Thermodynamic Analysis of the Effect of Cholesterol on Dipalmitoylphosphatidylcholine Lipid Membranes

Cholesterol is an important component of eukaryotic cellular membranes. Despite extensive literature on the physiochemical effects of cholesterol on membranes, much remains unknown about the precise role of cholesterol and its molecular interactions in membranes. Regular thermal fluctuations of lipids normal to the plane of the membrane are biologically relevant for many processes, such as interactions with enzymes, elastic properties, and hydrophobic matching, while larger fluctuations are involved in vesicle budding and fusion, passive lipid flip-flop, and pore formation. Here we used molecular dynamics simulations to investigate the thermodynamic effect of the cholesterol concentration on dipalmitoylphosphatidylcholine (DPPC) bilayers. We calculated the potentials of mean force for DPPC partitioning in DPPC bilayers containing 20 and 40 mol % cholesterol. Increasing the cholesterol content increases the free energy barrier for transferring the headgroup of DPPC to the center of the bilayer and slows the rate of DPPC flip-flop by orders of magnitude. Cholesterol increases the order, thickness, and rigidity of the bilayers, which restricts bilayer deformations and prevents pore formation. While DPPC flip-flop is pore-mediated in a pure bilayer, we do not observe pores in the 20 and 40 mol % bilayers. Increasing the cholesterol concentration causes a decrease in the free energy to transfer DPPC from its equilibrium position into bulk water-indicating that DPPC prefers to be in cholesterol-free bilayers. We also observe a reduction in small fluctuations of DPPC normal to the bilayer as the cholesterol concentration is increased.