Stabilizing effect of cholesterol on phosphatidylcholine vesicles observed by ultrasonic velocity measurement

The temperature dependence of the ultrasonic velocity was measured in sonicated vesicles of dipalmitoylphosphatidylcholine by varying the content of cholesterol. When cholesterol is incorporated, an anomalous dip of the ultrasonic velocity gradually smeared out. At the same time, the ultrasonic velocity of the membrane increased remarkably above 30 degrees C due to the increase of the bulk modulus by about 15%. On the other hand, the ultrasonic velocity and the bulk modulus decreased below 30 degrees C. Comparing the cholesterol-incorporated membrane with vesicles of bovine brain sphingomyelin and human erythrocyte membrane, we discuss the role of cholesterol in biological membranes in terms of the stability of the membrane as a barrier.