Spectroscopic investigation of hard and soft metal binding sites in synthetic melanin

Understanding the interaction of metal ions with melanin structures is essential to know some intricate functions of this kind of compounds in the living systems. The coordination chemistry of vanadyl oxycation, VO(II), and copper, Cu(II), in a synthetic l-dopa melanin (SM) was investigated using electron paramagnetic resonance (EPR), Fourier transform infrared (FT IR) and ultraviolet–visible (UV–Vis) spectroscopy. The EPR and FT IR spectra indicated axially coordinated VO(II) ions bound to catecholate functional groups, and a fraction of the Cu(II) ions in a square planar structure partially coordinated to carboxylate functional groups. The resolved EPR perpendicular super hyperfine splitting constant, AN⊥, and the Cu(II) complex EPR parallel parameters g‖ and A‖ values indicated two magnetically equivalent nitrogen atoms in a plane with the coordination sphere, CuN2O2. The ultraviolet–visible (UV–Vis) spectra of solid samples indicate supramolecular structure constitution for the synthesized VO(II)SM and Cu(II)SM complexes.