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Quantifying Pressure Effects on Water-Mediated Ion-Ion Interactions

Sapna Sarupria, Caitlin Scott, and Shekhar Garde. Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180

In recent years, pressure has emerged as an important thermodynamic dimension to perturb proteins and study their response. The most interesting and counter-intuitive result of these studies is that proteins unfold when subjected to high hydrostatic pressures! While this behavior of globular proteins has been well-established, the molecular picture of pressure unfolding is not completely understood. Insights have been gained from studies of the fundamental interactions involved in protein folding. In particular, previous studies have focused on the pressure effects on hydrophobic interactions. It has been found that hydrophobic interactions are weakened with increasing pressure suggesting that pressure unfolding of proteins involves penetration of water molecules into protein interiors causing the protein to swell and unravel. Influence of high pressure on hydration and interactions of ions in water on the other hand, are relatively less studied. To this end, we have performed extensive molecular dynamics simulations of aqueous salt solutions in an extended pressure-temperature plane. We have looked at three different alkali-halide salts (NaF, NaCl and NaI) thereby sampling different charge densities. The key finding is that the effect of pressure on the strength of water mediated ion-pair interactions is dependent on the ion-pair. The results from this study are also relevant to other biological systems and processes such as protein-DNA binding, protein-ligand binding, and salting-in and salting-out of proteins.