Reinvestigation of the copper(II)–carcinine equilibrium system: “two-dimensional” EPR simulation and NMR relaxation studies for determining the formation constants and coordination modes

Abstract

The equilibria and solution structure of complexes formed between copper(II) and carcinine (β-alanyl-histamine) at 2 ⩽ pH ⩽ 11.2 have been studied by EPR and NMR relaxation methods. Beside the species that have already been described in the literature from pH-potentiometric measurements, several new complexes have been identified and/or structurally characterized. The singlet on the EPR spectrum detected in equimolar solutions at pH 7, indicates the formation of an oligomerized ( CuL ) n 2 n + complex, with {NH 2, N im} coordination. The oligomerization is probably associated with the low stability of the ten-membered macrochelate ring, which would form in the mononuclear complex CuL 2+. In presence of moderate excess of ligand the formation of four new bis-complexes ( CuL 2 H n 2 + n , n = 2,1 and 0/−1) was detected with {N im}{N im}, {NH 2, N im}{N im} and {NH 2, N −, N im}{N im} type co-ordination modes, respectively. At higher excess of ligand ([L]/[Cu 2+] > 10) and at pH ∼ 7, the predominant species is CuL 4 H 2 4 + . The 1H and 13C relaxation measurements of carcinine solutions (0.6 M) in presence of 0 mM ⩽ [Cu 2+] tot ⩽ 5 mM at pH = 6.8, allowed us to extract the carbon-to-metal distances, the electronic relaxation and tumbling correlation times, as well as the ligand exchange rate for the species CuL 4 H 2 4 + . According to these results, the metal ion is {4N im} co-ordinated in the equatorial plane, while the neutral amino groups are unbounded. Since naturally occurring carcinine shows in vivo antioxidant property, the SOD-like activity of the copper(II)–carcinine system has also been investigated and the complex CuLH −1 was found to be highly active.