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

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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.