Interactions of cholesterol with lipid membranes and cyclodextrin characterized by calorimetry

Interactions of cholesterol ( cho) with different lipids are commonly believed to play a key role in the formation of functional domains in membranes. We introduce a novel approach to characterize cho- lipid interactions by isothermal titration calorimetry. Cho is solubilized in the aqueous phase by reversible complexation with methyl- beta- cyclodextrin ( cyd). Uptake of cho into the membrane is measured upon a series of injections of lipid vesicles into a cyd/ cho solution. As an independent assay, cho release from membranes is measured upon titrating lipid/ cho mixed vesicles into a cyd solution. The most consistent fit to the data is obtained with a mole fraction ( rather than mole ratio) partition coefficient and considering a cho/ cyd stoichiometry of 1: 2. The results are discussed in terms of contributions from 1), the transfer of cho from cyd into a hypothetical, ideally mixed membrane and 2), from nonideal interactions with POPC. The latter are exothermic but opposed by a strong loss in entropy, in agreement with cho- induced acyl chain ordering and membrane condensation. They are accompanied by a positive heat capacity change which cannot be interpreted in terms of the hydrophobic effect, suggesting that additive- induced chain ordering itself increases the heat capacity. The new assays have a great potential for a better understanding of sterol- lipid interactions and yield suggestions how to optimize cho extraction from membranes.