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 combined action of valinomycin and substituted phenols have been measured with potassium ions (tau K) and sodium ions (tau Na) in soyabean phospholipid vesicle suspensions (delta pH = pH difference across vesicular membrane). In these experiments the compensating charge flux of alkali metal ions limits the rate of delta pH decay. The large deviation of the ratio tau Na/tau K from the concentration ratio [valinomycin-K+]/[valinomycin-Na+] reflects the change in the cation selectivity of valinomycin on complexing with phenolates. The observed reduction in the cation selectivity can be understood by noting that the binding sites of 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 factors associated with the anions affect the magnitude of electrostatic interaction responsible for the increased stability of the sodium complex.
Prabhananda
B S
BS
Chemical Physics Group, Tata Institute of Fundamental Research, Bombay, India.
Kombrabail
M H
MH
D002414:Cations, Monovalent; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D007476:Ionophores / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism*; D009550:Nigericin / Q000378:metabolism; D010636:Phenols; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D013025:Glycine max; D014634:Valinomycin / Q000378:metabolism*
D002414:Cations, Monovalent; D007476:Ionophores; D008563:Membrane Lipids; D010636:Phenols; D014634:Valinomycin; D012964:Sodium; D009550:Nigericin; D011188:Potassium