Effects of Chain Structure on Surface Pressure−Area Behavior and Membrane Elasticity of Lipid Monolayers at the Air−Water Interface: Application of an Equation of State

Abstract

A recently developed equation of state (Feng, S.-S.; MacDonald, R. C. Biophys. J. 1995, 69, 460) for lipid monolayers at the air/water or oil/water interface was employed to fit experimental π−A curves of various lipids with different chain number, chain length, and chain unsaturation. In this kind of equation of state the distributional entropy of the lipid headgroups within the membrane plane and the conformational energy of the lipid chains of various unsaturation can be exactly calculated. The three parameters in the equation of state, namely, am/a0, ratio of the minimal molecular surface area to the cross-sectional area of the two chains of a single lipid molecule, γb, the ratio of the effective to the actual projected surface area increment, and π3, the residual part in the total surface pressure, were determined and used to reproduce the experimental data. Such an equation of state can accurately describe the properties of most lipid monolayers in their liquid-expanded state. To overcome the unreli...