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

Abstract

The black we wear: Why nature selected 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) to synthesize (photo)protective eumelanin pigments is an enigma. Synthetic DHICA eumelanin has now been shown to be a highly efficient free‐radical scavenger in the solid state, which is due to a conformationally interrupted π‐electron network associated with atypical optical, paramagnetic, and aggregation properties.

Polyelectrolytes to produce nanosized polydopamine

“…This shoulder is reproducible and identical to that observed in Ref. [16] but its chemical origin is not yet explained. It cannot be attributed to the presence of 5,6-dihydroxyindole-2-carboxilic acid (DHICA) which can only be obtained from L-DOPA solutions.…”

Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

“…[11,26,27] DHICA-melanin exhibits hydroxyl radicalscavenging properties in the Fenton reaction, differently from DHI-melanin. [12,28] DHICA-melanin features relatively homogeneous free-radical species, i.e., spatially confined within restricted segments of the polymer, in contrast to the broader variety of free-radical species generated within the delocalized π-electron systems of the DHI-melanin.…”

“…In addition, DHICA-melanin has a lower optical absorption in the visible. [12] Melanin contains intrinsic stationary-free radicals (observed predominantly in the solid state and likely carbon-centered species associated with defects in the polymer backbone), [19][20][21] with additional extrinsic free radicals generated under UV or visible irradiation or in high hydration conditions. [22][23][24][25] The presence of intrinsic radicals is independent of the pH value.…”

Melanin Chemistry and the Ecology of Stress

“…In contrast, the oxidative coupling of DHICA/IQCA leads to 4,4′-biindolyl, 4,7′-biindolyl, and other minor dimers due to the influence of the carboxyl group at the 2 position of the indole ring, which limits the range of reactive sites available for oxidative coupling (Aroca et al 1992;Ito and Wakamatsu 2008). As a consequence, the DHI/IQ pathway rapidly leads to large branched polymers that absorb light uniformly across a wide range of wavelengths, while the DHICA/IQCA pathway may lead to smaller linear polymers that exhibit distinct absorbance peaks (Panzella et al 2013). Thus, producing eumelanins with a high proportion of DHI/IQ implies greater final protection against UV radiation than producing eumelanins with a high proportion of DHICA/IQCA but more ROS production during melanogenesis and therefore more cytotoxicity and less pigment-producing melanocytes .…”