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Model plasma membrane indicates the inner leaflet is poised to initiate compositional heterogeneities

We investigate a model of an asymmetric bilayer consisting of sphigomyelin, phosphatidylcholine, and cholesterol in the outer leaflet, and phosphatidyl\-ethanolamine, (PE), phosphatidylserine, and cholesterol in the inner leaflet. Composition fluctuations are coupled to membrane fluctuations via the bending energy which depends upon the local spontaneous curvature dominated by the PE in the inner leaflet. This brings about a microemulsion in that leaflet with a characteristic wavelength of 37 nm as shown by the PE-PE structure function. Thus the inner leaflet will respond to an external perturbation most strongly at this length. However the correlation length is approximately 1 nm so that composition variations are not seen in the inner leaflet itself. Depending upon the strength of the coupling between leaflets, the outer leaflet is either a normal fluid or is also a microemulsion. In this model then, the lipids of the plasma membrane would not evince inhomogeneities, but would be primed to create them from the inner leaflet in response to non-lipid perturbations.