Interactions of cholesterol with lipid bilayers: The preferred configuration and fluctuations

The free energy difference associated with the transfer of a single cholest erol molecule from the aqueous phase into a lipid bilayer depends on its fi nal location, namely on its insertion depth and orientation within the bila yer. We calculated desolvation and lipid bilayer perturbation contributions to the water-to-membrane transfer free energy, thus allowing us to determi ne the most favorable location of cholesterol in the membrane and the exten t of fluctuations around it. The electrostatic and nonpolar contributions t o the solvation free energy were calculated using continuum solvent models. Lipid layer perturbations, resulting from both conformational restrictions of the lipid chains in the vicinity of the (rigid) cholesterol backbone an d from cholesterol-induced elastic deformations, were calculated using a si mple director model and elasticity theory, respectively. As expected from t he amphipathic nature of cholesterol and in agreement with the available ex perimental data, our results show that at the energetically favorable state , cholesterol's hydrophobic core is buried within the hydrocarbon region of the bilayer. At this state, cholesterol spans approximately one leaflet of the membrane, with its OH group protruding into the polar (headgroup) regi on of the bilayer, thus avoiding an electrostatic desolvation penalty. We f ound that the transfer of cholesterol into a membrane is mainly driven by t he favorable nonpolar contributions to the solvation free energy, whereas o nly a small opposing contribution is caused by conformational restrictions of the lipid chains. Our calculations also predict a strong tendency of the lipid layer to elastically respond to (thermally excited) vertical fluctua tions of cholesterol so as to fully match the hydrophobic height of the sol ute. However, orientational fluctuations of cholesterol were found to be ac companied by both an elastic adjustment of the surrounding lipids and by a partial exposure of the hydrophobic cholesterol backbone to the polar (head group) environment. Our calculations of the molecular order parameter, whic h reflects the extent of orientational fluctuations of cholesterol in the m embrane, are in good agreement with available experimental data.