BENDING ELASTICITY AND THERMAL EXCITATIONS OF LIPID BILAYER VESICLES - MODULATION BY SOLUTES

We present high-precision measurements of the bending elastic moduli of bilayers of a variety of different lipids and of modifications of the flexural rigidity by solutes. The measurements are based on the Fourier analysis of thermally excited membrane undulations (vesicle shape fluctuations) using a recently developed dynamic image processing method. Measurements of the bending modulus as a function of the undulation wave vector provide information on the limitation of the excitations by the constraint of finite membrane area (surface tension effects) and by transient lateral tensions arising in each monolayer by restricted diffusion at high wave vectors. Measurements of the autocorrelation function of the undulation amplitudes provide a further test of the theoretical models. Studies of the effect of solutes show that cholesterol increases the bending modulus of dimyristoylphosphatidylcholine from Kc = 1. 1 × 10-12 erg to 4.2 × 10-12 erg (at 30 mol%). Incorporation of a short bipolar lipid reduces Kc to the order of kT. Finally we present a variety of shape changes of vesicle and provide evidence for the stabilization of metastable non-equilibrium shapes by lateral phase separation. © 1990.