Potentiometric and spectrophotometric studies of the copper(II) complexes of methyldopa, methyltyrosine, and catechol in aqueous solution

Abstract

The system of copper(II) ions in aqueous solution in the presence of L-3-(3′,4′-dihydroxyphenyl)-2-methylalanine (methyldopa), (1), at 25 °C and I= 0.150 mol dm^–3(Na)[ClO₄] has been characterized using glass-electrode potentiometry. Both mononuclear and oligonuclear complexes are found to occur, the former including a series of successively deprotonated species in the titrations at higher ligand-to-metal ratios, the latter including a cyclic dimer in 1 : 1 ligand-to-metal solutions. Interpretation of the results was facilitated by (i) glass-electrode potentiometric measurements on DL-3-(3′-hydroxyphenyl)-2-methylalanine (methyltyrosine) and catechol in the presence of copper(II) ions, (ii) visible spectrophotometry of solutions containing copper(II), (1), or alanine or catechol at various pH, and (iii)¹H n.m.r. measurements of (1) in ²H₂O solutions. Formation constants are given for three proton and 11 copper complexes of (1), three proton and four copper complexes of methyltyrosine, and one proton and two copper complexes of catechol.