Structural and kinetic studies of lasalocid A (X537A) and its silver, sodium, and barium salts in nonpolar solvents.

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Abstract
The ionophore lasalocid A (X537A) and its metal salts have been investigated by high resolution (270 MHz and 360 MHz) proton nuclear magnetic resonance spectroscopy to obtain structural and kinetic information in nonpolar solution. The proton resonances were assigned from double resonance studies on lasalocid A and on its salts, homologs, isomers, and chemically modified derivatives. Studies of proton and carbon longitudinal relaxation time suggest that lasalocid A exists as a monomer, whereas the sodium and barium salts exist as dimers in nonpolar solvents. A study of the magnitude of the vicinal proton coupling constants and the chemical shifts and linewidths of the hydroxyl resonances suggest that the backbone conformation and intramolecular hydrogen bonds are similar for lasalocid A and its sodium and barium salts in nonpolar solvents. Nuclear magnetic resonance studies on the role of bound solvent molecules suggest a tightly bound water molecule in the barium complex dimer (crystallized from water-ethanol) and a weakly bound ethanol molecule in the lasalocid A monomer (crystallized from ethanol) in cyclohexane. The selective changes in proton chemical shift on complexation [where the polar faces of two lasalocid anions coordinate the metal cation(s) in nonpolar solvents have been analyzed in terms of the proximity of the resonances to the cation, their linkage to the coordinating oxygen atoms, and the magnetic anisotropy effects of the polar groups of one ligand on the resonances of its partner in the dimer. The nuclear magnetic resonance studies in solution are compared with earlier observations on lasalocid A and its salts in the crystalline state. Thus, the short Ag-C5 distance in the crystal structure of silver complex dimer is also observed in the solution structure. The kinetic parameters associated with the exchange between lasalocid A and its barium complex in chloroform have been measured from an analysis of the resonance line shapes as a function of temperature.