Net Interactions That Push Cholesterol Away from Unsaturated Phospholipids Are Driven by Enthalpy

The exchangeable unsaturated phospholipids c₁-Phos and c₃-Phos, which bear one and three permanent kinks, respectively, in their acyl chains, are mimics of the biologically important, low-melting phosphatidylcholines (PCs) having one and three cis double bonds in their sn-2 chains (i.e., 16:0,18:1 PC and 16:0,18:3 PC, respectively). The net interaction of an exchangeable form of cholesterol (Chol) with c₁-Phos and with c₃-Phos has been investigated using the nearest-neighbor recognition method. These interactions were found to be unfavorable in both cases having a positive free energy, ω, for replacing like by unlike nearest-neighbor contacts. The values for this free energy (or interaction parameter) were 165 cal/mol between Chol and c₁-Phos and 395 cal/mol between Chol and c₃-Phos. We now report the temperature dependence of these interactions in liquid-disordered bilayers. Their experimentally determined temperature dependencies, in combination with Monte Carlo simulations, revealed that the interaction parameter ω is dominated in both cases by enthalpy. These findings have important implications for the distribution of lipids in natural membranes and for the formation of lipid rafts.