Ph-sensitive copper(II) binding mode of carcinine: some insights by vibrational spectroscopy.

Carcinine ((b-alanyl-histamine) (Carc), a natural imidazole-containing compound, has antioxidative properties. One of the mechanism by which it can act is metal chelation, since transition metal ions, such as Cu(II), can catalyze the formation of harmful reactive oxygen species. In addition, imidazole ring is a metal-ion binding site in many enzymes. To obtain more information on the Cu(II) binding of Carc, a vibrational study of Carc and its complexes with Cu(II) ions was carried out at different pH. The complexes, obtained by mixing aqueous solutions of CuCl2 and Carc in the 1:1 molar ratio at different pH (from 7 to 10) and then lyophilized, were examined by FT-IR and FT-Raman spectroscopies. Both the Raman and IR spectra present marker bands useful for the identifìcation of the predominant complex. Tautomer I of thè imidazolic ring (NT-H) is preferred in free Carc. The presence of Cu ions does not affect the tautomeric equilibrium at pH 7, whereas a pH increase strongly induces the deprotonation of both N-imidazolic nitrogens, giving rise to metal-imidazolate bridges. Thus, at neutral pH Carc acts as a tri-dentate ligand, mainly giving rise to a monomeric complex, whereas at higher pH Carc can be a tetradentate ligand and form oligomeric complexes. In conclusion, various complexes can be formed between Carc and Cu(II) and their prevalent form is affected by pH which determinates different structural rearrangements.