Abstract
In this work, we measured the metal-binding sites of natural and synthetic dihydroxyindole (DHI) melanins and their respective interactions with Fe(III) ions. Besides the two acid groups detected for the DHI system: catechol (Cat) and quinone-imine (QI), acetate groups were detected in the natural oligomer by potentiometric titrations. At acidic pH values, Fe(III) complexation with synthetic melanin was detected in an Fe(OH)(CatH2Cat) interaction. With an increase of pH, three new interactions occurred: dihydroxide diprotonated catechol, Fe(OH)2(CatH2Cat)−, dihydroxide monoprotonated catechol, [Fe(OH)2(CatHCat)]2−, and an interaction resulting from the association of one quinone-imine and a catechol group, [Fe(OH)2(Qi−)(CatHCat)]3−. In the natural melanin system, we detected the same interactions involving catechol and quinone-imine groups but also the metal interacts with acetate group at pH values lower than 4.0. Furthermore, interactions in the synthetic system were also characterized by infrared spectroscopy by using the characteristic vibrations of catechol and quinone-imine groups. Finally, scanning electronic microscopy (SEM) and energy-dispersive X-ray (EDS) analysis were used to examine the differences in morphology of these two systems in the absence and presence of Fe(III) ions. The mole ratio of metal and donor atoms was obtained by the EDS analysis.
Highlights
IntroductionMelanin is an ubiquitous oligomeric pigment found in plants and in the hair, skin (eumelanins), and brain (neuromelanins) of animals
Melanin is an ubiquitous oligomeric pigment found in plants and in the hair, skin, and brain of animals
We have shown that metal ion complexes, which induce passive uptake of the metal ions into cells, show significant toxicity towards melanin-producing melanoma cells [16]
Summary
Melanin is an ubiquitous oligomeric pigment found in plants and in the hair, skin (eumelanins), and brain (neuromelanins) of animals. Other researchers have studied the binding of Fe(III) by melanins [8,9,10] and shown that this ion accelerates the air oxidation of DHI and DHICA and suggested that a similar increase in iron-promoted oxidative stress in the substantia nigra is linked to the loss of melanin observed during Parkinson’s disease [11,12,13,14,15]. The precipitate was suspended in 30 mL of 0.1 M phosphate buffer pH 7.5, with papain (10 mg) and dithiothreitol (50 mg) This mixture was stirred for 18 h at 37◦C under argon and centrifuged as earlier. The black pellet collected, after 6 washings with water, was resuspended in 10 mL of 0.1 M phosphate buffer, pH 7.5, with protease (10 mg) and dithiothreitol (20 mg) added, and the mixture was stirred for 18 h at 37◦C under an argon stream. Sites and to see the morphological differences in these types of melanin
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