On the molecular mechanism of ion specific Hofmeister series

Hofmeister series ranks the ability of salt ions in influencing a variety of properties and processes in aqueous solutions. In this review, we reexamine how these ions and some other small molecules affect water structure and thermodynamic properties, such as surface tension and protein backbone solvation. We illustrate the difficulties in interpreting the thermodynamic information based on structural and dynamic arguments. As an alternative, we show that the solvation properties of ions and proteins/small molecules can be used to explain the salt effects on the thermodynamic properties of the solutions. Our analysis shows that the often neglected cation-anion cooperativity plays a very important role in these effects. We also argue that the change of hydrogen donor/acceptor equilibrium by added cosolutes/cosolvents can be used to explain their effects on protein secondary structure denaturation/protection: those increase hydrogen donor concentrations such as urea and salts with strongly solvated cations/weakly hydrated anions tend to dissolve protein backbone acting as secondary structure denaturants, whereas those lack of hydrogen donors but rich in acceptors have the opposite effect.