Apparent activation volumes of hydrophobic ions and carriers in planar lipid bilayers

A gas-free high-pressure cell has been developed to measure planar bilayer conductances induced by hydrophobic ions and ionophores as a function of hydrostatic pressure. Plots of log conductance versus pressure for valinomycin and nonactin-mediated potassium transport in egg phosphatidyl cholinedecane membranes are essentially linear over a pressure range of 1 to 818 atm. Calculated activation volumes give similar results for both nonactin and valinomycin yielding values of + 48 and + 42 cc/mole, respectively. The valinomycin activation volume agrees reasonably well with the results obtained by Johnson and Miller (Biochim. Biophys. Acta 375:286-291, 1975) for K+-valinomycin transport in liposomes. In contrast to the activation volumes for nonactin and valinomycin, relaxation measurements of tetraphenyl boron (TPB) and dipicrylamine (DPA) give very small values of less than 5 cc/mole for the translocation rate constant, ki. Similarly, steady-state conductance measurements on tetraphenyl arsonium (TPA) and carbonylcyanide m-chlorophenylhydrazone (CCCP), give small values of 6 and 7 cc/mole, respectively. These low figures do not support transport theories based on the formation of bilayer holes or kinks (H. Träuble, J. Membrane Biol. 4:193-208, 1971). The low values for TPB and TPA are especially interesting because their cross-sectional areas are not much different than those of valinomycin and nonactin. Pressure-induced changes in membrane dielectric constant and thickness which lower the bilayer electrostatic barrier could explain the low values for the hydrophobic ions. Additionally, larger activation volumes might be expected for carriers such as nonactin and valinomycin that undergo significant rearrangement and change in hydration during surface complexation of cations.