Multinuclear magnetic resonance study of equilibria between aluminum(III) and sulfur-containing amino acids in aqueous solutions
Aluminum(III) is generally found in small amounts in living organisms where it interacts with several biomolecules. Low mass biomolecules can form complexes associated with absorption and distribution processes of this toxic ion. However, investigating aluminum(III) complexes in solution poses singular difficulties, because it is highly hydrolyzable. Amino acids constitute possible ligands for aluminum(III). Methionine, cysteine and homocysteine are found in healthy human body. Penicillamine is a drug used in several circumstances, such as in treating metal poisoning. These four biomolecules share similar donor atoms involved in complexation reactions: a carboxylate oxygen, an amine nitrogen and sulfur. In this study, four binary complexes formed by aluminum(III) and amino acids in solution at a metal-to-ligand ratio of 1:1 were analyzed through potentiometry and multinuclear magnetic resonance (13C and 27Al). Potentiometry data consistently demonstrated complexion formation. Distributions of species revealed the coexistence of several species in ample pH ranges and pointed out the preferable value to perform the NMR analyses. 13C NMR data were used to confirm complexion formation, while 27Al NMR data indicated the geometric arrangement adopted by the complexes. Structures were proposed for each complex.