A new interpretation of eutectic behavior for distearoylphosphatidylcholine-cholesterol binary bilayer membrane

Area of Science:

• Biophysics
• Materials Science
• Physical Chemistry

Background:

• Biological membranes are complex structures whose properties are influenced by lipid and cholesterol composition.
• Understanding the phase behavior of lipid bilayers is crucial for deciphering membrane function and integrity.

Purpose of the Study:

• To investigate the thermotropic phase behavior of distearoylphosphatidylcholine (DSPC)-cholesterol binary membranes.
• To determine the influence of cholesterol composition (X(ch)) on membrane phase transitions and structures.

Main Methods:

• Fluorescence spectroscopy using 6-propionyl-2-(dimethylamino)naphthalene (Prodan) as a probe.
• Differential scanning calorimetry (DSC) to detect thermal events.

Main Results:

• Distinct fluorescence spectra and transition temperatures correlated with specific bilayer phases (L(beta)', P(beta)', L(alpha), L(o)).
• The pretransition phase disappeared around X(ch)=0.035.
• A temperature-X(ch) phase diagram revealed eutectic behavior and characteristic points at X(ch)=0.08 (congruent melting) and X(ch)=0.15 (peritectic-like).
• Cholesterol distribution within a hexagonal lattice model explained the observed phase behavior and eutectic points.
• The liquid ordered (L(o)) phase was prevalent at higher cholesterol concentrations (X(ch)>0.15).

Conclusions:

• The phase diagram of DSPC-cholesterol membranes exhibits behavior similar to binary systems with complete miscibility in the liquid phase and immiscibility in the solid phase.
• The observed phase separation is explained by distinct regular distributions of cholesterol molecules within the bilayer.
• Specific cholesterol compositions (0.035, 0.08, 0.15) correspond to defined arrangements of cholesterol and DSPC molecules.
• A single liquid ordered (L(o)) phase can exist at high cholesterol concentrations (X(ch)>0.50).