Barriers to Translocation of Organic Ions in Phospholipid Membranes

Abstract

The hypothesis that a dominant barrier to the translocation of organic ions (such as carbonyl cyanide m-chlorophenylhydrazone (CCCP)) across phospholipid bilayer membranes is located between the “polar region” and “nonpolar region” of the membrane layer has been confirmed by the following observation: The rate of decay of the pH difference across soybean phospholipid vesicular membrane (ΔpH) increased substantially when the above substep was specifically catalyzed. In our strategy, the formation of the electroneutral ternary complex valinomycin−M+−CCCP- (M+ = K+, Cs+) affects this substep. Also, CCCP- ion translocation is facilitated by having an alternate path for the compensating charge flux of alkali metal ions through gramicidin channels. The observed enhancement behaviors on changing the lipid composition from soybean phospholipid (SBPL) to a mixture of PC + 6% PA, weak acid from CCCP to carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP), and the metal ion from K+ to Cs+ are consistent with the predictions of the suggested mechanisms. The increase in the ΔpH decay rate on increasing the ionic strength confirms the Coulombic nature of a dominant interaction between the CCCP- ion and the lipid contributing to the kinetic barrier. Even though the gross features of the ion translocation barrier profile inferred by us are similar to that given in the literature, our results suggest the need to consider ion translocation across the phospholipid membrane to involve at least three substeps. This requires a refinement of the barrier profile given in the literature. In our experiments, the ΔpH was created by temperature jump, and observations on the ΔpH decay were made by monitoring the fluorescence from the pyranine entrapped inside the vesicles.