Study of the coordination of ortho-tyrosine and trans-4-hydroxyproline with aluminum(III) and iron(III)

The coordination of amino acids ortho-tyrosine (o-Tyr, 1) and trans-4-hydroxyproline (HPro, 2) with the biologically important trivalent metal ions Al(III) and Fe(III) in aqueous solution was investigated by 1H and 13C high resolution NMR, Laser Desorption Mass Spectrometry (LD-MS), and MS/MS experiments. Potentiometric measurements were also carried out in 0.16 M NaCl, 37 °C, by varying the pH between 2.0 and 3.5. NMR spectra recorded on aqueous solutions of amino acids 1 or 2 in the presence of the appropriate trivalent metal chloride suggested that binding of Al(III) and Fe(III) involved the COOH and NH2 functional groups of ligands, while their phenolic and alcoholic groups which did not participate in the metal coordination. Potentiometry indicated the prevalent formation of complexes having the general stoichiometry ML and ML2. Laser desorption mass spectrometry (LD-MS) and MS/MS investigation proved the isotopic compositions of ML and ML2 complexes, demonstrating their structures.