Microcalorimetry has become a method of choice for sensitive characterization of biomolecular interactions. In this study, isothermal titration calorimetry (ITC) was used to measure the partitioning of 1-octanol into lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a semi-unsaturated lipid, and cholesterol, a steroid, as a function of cholesterol molar concentration. The ITC instrument measures the heat evolved or absorbed upon titration of a liposome dispersion, at concentrations ranging from 0 to 40% cholesterol, into a suspension of 1-octanol in water. A model function was fit to the data in order to determine the partition coefficient of octanol into POPC bilayers and the enthalpy of interaction. I found that the partition coefficient increases and the heat of interaction becomes less negative with increasing cholesterol content, in contrast to results found by other groups for partitioning of alcohols into lipid-cholesterol bilayers containing saturated lipids. The heat of dilution of vesicles was also measured.
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