NMR Studies of Melanins: Characterization of a Soluble Melanin Free Acid From Sepia Ink

This paper deals with the nuclear magnetic resonance characterization of a soluble derivative (melanin free acid) of Sepia melanin obtained by a peroxidative treatment of the parent (insoluble) species. High resolution 13 C and 15 N solid state NMR spectroscopies allow the assessment of the chemical changes occurring in the macromolecule upon solubilization. 1 H and 13 C NMR solution spectra are discussed in light of the results obtained from the solid state spectra. Furthermore, the coordination properties of melanin have been investigated through 27 Al NMR spectroscopy and proton relaxation enhancement studies of the paramagnetic gadolinium complex of melanin free acid. Through these experiments it has been possible to evaluate the molecular reorientational time τR (and from it an estimated molecular weight close to 20 KDa) and the strength of the metal‐macromolecule interaction.