Transport of Biological Lipophiles: Effect of Lipophile Structure

A systematic study of the structural determinants involved in the spontaneous transfer of molecules between single bilayer vesicles of phosphatidylcholine is reported. All of the molecules studied contain a pyrenyl moiety whose excimer fluorescence provides a direct measure of the changes in its microscopic concentration. These compounds include pyrenyl alkanes, alcohols, carboxylic acids, and their methyl esters. In each group, transfer between vesicles occurs via the intervening aqueous phase. The rate of transfer is a function of both the hydrophobicity (chain length) and the hydrophilicity (polar or nonpolar) of the transferred species. The rates of transfer can be expressed in terms of a free energy of activation, ΔG^‡, which is calculated from absolute rate theory. A good correlation exists between ΔG^‡ and ΔG_t, the free energy of molecular transfer from a hydrophobic environment to the aqueous phase. The rate of transfer increased both with decreasing chain length in a given homologous series and with the polarity of the substituents if the number of methylene units is constant. The incremental ΔG^‡ for the polar compounds was ≃740 cal/methylene unit, whereas the corresponding value for the alkyl pyrenes is ∼900 cal/methylene unit. These values are similar to the reported ΔG_t per methylene unit calculated from equilibrium measurements. The ΔG^‡ per methylene unit of the polar compounds reflected changes in the ΔH^‡ since ΔS^‡ was independent of chain length. By contrast, the alkyl pyrenes exhibited very large changes in ΔG^‡ with increasing chain length (≃2 kcal/methylene unit) that are, in part, compensated by changes in ΔS^‡. As a consequence, only a small difference in the contribution of each methylene to ΔG^‡ of transfer of alkanes and amphiphiles is predicted.