A microscopic model for lipid/protein bilayers with critical mixing

Area of Science:

• Statistical mechanics
• Biophysics
• Materials science

Background:

• Phospholipid bilayers exhibit complex phase transitions, crucial for membrane function.
• Transmembrane proteins can significantly alter bilayer properties and phase behavior.
• Existing models often simplify protein-lipid interactions and thermal fluctuations.

Purpose of the Study:

• To develop a statistical mechanical lattice model for phospholipid bilayers containing small transmembrane proteins or polypeptides.
• To investigate the influence of proteins on the gel-fluid phase transition of lipid bilayers.
• To accurately predict the phase diagram and thermodynamic properties.

Main Methods:

• Utilized an extended Pink-Green-Chapman model as the basis for pure lipid bilayers.
• Incorporated hydrophobic matching to model protein-lipid interactions.
• Employed computer simulations to derive the phase diagram, accounting for thermal fluctuations and free energy.

Main Results:

• Predicted a closed loop of gel-fluid coexistence with a lower critical mixing point.
• Generated specific-heat traces across the phase diagram.
• Demonstrated agreement between theoretical predictions and experimental data for mixed bilayers.

Conclusions:

• The statistical mechanical model accurately describes the phase diagram of phospholipid bilayers with transmembrane proteins.
• Hydrophobic matching is a key factor in protein-lipid interactions influencing phase behavior.
• The model provides a framework for interpreting experimental results on mixed lipid-protein systems.