Enhanced fluctuations in small phospholipid bilayer vesicles containing cholesterol

Area of Science:

• Biophysics
• Materials Science
• Physical Chemistry

Background:

• Small unilamellar vesicles (SUVs) composed of dimyristoylphosphatidylcholine (DMPC) are model systems for cell membranes.
• Cholesterol is a key component in biological membranes, influencing their physical properties.
• Understanding lipid-bilayer phase transitions and molecular organization is crucial for membrane function.

Purpose of the Study:

• To investigate the effects of cholesterol on the thermal transitions and molecular behavior of DMPC SUVs.
• To characterize the nature of thermal transitions in DMPC SUVs with varying cholesterol content.
• To determine how cholesterol influences lipid clustering and membrane dynamics.

Main Methods:

• Ultrasonic and calorimetric studies were performed on homogeneously-sized DMPC SUVs.
• Vesicles with varying mol% of cholesterol were analyzed.
• Thermal transitions and lipid cluster sizes were measured.

Main Results:

• Two distinct thermal transitions, Tc1 and Tc2, were observed in DMPC SUVs, with Tc2 near the phase transition temperature of large vesicles.
• The transition at Tc2 was identified as an order-disorder transition, not vesicle fusion.
• Increasing cholesterol content reduced the difference between Tc1 and Tc2 and increased lipid cluster size at Tc2 (from ~85 to ~180 molecules).

Conclusions:

• Cholesterol incorporation into DMPC SUVs modifies their thermal behavior and phase transition characteristics.
• Cholesterol leads to an increase in the size of lipid clusters, indicating enhanced molecular organization.
• The study demonstrates that cholesterol incorporation enhances membrane fluctuations in this model system.