Atypical Structural and π-Electron Features of a Melanin Polymer That Lead to Superior Free-Radical-Scavenging Properties

摘要

Black and insoluble eumelanin biopolymers are the key determinants of skin and hair pigmentation in dark human phenotypes, and are believed to play a major antioxidant and photoprotective role. This latter property is attributed to the peculiar physicochemical properties of eumelanins, including a broad-band UV/Vis absorption, a distinct electron paramagnetic resonance (EPR) signal, and hydration-dependent electronic-ionic-hybrid conductor behavior. The peculiar bio-optoelectronic, dielectric, metal-binding and free-radical properties of eumelanins have recently spurred intensive research that is aimed at their exploitation as soft, bioinspired, and biocompatible multifunctional (nano)materials for a diverse range of technological applications, for example, in organoelectronics to prepare nanoparticles with free-radical-scavenging properties, and as antioxidants for the thermooxidative stabilization of polymers. In human melanocytes, eumelanin synthesis involves enzymatic oxidation of tyrosine or dopa to give dopachrome, which undergoes tyrosinase-related protein (Tyrp2)-assisted isomerization to 5,6-dihydroxyindole-2-carboxylic acid (DHICA), a major circulating melanogen. In the absence of enzymatic assistance, a spontaneous decarboxylation occurs instead to give mainly 5,6-dihydroxyindole (DHI; Scheme 1). Thus, whereas natural eumelanins consist of DHICA-derived units of more than 50 %, synthetic dopa melanin contains mainly DHI, with only 10% of DHICA.