Chemistry of melanin

Eumelanins are black insoluble natural pigments which their chemistry is poorly understood. Since natural melanins vary in composition and structure depending on their source, synthetic melanin derived from 5,6-dihydroxyindole (DHI) is used as a simplified model of eumelanin. DHI-melanin films, formed on electrode surfaces by oxidative polymerization of 5,6-dihydroxyindole solution, were used to directly measure the chromophore's redox reactivity. Films on optically transparent ITO electrodes allow correlation of spectral changes with electrochemical potential. Spectroelectrochemical titrations give evidence to formation of a unique quinone-imine (QI) chromophore upon oxidation of melanin. Further proof for the quinone-imine intermediate comes from potentiometric titration of colloidal DHI-melanin. Two types of acidic functionalities are assignable: catechol groups with pK_a between 9 and 13, and quinone-imine with pK_a of 6.3. Melanin solutions obtained from N-methylated DHI lack the pK_a 6.3 buffer, consistent with its inability to form quinone-imine tautomer. To study a small molecule model of the quinone-imine intermediate, a ruthenium complex of DHI was synthesized. Ruthenium is an inert metal which by slowing down the oxidation of DHI allows us to study the intermediates of the reaction. Upon oxidation of this complex with potassium ferricyanide, a dimer of the quinone-imine intermediate is identified by mass spectrometry.; Melanins are known to have high affinity for metal ions. To study this aspect of chemistry of melanin, films pretreated with CuCl₂ and Zn(CH₃COO)₂ were investigated. These samples show a different spectral behavior. They have elevated absorbances compared to none-treated films indicating stabilization of the quinone-imine form of melanin by coordination to the cations coordination to the metals. To study the nature of the metal-binding sites in melanin, potentiometric titrations were preformed on colloidal DHI solutions in presence of different metal ions. The changes in titration curves after addition of metal ions were analyzed to obtain the binding constant and stoichiometry of metal-melanin complexes.; Melanin is also known to react with oxygen under different conditions. The interaction of oxygen with DHI-melanin film is followed spectrophotometrically. The results show a reversible interaction of the pigment with oxygen associated with an irreversible bleaching at higher wavelengths. Superoxide, peroxide, and hydroxy radicals are identified as the products of the reaction of melanin with oxygen. Binding to metal ions enhances the formation of reactive oxygen species under similar conditions.; To study the structural units of melanin, colloidal solution of auto-oxidized DHI-melanin as well as electrochemically deposited DHI-melanin particles are investigated by atomic force microscopy (AFM). The results support the recently proposed graphite like structure for melanin.