Institut für Angewandte Physik, Universität Tübingen, Tübingen, Germany.
J Phys Chem B. 2013 May 9;117(18):5777-87. doi: 10.1021/jp401874t. Epub 2013 May 1.
Tuning of protein surface charge is a fundamental mechanism in biological systems. Protein charge is regulated in a physiological context by pH and interaction with counterions. We report on charge inversion and the related reentrant condensation in solutions of globular proteins with different multivalent metal cations. In particular, we focus on the changes in phase behavior and charge regulation due to pH effects caused by hydrolysis of metal ions. For several proteins and metal salts, charge inversion as measured by electrophoretic light scattering is found to be a universal phenomenon, the extent of which is dependent on the specific protein-salt combination. Reentrant phase diagrams show a much narrower phase-separated regime for acidic salts such as AlCl3 and FeCl3 compared to neutral salts such as YCl3 or LaCl3. The differences between acidic and neutral salts can be explained by the interplay of pH effects and binding of the multivalent counterions. The experimental findings are reproduced with good agreement by an analytical model for protein charging taking into account ion condensation, metal ion hydrolysis, and interaction with charged amino acid side chains on the protein surface. Finally, the relationship of charge inversion and reentrant condensation is discussed, suggesting that pH variation in combination with multivalent cations provides control over both attractive and repulsive interactions between proteins.
蛋白质表面电荷的调节是生物系统中的一个基本机制。在生理环境中，蛋白质电荷通过 pH 值和与抗衡离子的相互作用来调节。我们报告了带有不同多价金属阳离子的球状蛋白质溶液中的电荷反转和相关的再进入凝聚。特别是，我们关注由于金属离子水解引起的 pH 效应导致的相行为和电荷调节的变化。对于几种蛋白质和金属盐，通过电泳光散射测量的电荷反转被发现是一种普遍现象，其程度取决于特定的蛋白质-盐组合。再进入相图显示，对于酸性盐（如 AlCl3 和 FeCl3），与中性盐（如 YCl3 或 LaCl3）相比，相分离区域要窄得多。酸性盐和中性盐之间的差异可以通过 pH 值效应和多价抗衡离子的结合相互作用来解释。实验结果与考虑离子凝聚、金属离子水解以及与蛋白质表面带电氨基酸侧链相互作用的蛋白质充电分析模型很好地吻合。最后，讨论了电荷反转和再进入凝聚的关系，表明 pH 值变化与多价阳离子相结合，可以控制蛋白质之间的吸引和排斥相互作用。
