To switch or not to switch: the effects of potassium and sodium ions on alpha-poly-L-glutamate conformations in aqueous solutions.

Molecular dynamics simulations demonstrate that differences in the interaction of sodium and potassium with the carboxylate side chains of alpha-poly-L-glutamate (alpha-PGA) have a dramatic effect on the conformational properties of the polypeptide. Potassium ions cluster mainly in the second and third solvation shells of alpha-PGA because their low charge density makes the electrostatic interactions between them and alpha-PGA too weak for K(+) to compete with water for the first solvation shell of the alpha-PGA glutamic acid residuals. Unlike sodium ions, they do not switch the conformation of alpha-PGA from extended to alpha-helical. Potentials of mean force for pure water, sodium ion solutions, and potassium ion solutions show marked differences in ion association behavior. This supports the idea that Hofmeister effects depend upon direct ion-macromolecule interactions as well as interactions with water molecules in the first solvation shell rather than bulk water structuring.