Thermal fluctuations of large quasi-spherical bimolecular phospholipid vesicles

The time correlation function of the fluctuations in shape of large ( ~ 10 03BCm) quasi-spherical hydrated phospholipid membrane vesicles consisting of one to several bimolecular layers is measured. These membranes are flaccid, so the vesicle area and volume remain constant and the only contribution to the energy of the fluctuating shape is from the excess curvature of a membrane element. A value for the curvature elastic modulus, Kc, is obtained from the mean-square amplitude of normal modes of the fluctuations using the equipartition theorem. An expression for the correlation time is found by solving the dynamics of membrane relaxation against the low Reynolds number viscous drag of the water. The restoring force of the membrane is calculated following the theory of Jenkins [1] which treats the membrane as a two dimensional incompressible fluid. The correlation time is a function of Kc and d0, the two dimensional pressure in the membrane plane. The measurements yield Kc ~ 1-2 10-12 ergs, in agreement with other experiments on artificial vesicles [2, 3], and values for d0 in agreement with the theoretical range of predicted values [1]. A reason for the lower published value of Kc deduced from experiments [4] on the red blood cell is suggested.