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The binding of alkali cations by the ionophorous antibiotics valinomycin, nigericin, alamethicin, and the macrotetralide actins has been shown to occur, in aqueous media, by the use of the fluorescent probes 1-anilino-8-naphthalene sulfonate and 2-p-toluidinyl-6-naphthalene sulfonate. The interaction of the ionophore-cation complexes with the fluorescent dyes produced enhanced fluorescence emission, increased lifetime and polarization, and a significant blue-shift of the emission maxima of the fluorescence spectrum. At constant antibiotic and fluorophore concentrations in water, the intensity of the fluorescence emission was found to be a function of the cation concentration. This permitted relative cation affinities to be determined for alamethicin (Na(+) congruent with K(+)), valinomycin (Rb(+) > K(+) > Cs(+)), nigericin (K(+) > Rb(+) > Na(+) > Cs(+)) and trinactin (NH(4) (+) > K(+) > Rb(+) > Cs(+)).
Feinstein et al. (Wed,) studied this question.