Partitioning of nonpolar solutes into bilayers and amorphous n-alkanes

We have measured the partition coefficients for hexane between an aqueous solvent and phospholipid bilayer membranes as a function of the surface density of the bilayer chains. The surface density was varied by temperature, phospholipid chain length, and the incorporation of cholesterol and was monitored by ²H NMR. We observe that increasing surface density leads to expulsion of the solute: hexane partitioning decreases by a factor of 9 as the surface density of the bilayer chains increases from 50% to 90% of its maximum value, a range readily accessible in biomembranes under physiological conditions. Benzene as solute shows similar behavior; thus, the dependence of partitioning on surface density appears to be a general property of the organization of the bilayer chains. In order to determine the relative contributions to partitioning of (i) chain ordering and (ii) contact interactions, we have also performed reference-state experiments for benzene transfer between water and amorphous n-alkanes of different chain lengths. The widely accepted value of 25 cal/mol per Ų of surface area for the free energy of oil/water transfer per methylene group is appropriate only when solute and solvent molecules are of the same size. We find that Flory-Huggins theory satisfactorily corrects for the molecular size differences of solutes and solvents. Comparisons of the bilayer/water and oil/water temperature dependencies of benzene partitioning show that there is an additional entropic expulsion of solute from the bilayer, consistent with recent statistical thermodynamic theory suggesting the importance of the chain ordering in the bilayer.