A simple model of directional interactions for proteins

Area of Science:

• Soft Matter Physics
• Computational Chemistry
• Biophysics

Background:

• Understanding protein solution behavior is crucial for biological processes and materials science.
• Protein interactions are complex, involving both short-range repulsions and longer-range attractions.
• Predicting the phase behavior and solid structures of proteins remains a significant challenge.

Purpose of the Study:

• To investigate the phase diagram and crystal structures of a simplified two-patch model for globular protein solutions.
• To elucidate the conditions under which fluid-fluid phase separation and various crystalline states occur.
• To determine the stability of different crystal lattices (bcc-o, fcc-o, fcc-d) as a function of temperature and pressure.

Main Methods:

• Utilizing a two-patch model with hard sphere repulsion and square-well isotropic/anisotropic attractions.
• Calculating the phase diagram to identify fluid-fluid phase separation regions.
• Determining stable crystal structures through energy minimization and stability analysis at various thermodynamic conditions.

Main Results:

• A metastable fluid-fluid phase separation curve was identified in the phase diagram.
• The orientationally ordered body-centered cubic (bcc-o) lattice is the lowest energy crystal structure, stable at low temperatures and moderate pressures.
• The orientationally ordered face-centered cubic (fcc-o) lattice becomes stable at high pressures and low temperatures, while an orientationally disordered fcc (fcc-d) lattice is stable at high temperatures.

Conclusions:

• The two-patch model successfully predicts complex phase behavior, including fluid-fluid separation and distinct crystalline phases.
• Thermodynamic conditions (temperature and pressure) dictate the stability of different protein crystal structures.
• Entropy plays a critical role in the transition from ordered to disordered crystalline states at elevated temperatures.