Solvation Behavior of Elastin-like Polypeptides in Divalent Metal Salt Solutions

The effects of CaCl₂ and MgCl₂ on the cloud point temperature of two different elastin-like polypeptides (ELPs) were studied using a combination of cloud point measurements, molecular dynamics simulations, and infrared spectroscopy. Changes in the cloud point for the ELPs in aqueous divalent metal cation solutions were primarily governed by two competing interactions: the cation-amide oxygen electrostatic interaction and the hydration of the cation. In particular, Ca²⁺ cations can more readily shed their hydration shells and directly contact two amide oxygens by the formation of ion bridges. By contrast, Mg²⁺ cations were more strongly hydrated and preferred to partition toward the amide oxygens along with their hydration shells. In fact, although hydrophilic ELP V₅A₂G₃ was salted-out at low concentrations of MgCl₂, it was salted-in at higher salt concentrations. By contrast, CaCl₂ salted the ELP sharply out of solution at higher salt concentrations because of the bridging effect.