Phospholipid-cholesterol bilayers under osmotic stress.

Area of Science:

• Lipid bilayer thermodynamics
• Biophysical chemistry
• Membrane biophysics

Background:

• Understanding lipid-cholesterol interactions is crucial for cell membrane function.
• Cholesterol's role in modulating lipid phase behavior under varying conditions remains an active research area.
• Osmotic pressure significantly influences lipid phase diagrams.

Purpose of the Study:

• To investigate the isothermal phase behavior of dimyristoyl phosphatidyl choline-cholesterol mixtures at 27°C under varying osmotic pressures and cholesterol contents.
• To construct phase diagrams based on thermodynamic data.
• To elucidate the influence of cholesterol on lipid phase stability and transitions.

Main Methods:

• Isothermal sorption microcalorimetry for simultaneous measurement of enthalpy and osmotic pressure of water.
• 2H-nuclear magnetic resonance spectroscopy.
• Construction of Pi(osm) - X(chol) and ternary composition phase diagrams.

Main Results:

• Similarities observed between osmotic pressure-cholesterol (Pi(osm) - X(chol)) and temperature-cholesterol (T - X(chol)) phase diagrams.
• A stable liquid ordered (L(alpha)(o)) phase exists across all water contents at high cholesterol concentrations, indicating stability against increasing osmotic pressure.
• Cholesterol exhibits minimal melting point depression at low concentrations.
• The P(beta) phase dissolves cholesterol more readily than the L(beta) phase, and cholesterol addition induces the P(beta) phase.

Conclusions:

• The liquid ordered (L(alpha)(o)) phase demonstrates remarkable stability under increasing osmotic pressure.
• Cholesterol's interaction with lipid phases is dependent on the phase type, with P(beta) being more receptive.
• The study provides thermodynamic insights into lipid-cholesterol mixtures under osmotic stress.