Vibrational Investigation on the copper(II) binding mode of carcinine and its pH dependence

A comparative FT-Raman and FT-IR study of Carcinine (Carc), a natural imidazole dipeptide, and its complexes with Cu(II) ions was performed at different pH's. Both Raman and IR spectra present marker bands useful for the identification of the predominant complexes; in particular, Raman spectroscopy appears useful for identifying the metal-coordination site of the imidazole ring (Np or Ns atoms) of Carc. Free Carc shows a strong network of H-bonds and tautomer I (Ns-H) is the preferred form of the imidazolic ring (bands at 1578, 1292 and 988 cm1). The presence of Cu(II) does not affect the tautomeric equilibrium at pH 7, whereas the deprotonation of both N-imidazolic nitrogens is strongly induced at higher pH. Under neutral and alkaline conditions the primary amino group takes part to the Cu(II) chelation, whereas all the peptidic moieties are involved in coordination only at pH 7. Thus, Carc acts as a tri-dentate ligand at neutral pH, mainly giving a monomeric complex, [CuLH1], containing tautomer I, whereas an oligonuclear complex, probably [Cu4L4H8], where metal-imidazolate ions connect different ligand molecules, predominates at alkaline pH.