Independence of substituent contributions to the transport of small-molecule permeants in lipid bilayer

Abstract

Purpose: To explore the independence of functional group contributions to permeability of nonelectrolytes across egg lecithin bilayers. Methods. The transport rates were measured of a series of α-substituted p-methylhippuric acids (-H, -Cl, -OCH3, -CN, -OH, -COOH, and -CONH2) across egg lecithin lipid bilayers, in the form of large unilamellar vesicles (LUVs) at 25EC. Intrinsic permeability coefficients (PHA) were calculated from apparent permeability coefficients (Papp) measured as a function of pH. Group contributions to the free energy of transfer from water into the barrier domain, Δ(ΔGE)P,X, were calculated for the substituents and compared to the contributions of these groups when attached to p-toluic acid measured earlier. The Δ(ΔGE)P,X values from permeability data were also correlated with Δ(ΔGE)PC,X values of partitioning from water into organic solvents to determine the physicochemical selectivity of the barrier domain. Results. Papp values in LUVs were found to vary approximately linearly with the fraction of neutral permeant over a pH range of 5.5 to 10.5, suggesting that the transport of the ionized species is negligible over this pH range. The Δ(ΔGE)P,X values from the 2 series of compounds appear to be the same, indicating that the functional group contributions are independent. 1,9-Decadiene was found to be the most similar to the chemical environment of the barrier domain. Conclusions. Functional group contributions to transport across egg lecithin bilayers appear to be independent of the compound to which they are attached, even though the thickness of the barrier domain in lipid bilayers is approximately the same as the extended length of the permeant.