A thermodynamic study of the partition of [formula omitted] into phosphatidylcholine and phosphatidylcholinecholesterol bilayers

Abstract

The partition coefficient of n- hexane from water to bilayers of dioleoyl phosphatidylcholine and egg lecithin were measured as a function of temperature. Over a 50° C temperature range, the partition coefficient decreased linearly with temperature. The addition of cholesterol (1 : 1 mol ratio) to those bilayers reduced the partition coefficient by, at most, a factor of 2.5 at 25° C. The thermodynamic transfer parameters so obtained were compared to those of bulk hydrocarbon liquids and sodium dodecyl sulphate micelles. The results indicate that a bulk hydrocarbon liquid is not a good model for the interior of a bilayer whereas sodium dodecyl sulphate has approximately the same thermodynamic transfer parameters as egg lecithin. A comparison of the thermodynamic transfer parameters of n- hexane from a hydrocarbon liquid to either egg lecithin or dioleoyl phosphatidylcholine at 25° C shows that energetically and entropically the n- hexane prefers the bulk hydrocarbon phase whereas enthalpically it prefers the bilayer. These data imply that the n- hexane is aligned, on the average, parallel to the acyl chains. The partition coefficient, K, of n- hexane from water into various solvents follows the sequence K HC > K EL ≈ K SDS ⩾ K DP > K Oct , where the subscript HC represents hydrocarbon; EL, egg lecithin; SDS, sodium dodecyl sulfate; DP, dioleoyl phosphatidlcholine; DPC, dioleoyl phosphatidylcholine cholesterol; and Oct, n- octanol .