Interpretation of nonelectrolyte partition coefficients between dimyristoyl lecithin and water
Nonelectrolyte partition coefficients (K's) and free energies of solution (ΔF l 's) in dimyristoyl lecithin liposomes and in bulk nonpolar solvents were compared. Individual substituent groups tend to have consistent effects onK, permitting the extraction of incremental free energies (δΔF), enthalpies (δΔH), and entropies (δΔS) of partition and of solution. Values of the selectivity constants and of δΔF l for the −CH2−and −OH groups in lecithin suggest that partitioned solutes are mainly located in a region slightly less hydrophobic than octanol and similar to C5H11OH in its solvent properties. Lecithin discriminates against branched solutes more than does a bulk solvent with the sames value. Below the endothermic phase-transition temperature (i.e., when the hydrocarbon tails “freeze”), ΔS and ΔH of partition increase 10-fold,K jumps down slightly, ΔS and ΔH of solution reverse in sign from negative to positive, and the Barclay-Butler constants become more positive. Partition in lecithin and in erythrocytes is similar, except for the absence of surface charge effects in lecithin. Resistance to nonelectrolyte permeation is inhomogeneously distributed through the bilayer, and the region of maximum partition does not provide the rate-limiting barrier. An appendix derives a simple general expression for the nonelectrolyte permeability of a membrane that may be asymmetrical, may have position-dependent partition coefficients and diffusion coefficients, and may have significant interfacial resistances.