Thermal fluctuations of large cylindrical phospholipid vesicles.

The time correlation function of the shape fluctuations of large (greater than 10 micron), cylindrical, hydrated, phospholipid-membrane vesicles consisting of one bimolecular layer was measured. The restoring force of the membrane was due to the excess curvature of a membrane element. A value for the curvature elastic modulus, Kc, was obtained from the mean-square amplitude of the normal modes of the fluctuations using the equipartition theorem. An expression for the correlation time was found by solving the dynamics of the membrane's relaxation against the low Reynolds number viscous drag of the surrounding fluid. The amplitudes and correlation times of the fundamental bending mode of the cylindrical vesicles both yield Kc = 1-2 X 10(-12) ergs.