MODIFICATION OF CATION SELECTIVITY OF VALINOMYCIN BY COMPLEXING IT WITH AN ANION - DELTA-PH DECAY STUDIES

The relaxation times associated with the Delta pH decay by the combine d action of valinomycin and substituted phenols have been measured wit h potassium ions (tau(K)) and sodium ions (tau(Na)) in soyabean phosph olipid vesicle suspensions (Delta pH=pH difference across vesicular me mbrane). In these experiments the compensating charge flux of alkali m etal ions limits the rate of Delta pH decay. The large deviation of th e ratio tau(Na)/tau(K) from the concentration ratio [valinomycin-K+]/[ valinomycin-Na+] reflects the change in the cation selectivity of vali nomycin on complexing with phenolates. The observed reduction in the c ation selectivity can be understood by noting that the binding sites o f Na+ and K+ on valinomycin are different and that the anion binds at a site closer to that of Na+ Our data also suggest that the steric fac tors associated with the anions affect the magnitude of electrostatic interaction responsible for the increased stability of the sodium comp lex.