Experimental and theoretical Raman investigation on interactions of Cu(II) with histamine

A vibrational study of histamine (Hm), a small imidazole-containing hormone involved in many biological processes, and its complexes with copper (II) was carried out at pH 9. Both the Raman and IR spectra present marker bands yielding information on the metal coordination sites of Hm and, as a consequence, on the main species existing in the system. At basic pH Hm gives rise to two predominant complexes: a water-insoluble polymeric species, [CuLH-1]n, and a monomer, [CuL] or [CuL2]. In the first the imidazole ring takes part in the Cu(II) coordination in a fully deprotonated form, whereas in the second the ring is in the tautomeric I form. In addition, the neutral aminic group participates in the Cu(II) chelation in both species, whereas some ClO4- ions complete the coordination number of Cu(II) to six only in the monomeric complex, probably by replacing H2O or OH- in the coordination sphere. Finally, the Raman spectra were satisfactorily simulated with the aid of the results of theoretical calculations based on the Density Functional Theory, allowing a reliable prediction of the bands of the species supposed to exist in the system.