Understanding the phase behavior of a protobiomembrane.

Area of Science:

• Physical Chemistry
• Biophysics
• Materials Science

Background:

• Lipid bilayers exhibit complex thermotropic behavior, crucial for biological membranes.
• Recent experiments show a significant decrease in the tilt modulus of DMPC lipid bilayers as temperature approaches the main transition (T_M).
• Existing theories have not fully explained this observed tilt modulus behavior.

Purpose of the Study:

• To develop a phenomenological theory explaining the dramatic decrease in tilt modulus in DMPC lipid bilayers.
• To integrate this theory with existing models of chain melting and lipid bilayer transitions.
• To extend the theory to encompass the experimentally observed ripple phase.

Main Methods:

• Introduction of a free energy functional for molecular tilt, coupled to the area per molecule.
• Combination with a chain melting free energy functional where area is the primary order parameter.
• Comparison of theoretical predictions with recent experimental data on DMPC lipid bilayers, including ripple phase structures.

Main Results:

• The proposed free energy functional successfully explains the experimentally observed decrease in tilt modulus.
• The model achieves satisfactory agreement with experimental data using experimentally determined parameter values.
• The theory predicts a direct transition into the gel phase, with extensions accounting for the ripple phase.

Conclusions:

• A unified theoretical framework can explain the thermotropic behavior of lipid bilayers, including tilt modulus changes and the ripple phase.
• Coupling of molecular tilt to area per molecule is key to understanding the observed phenomena.
• The developed theory provides a valuable tool for interpreting experimental findings in lipid bilayer research.