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

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Abstract
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. The ultrasonic velocity of the membrane increased remarkably above 30.degree. C due to the increase of the bulk modulus by about 15%. The ultrasonic velocity and bulk modulus decreased below 30.degree. C. Comparing the cholesterol-incorporated membrane with vesicles of bovine brain sphingomyelin and human erythrocyte membrane, the role of cholesterol in biological membranes is discussed in terms of the stability of the membrane as a barrier.