Binding of Cu(II) to carnosine: a Raman and IR spectroscopic study

A comparative Raman and FTIR study of carnosine, a dipeptide present in several mammalian tissues, and its complexes with copper(II) at different pH values was carried out. The neutral imidazole ring gives rise to some bands that appear at different wavenumbers, depending on whether the imidazole ring is in the tautomeric form II or I. At pH 7 and 9 the molecule exists in equilibrium between the two tautomeric forms; tautomer I is predominant. Metal coordination is a factor that affects the tautomeric equilibrium, and the copper(II) coordination site can be monitored by using some Raman marker bands such as the nu C-4=C-5 band. On the basis of the vibrational results, conclusions can be drawn on the functional groups involved in the Cu(II) chelation and on the species existing in the Cu(II)-carnosine system. At neutral and basic pH the most relevant species formed when the Cu(II)/carnosine molar ratio is not very different from unity is a dimer, [Cu2L2H-2](0). In this complex the ligand coordinates the metal via the N (amino), O (carboxylate), and N (amide) donor atoms while the N, nitrogen atoms of the imidazole rings (tautomer II) bridge the copper(II) ions. At a slightly acidic pH the two monomeric complexes [CuLH](2+) and [CuL](+) were present. In the former the imidazole ring takes part in the Cu(II) coordination in the tautomeric I form whereas in the latter it is protonated and not bound to Cu(II)