Solution chemistry of lanthanide complexes—I. Spectroscopic studies of the complexes formed between lanthanide ions and hydroxy amino acids

Abstract

Intermolecular energy transfer from Tb 3+ to Eu 3+, luminescence intensity measurements, potentiometric titrations, differential absorption spectroscopy, and spectroscopic titrations were all used to study the binding of lanthanide ions of serine and threonine. At low pH (3.0–6.0) the complexes are mononuclear and ligand is only weakly bound. In the pH interval of 6.0–8.5 stronger interaction takes place between the ligand and the metal (with possible coordination of the undissociated hydroxyl group), and self-association of complexes becomes important. Above pH 8.5, base hydrolysis of the complexes leads to highly associated species in solution and shortly above this pH an insoluble precipitate is formed. It was found that energy could be transferred from Tb 3+ to Eu 3+ more efficiently among complexes prepared from racemic ligands than in complexes made from resolved ligand, but this stereoselectivity was only observed at pH values greater than 6.5 and in solutions having a 1:10 ratio of metal-to-ligand. No stereoselectivity was found in solutions having 1:5 rations, and this observation was explained by the existence of 1:2 metal-ligand complexes existing in solutions having the higher ratio of metal-to-ligand (only 1:1 complexes are then found at lower ratios of metal-to-ligand).