Thallium-205 and proton nuclear magnetic resonance investigation of the complexation of thallium by the ionophores monensin and nigericin.

A thallium-205 and proton nuclear magnetic resonance study of the ionophores monensin and nigericin is reported. Evidence is presented for the simultaneous existence of two forms of monensin free acid in chloroform solution, one containing a water molecule in the central cavity and one with a water molecule on the periphery, linking the ends of the monensin molecule by hydrogen bonding. There appear to be three structurally different monensin complexes of thallium, two in the salt form and one in the acid form. In free nigericin acid, both terminal hydroxyl moieties hydrogen bond to the terminal carboxylic acid oxygens; in the Na+ salt, the binding of one of the carboxylate oxygens to the metal ion allows but one of the terminal OH's to hydrogen bond strongly. Thallium-205 chemical shifts and spin-lattice relaxation times in the title complexes are discussed and compared to those in ionophores studied previously.