Influence of solvent and of cation size on the conformations of lasalocid A-lanthanide(III) ion complexes: circular dichroism and fluorescence studies

Abstract

The interaction of lanthanide(III) nitrates (La³⁺ to Lu³⁺) with the carboxylic ionophore lasalocid A (LS) has been studied by circular dichroism (CD) and fluorescence spectroscopic techniques in acetonitrile and in methanol. Analysis of the CD data in acetonitrile has revealed the coexistence of both 1:l (ionophorecation) and 2: 1 complexes in solution. For 1.22 .Å > ionic radius > 1.13 Å, 1:1 complexes are preferred, and for 1.13 Å > ionic radius > 1.03 Å, 2:l complexes are preferred. Induced CD bands for Ln³⁺ions have been observed upon binding to LS in acetonitrile. The LS-Ln³⁺ complexes are less stable in methanol than in acetonitrile. CD spectral changes showed that the conformations of the complexes in methanol are different from those in acetonitrile. The complexes have rather open conformations in methanol compared to those in acetonitrile. The results underscore the importance of ionic radius, solvent environment, and ionization state of LS in determining the conformations of the ionophore-cation complexes.