Thermodynamicall Dominant Structures in the Hydration of Aqueous Ions

The wealth of structures and processes made possible by biological macromolecules rests in considerable measure on the poly-electrolyte character of the macromolecule. To compensate for the charge on the macromolecule, small inorganic ions are always present in the aqueous medium. However, these ions also participate in ion specific ways in the structure and function of the macromolecule. To understand the basis of ion specific effects, it is necessary to understand how the ions are hydrated and how they influence the solvent matrix. We reformulate the quasichemical theory of solution to study how each stepwise increment in the hydration state of the ion accounts for the thermodynamics of hydration. This molecular aufbau approach leads to the surprising insight that only a small subset of water molecules in the first hydration shell of the ion decisively contribute to ion specificity. Further, we find that the distribution of water occupation in a cavity the size and shape of the hydration shell itself is an important element in determining the hydration structure of the ion. These ideas are developed to study the range of ion-specific effects in water and the implications for ion specific effects in biology.