Enhanced fluctuations in small phospholipid bilayer vesicles containing cholesterol.

Ultrasonic and calorimetric studies of small homogeneously-sized DMPC unilamellar vesicles showed two thermal transitions at temperatures Tc1 and Tc2 (Tc2 greater than or equal to Tc1); Tc2 is close to the phase transition temperature, Tc, of large vesicles. The process at Tc2 is not a fusion of vesicles and is interpreted as characterizing an order-disorder transition essentially similar to that of large vesicles. The temperatures Tc1 and Tc2 become increasingly similar as the cholesterol content is increased, while the clusters at Tc2 (congruent to 85 lipid molecules in pure DMPC) increase in size up to approximately 180 lipid molecules at 12 mol% cholesterol. Incorporation of cholesterol thus brings about enhanced fluctuations in this model system of a membrane.