The mechanism of ion conduction by valinomycin: analysis of charge pulse responses

Even though valinomycin has been employed and studied extensively for over 30 years, the attempts to explain its mechanism have not been entirely successful. The basic carrier model uses four rate constants that describe association of an ion and carrier, transfer of the complex across the membrane, dissociation of the complex, and transfer of the free carrier back across the membrane. If the basic model is correct all of these constants are independent of ion concentration. In previous work with rubidium the rate constants for transfer of free carrier, transfer of complexes, and dissociation were independent of the concentration, but the rate constant for association varied markedly. No satisfactory explanation for these observations was proposed. In this study current relaxations after charge pulses have been analyzed using digital data acquisition, a Bayesian algorithm, and inspection of linear plots of residuals. In agreement with previous results the relaxations for sufficiently high rubidium or potassium concentrations contain three exponential components, but the rate constants for association and dissociation decrease to similar extents as ion concentration increases. A simple extension of the carrier model to allow a more realistic description of association and dissociation is in good agreement with the rate constants fitted in the present study but not those for low ion concentrations found in previous work. At high ion concentrations the rate-limiting step in association appears to be a change in the conformation of the free carrier preceding the bimolecular association reaction. Transfer of neutral, free valinomycin between the surfaces is slower than the transfer of the charged ion-valinomycin complexes. Transfer of the complex may be hastened by deformation of the membrane, or transfer of the free carrier may be slowed by a need for conformation changes