Interactions between metal cations and the ionophore lasalocid. Part 1.—Complexation of alkaline-earth-metal cations by lasalocid, bromolasalocid and salicylic acid in methanol

Abstract

Interactions of the ionophore lasalocid (X-547 A), 5-bromolasalocid and model compounds, salicylic acid and o-methoxybenzoic acid with alkaline-earth-metal cations (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) have been studied in methanol using a potentiometric method. For all the acids (AH) investigated, successive complexes AM⁺ and A₂M are formed with M²⁺ cations, but the strength of complexation is stronger for the AM⁺ complexes. Gibbs energies of interaction of ionophore anions with alkaline-earth-metal cations are very close for lasalocid and its bromo-derivative. Comparison between these and model acids shows that the salicylic group is the leading site of ligation for these cations and possibly the sole site for Mg²⁺. As the size of the cation increases, other oxygen sites participate in complexation.