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

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)n2n+ complex, with {NH2, Nim} coordination. The oligomerization is probably associated with the low stability of the ten-membered macrochelate ring, which would form in the mononuclear complex CuL2+. In presence of moderate excess of ligand the formation of four new bis-complexes (CuL2Hn2+n, n = 2,1 and 0/−1) was detected with {Nim}{Nim}, {NH2, Nim}{Nim} and {NH2, N−, Nim}{Nim} type co-ordination modes, respectively. At higher excess of ligand ([L]/[Cu2+] > 10) and at pH ∼ 7, the predominant species is CuL4H24+. The 1H and 13C relaxation measurements of carcinine solutions (0.6 M) in presence of 0 mM ⩽ [Cu2+]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 CuL4H24+. According to these results, the metal ion is {4Nim} 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.