Spectroscopy and theoretical studies of natural melanin (eumelanin) and its complexation by iron(III)

Abstract

Eumelanin is an oligomeric pigment that has a high affinity for metal ions, which induces the formation of reactive oxygen species, causing melanoma cell apoptosis due to the acceleration of intracellular or extracellular oxidative stress. Melanin in the skin and in dark hair, known as eumelanin, has three main groups that serve as donors: carboxylic acid, catechol, and quinone-imine. In this study, eumelanin was extracted and purified from dark hair using a modified Prota method and characterized by elemental analysis. The sample shows the absence of sulfur-containing groups, and the infrared spectrum shows characteristic νO–H, νC–H, νC=C, νC=O, and νC–O stretches, which are confirmed by electronic structure calculations. The major interactions with Fe(III) in solution are at acidic pH values: [Fe(Ac)]2+ and [Fe(Qi)]2+, and at neutral and alkaline pH values: [Fe(OH)(Cat)] and [Fe(OH)2(Cat)2]3−, evaluated by electronic structure calculations. Electron paramagnetic resonance measurements in the solid state showed that the species isolated at acidic pH provided g = 4.3, characteristic for high-spin Fe(III) and the presence of a discrete semi-quinone with g = 2.003; at alkaline pH, it was observed that g = 4.3, and there was also a large increase in the radical species, suggesting interaction of the metal ion with the catechol.