Effect of Hofmeister ions on protein thermal stability: Roles of ion hydration and peptide groups?

We have systematically explored the Hofmeister effects of cations and anions (0.3–1.75 M range) for acidic Desulfovibrio desulfuricans apoflavodoxin (net charge −19, pH 7) and basic horse heart cytochrome c (net charge +17, pH 4.5). The Hofmeister effect of the ions on protein thermal stability was assessed by the parameter dTtrs/d[ion] (Ttrs; thermal midpoint). We show that dTtrs/d[ion] correlates with ion partition coefficients between surface and bulk water and ion surface tension effects: this suggests direct interactions between ions and proteins. Surprisingly, the stability effects of the different ions on the two model proteins are similar, implying a major role of the peptide backbone, instead of charged groups, in mediation of the interactions. Upon assessing chemical/physical properties of the ions responsible for the Hofmeister effects on protein stability, ion charge density was identified as most important. Taken together, our study suggests key roles for ion hydration and the peptide group in facilitating interactions between Hofmeister ions and proteins.