Alterations in Water Structure Induced by Guanidinium and Sodium Ions

Using molecular dynamics simulations, we have probed the ion-induced changes in the water structure as the concentration of ion is varied. We consider two ions, namely, sodium chloride and guanidinium chloride. As the concentration of sodium ions, Na+, is increased the water structure is greatly perturbed. When the mole fraction of water is less than about 0.93, the tetrahedral network of water is affected and there is a total disruption of the tetrahdedral structure of water, just as found in water at high pressures. The number of water−water hydrogen bonds per water molecule is greatly diminished as Na+ concentration increases. Surprisingly, we find that the number of water molecules that are coordinated to Na+ is nearly independent of the sodium chloride concentration. In contrast to the kosmotropic ion Na+, the weakly hydrated chaotropic ion Gdm+ does not alter the water structure as dramatically. At all Gdm+ concentrations examined here, the water hydrogen bonds are fully preserved. We find that the tetrahedral network of water is compromised only at the highest Gdm+ concentration. The differences in the interaction of these two ions with water as their concentration is varied are used to propose a mechanism by which Gdm+ denatures proteins.