Cation binding by valinomycin and trinactin at the air-water interface: Cooperativity in cation binding by valinomycin

A previous communication reported the uptake of monovalent cations by a valinomycin monolayer at the air-water interface (Colacicco, G., Gordon, E. E. and Berchenko, G. (1968) Biophys. J. 8,22a). A similar study has been done with trinactin. As in the case of valinomycin, an elevated surface potential is obtained when the cation-ionophore complex is formed. A surface potential of 0.82 V was obtained for the trinactin-cation complex, as compared with 0.54 V for uncomplexed trinactin. The observed cation selectivity NH₄⁺ > K⁺ > Rb⁺ > Cs⁺, Na⁺ and Li⁺ is in agreement with partition and bilayer conductance experiments.A minimum packing area of 130 Ų obtained for the trinactin-cation complex was in excellent agreement with the 125 Ų predicted from space filling models, reinforcing the suggestion that area-per-molecule calculations obtained at the air-wate interface can provide useful information on the molecular dimensions of these hydrophobic, relatively low molecular weight transport antibiotics.Comparison of the data obtained previously with valinomycin and with trinactin revealed two striking differences: (1) a large inflection in the force-area curve concurrent with cation binding and indicative of a conformational change was obtained with valinomycin,, but no evidence was found with trinactin; (2) the uptake of cations by trinactin could be predicted by simple equilibrium expressions, but the uptake of cations by valinomycin was strongly cooperative. Possible mechanisms for this cooperative association fo cations are discussed.