Low-density/high-density liquid phase transition for model globular proteins.

The effect of molecule size (excluded volume) and the range of interaction on the surface tension, phase diagram, and nucleation properties of a model globular protein is investigated using a combination of Monte Carlo simulations and finite temperature classical density functional theory calculations. We use a parametrized potential that can vary smoothly from the standard Lennard-Jones interaction characteristic of simple fluids to the ten Wolde-Frenkel model for the effective interaction of globular proteins in solution. We find that the large excluded volume characteristic of large macromolecules such as proteins is the dominant effect in determining the liquid-vapor surface tension and nucleation properties. The variation of the range of the potential is important only in the case of small excluded volumes such as for simple fluids. The DFT calculations are then used to study the homogeneous nucleation of the high-density phase from the low-density phase including the nucleation barriers, nucleation pathways, and rate. It is found that the nucleation barriers are typically only a few k(B)T and that the nucleation rates are substantially higher than would be predicted by classical nucleation theory.