Role of solvent in protein phase behavior: Influence of temperature dependent potential.

Among many factors that affect protein phase separation, solvent plays a pivotal role in the possible structuring of the solvent molecules around the protein. The effect of solvent structuring is influenced strongly by temperature because of the relative stability of hydrogen bonding at low temperatures. As a result, quantitative as well as qualitative changes in protein phase separation may be expected with change in temperature. Here, we use a temperature dependent pair potential to examine the effect of water in the phase separation of protein solutions. Using Gibbs ensemble Monte Carlo simulations, we observe both a lower critical solution temperature and an upper critical solution temperature, in good agreement with the experimental observations for a number of proteins and phenomenological, statistical thermodynamic arguments. It is found that the effect of solvent is significant at low temperatures as a result of the highly structured shell of water molecules around the protein molecules. Radial distribution functions also indicate that a thick shell of structured water exists around the protein molecules due to the formation of strong hydrogen bonds when temperature is low. The findings of this study suggest that a simple model with a reasonable physical basis can capture the general phase behavior of some proteins or biopolymers.