General features in the stoichiometry and stability of ionophore A23187-cation complexes in homogeneous solution

Abstract Existing literature describing the stoichiometry and stability of complexes between A23187 and divalent cations in solution has been extended to include additional transition series cations, the heavy-metal cations Cd 2+ and Pb 2+ , plus seven lanthanide series trivalent cations. Stability constants of 1:1 complexes between the ionophore and the divalent cations vary by 6.2 orders of magnitude between Cu 2+ and Ba 2+ which are the strongest and weakest complexes, respectively. Considering alkaline-earth and first-series transition cations together, the pattern of stability constants obeys the extended Irving-Williams series as is seen with many nonionophorous liganding agents. Cd 2+ and Pb 2+ are bound with an affinity similar to those of Mn 2+ and Zn 2+ , whereas the lanthanides are bound with little selectivity and slightly higher stability. Titration of the ionophore in the 10 −5 m concentration range with diand trivalent cations gives rise first to complexes of stoichiometry MA 2 and subsequently to MA as the metal concentration is increased. The second stepwise stability constants for formation of the MA 2 species exceeds the first constant by approximately 10-fold. With lanthanides, heavy metals, and transition-metal cations, OH − , at near physiological concentrations, competes significantly with free ionophore for binding to the 1:1 complexes. This competition is not apparent when Ca 2+ or Mg 2+ are the central cations. Possible implications of the 1:1 complex selectivity pattern, the ionophorehydroxide competitive binding equilibria, and potential ternary complexes involving 1:1 ionophore:cation complexes and other anions present in biological systems are discussed with respect to the ionophore's transport selectivity and biological actions.