Specific protein–metal interactions (PMIs) fulfill essential functions in cells and organic bodies, and activation of these functions in vivo are mostly modulated by the complex environmental factors, including pH value, small biomolecules, and salts. Specifically, the role of salts in promoting specific PMIs and their competition among various metals has remained untapped mainly due to the difficulty to distinguish nonspecific PMIs from specific PMIs by classic spectroscopic techniques. Herein, we report Hofmeister salts differentially promote the specific PMIs by combining nanoelectrospray ionization mass spectrometry and spectroscopic techniques (fluorescence measurement and circular dichroism). Furthermore, to explore the influence of salts in competitive binding between metalloproteins and various metals, we designed a series of competitive experiments and applied to a well-defined model system, the competitive binding of zinc (II) and arsenic (III) to holo-promyelocytic leukemia protein (PML). These experiments not only provided new insights at the molecular scale as complementary to previous NMR and spectroscopic results, but also deduced the relative binding ability between zinc finger proteins and metals at the molecular scale, which avoids the mass spectrometric titration-based determination of binding constants that is frequently affected and often degraded by variable solution conditions including salt contents.

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特定的蛋白质-金属相互作用（PMI）在细胞和有机体中完成基本功能，并且体内这些功能的激活主要受复杂的环境因素（包括pH值，小生物分子和盐）的调节。具体而言，盐在促进特定PMI及其在各种金属之间竞争中的作用仍未开发，这主要是由于难以通过经典的光谱技术将非特定PMI与特定PMI区分开。在这里，我们报告霍夫迈斯特盐通过结合纳米电喷雾电离质谱和光谱技术（荧光测量和圆二色性）差异地促进特定的PMI。此外，为了探讨盐对金属蛋白与各种金属之间竞争性结合的影响，我们设计了一系列竞争性实验并将其应用于定义明确的模型系统，即锌（II）和砷（III）与全早幼粒细胞白血病蛋白（PML）的竞争结合。这些实验不仅在分子规模上提供了与以前的NMR和光谱结果互补的新见解，而且推论了锌指蛋白与金属之间在分子规模上的相对结合能力，从而避免了基于质谱滴定法的结合常数测定它经常受包括盐含量在内的各种溶液条件的影响，并经常降解。

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