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We describe an approach for characterizing selective binding between oppositely charged ionic functional groups under biologically relevant conditions. Relative shifts in K-shell x-ray absorption spectra of aqueous cations and carboxylate anions indicate the corresponding binding strengths via perturbations of carbonyl antibonding orbitals. XAS spectra measured for aqueous formate and acetate solutions containing lithium, sodium, and potassium cations reveal monotonically stronger binding of the lighter metals, supporting recent results from simulations and other experiments. The carbon K-edge spectra of the acetate carbonyl feature centered near 290 eV clearly indicate a preferential interaction of sodium versus potassium, which was less apparent with formate. These results are in accord with the Law of Matching Water Affinities, relating relative hydration strengths of ions to their respective tendencies to form contact ion pairs. Density functional theory calculations of K-shell spectra support the experimental findings.Hofmeister effects ͉ ion interactions ͉ aqueous systems T he discovery of the selective interactions between simple ions and proteins dates back over a century to Hofmeister's studies with chicken egg protein; proteins could be selectively ''salted in'' or ''salted out'' by the addition of various salts to the solution (1). This ''Hofmeister effect'' has subsequently been observed with more salts and many more proteins, with relative magnitudes following the ''Hofmeister series'' ordering, as do various related phenomena (2, 3). Nevertheless, despite enormous effort, the origin of Hofmeister effects is not completely understood (4, 5).To rationalize biological ion specificity, such as the physiologically important preferential binding of sodium versus potassium with proteins, the Law of Matching Water Affinities was proposed by Collins (6, 7). Based on charge densities and electrostatic arguments, this law holds that ions with similar hydration free energies form the most stable (insoluble) contact ion pairs. In the case of proteins, the carboxylate group is considered to have a hydration energy much closer to that of sodium than to that of potassium, which is manifested in the sodium binding free energy being larger by 2.22 kcal/mol, as determined by Vrbka et al. (8) with simulations and conductivity measurements. The simulations indicated that the interaction is localized on the carboxylate groups of the protein. Conductivity measurements were performed on protein solutions for experimental support, revealing a larger relative decrease upon addition of sodium chloride compared with potassium chloride, indicative of sodium being more efficiently removed from solution than potassium. The rationalization of the preferential interaction was again that there was a closer match of hydration energy, which was reflected in more stable contact ion pairing between the protein carboxylate groups and sodium, versus potassium ions.In this article, we examine the selective interactions of alkali cations ...

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