Studies on chelation ofl-DOPA with metal ions and metal-ATP systems

The nature and strength of chelation ofl-DOPA with the metal ions, namely, Zn(II), Cu(II), Ni(II), Co(II), Fe(II, III), Mn(II), Ca(II), Mg(II), and Al(III) in both presence and absence of ATP were investigated by thermodynamic equilibrium studies at a constant ionic strength (0.12 M NaCl) and temperature, 25 ± 0.1°C, using the potentiometric pH method. Copper(II), Fe(III), and Al(III) interacted strongly withl-DOPA with the stability constants (logK_ML, 18.47 ± 0.13, 18.39 ± 0.31, and 19.60 ± 0.20, respectively. In the combined presence of ATP the metal ions were predominantly coordinated with ATP in the pH range 3–5, followed by interaction withl-DOPA above pH = 5 and the formation of the mixed ligand chelates. The stabilities (logK_MLA) for the chelation ofl-DOPA with the metal-ATP (1:1) systems varied from 2.32 ± 0.15 to 13.24 ± 0.20 for the different metals. Electronic absorption spectral studies of the metal-l-DOPA systems were undertaken to determine the relative involvement of the different donor groups of the ligand in metal binding. On the basis of the spectral and equilibrium data, the nature and extent of metal binding by the pyrocatechol and the alanine ends of thel-DOPA molecule over the pH range 3–10 are discussed.