。我们的结果提供了关于蛋白质/离子分散体在气-液界面处电荷反转的新信息。因此，我们认为我们的发现是澄清界面再进入冷凝效应及其与泡沫稳定性相关性的重要一步。
