Structure and Correlation Functions of an Asymmetric Model Membrane of Five Components Display No Rafts but Rather the Propensity to Create them

Abstract

We consider a model of an asymmetric lipid bilayer, one that consists of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Co) in the outer leaf and phosphatidyletanolamine (PE), phosphatidylserine (PS), and cholesterol (Ci) in the inner leaf. As cholesterol can interchange between leaves, its chemical potential is the same in each leaf.The spontaneous curvature of the PE depends upon the cholesterol concentration and decreases with sufficient cholesterol concentration. Height fluctuations couple to the local concentration of PE which has the largest magnitude of spontaneous curvature. At physiological temperatures, the membrane is in a disordered phase with somewhat more than half of the total cholesterol in the inner leaf. We calculate all independent structure functions. Many, but not all of them, display a peak at a non-zero value of the wavenumber, a characteristic of a microemulsion. These include the intraleaf PE-PE, PE-PS, Ci-Ci, and SM-SM, PC-PC, Co-Co, and SM-Co structure functions. The interleaf Ci-Co and SM-Ci structure functions have the same behavior. The peak occurs at a wavenumber corresponding to a wavelength on the order of 80 nm indicating that the system is most susceptible to disturbances of this size. However the correlation functions corresponding to these structure functions, modeled as exponentially damped oscillations, display a correlation length which is only a few nanometers, and shorter than the wavelength. Taken together, these results indicate that the bilayer evidences no “rafts”, but is primed to create them in response to an external perturbation.