Effect of transition metal binding on the tautomeric equilibrium of the carnosine imidazolic ring

A Raman study of carnosine (Carn) and its complexes with Cu(II), Zn(II) and Co(II) at different pH values was carried out. At pH 7 and 9, Carn exists in equilibrium between two tautomeric forms. Raman spectroscopy appears to be a useful tool for analysing the tautomeric equilibrium of the imidazole ring of Carn since the sites involved in metal chelation can be identi®ed by some bands (e.g. nC4yC5† that change in wavenumber depending on whether the imidazole ring takes the tautomeric form I or II. Form I (Np±H) is predominant in the free ligand, but the metal coordination can affect the tautomeric equilibrium. Although weak compared to those of aromatic residues, the Raman marker bands may be useful in analysing metal±histidine interaction in proteins