Ion specific effects on phase transitions in protein solutions

TL;DR

This study uses simulation data to map the phase diagrams of lysozyme solutions with different salts, revealing ion-specific effects on phase stability and critical points due to hydrophobic and repulsive interactions.

Contribution

It provides the first detailed phase diagrams of lysozyme solutions considering ion-specific interactions derived from Monte Carlo simulations.

Findings

Two phase diagrams have stable fluid-fluid critical points.

One phase diagram exhibits a metastable critical point.

Ion-specific interactions influence the effective range of protein attraction.

Abstract

A recent Monte Carlo simulation determined the potential of mean force between two lysozyme molecules in various aqueous solutions [M. Lund et al. Phys. Rev. Lett. 100, 258105 (2008)]. The study involved a combination of explicit solvent and continuum model simulations and showed that there are significant ion-specific protein-protein interactions due to hydrophobic patches on the protein surfaces. In this paper we use the results of their study to determine the phase diagram for lysozyme for aqueous solutions of NaCl and NaI. Two of the three phase diagrams have a stable fluid-fluid critical point, while the third has a slightly metastable critical point. This results from a secondary extremum in the potential associated with a repulsive interaction. This repulsive interaction reduces the effective range of the attractive interaction and produces a metastable critical point. We compare the results of one of these phase diagrams with that for a model that includes ion-dispersion forces, but does not contain solvent structural effects.