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 ent irely successful. The basic carrier model uses four rate constants tha t 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 a ll of these constants are independent of ion concentration, In previou s work with rubidium the rate constants for transfer of free carrier, transfer of complexes, and dissociation were independent of the concen tration, but the rate constant for association varied markedly. No sat isfactory explanation for these observations was proposed. In this stu dy current relaxations after charge pulses have been analyzed using di gital data acquisition, a Bayesian algorithm, and inspection of linear plots of residuals. In agreement with previous results the relaxation s for sufficiently high rubidium or potassium concentrations contain t hree exponential components, but the rate constants for association an d dissociation decrease to similar extents as ion concentration increa ses. 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 co nformation of the free carrier preceding the bimolecular association r eaction. Transfer of neutral, free valinomycin between the surfaces is slower than the transfer of the charged ion-valinomycin complexes, Tr ansfer of the complex may be hastened by deformation of the membrane, or transfer of the free carrier may be slowed by a need for conformati on changes.