Spectroelectrochemical Reverse Engineering DemonstratesThat Melanin’s Redox and Radical Scavenging Activities Are Linked

Abstract

Melanins are ubiquitous in nature but their biological activities and functions have been difficult to discern. Conventional approaches to determine material function start by resolving structure and then characterize relevant properties. These approaches have been less successful for melanins because of their complex structure and insolubility, and because their relevant properties are not readily characterized by conventional methods. Here, we report a novel spectroelectrochemical reverse engineering approach that focuses on redox and radical scavenging activities. In this method, the melanin is immobilized in a permeable hydrogel film adjacent to an electrode and this immobilized melanin is probed using diffusible mediators and complex electrical inputs. Response characteristics are measured using two modalities, electrochemical currents associated with the reaction of diffusible mediators, and optical absorbance associated with the presence of diffusible free radicals. Using this method, we observed that both Sepia and fungal melanins are redox active and can repeatedly exchange electrons to be switched between oxidized and reduced states. Further, we observed that these melanins can quench radicals either by donating or accepting electrons. Finally, we demonstrate that the melanins' radical scavenging activities are dependent on their redox state such that a melanin must be reduced to have donatable electrons to quench oxidative free radicals, or must be oxidized to accept electrons from reductive free radicals. While the observation that melanin is redox-active is consistent with their well-accepted beneficial (radical-scavenging) and detrimental (pro-oxidant) activities, these observations may also support less well-accepted proposed functions for melanin in energy harvesting and redox communication.

Eumelanin for nature‐inspired UV‐absorption enhancement of plastics

“…As this effect was associated in the literature with the radical scavenging properties of the biopigment, we make the hypothesis that the MFA treatment reduced the radical scavenging power of eumelanin. This reduction following the MFA treatment has already been reported in the literature …”

A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent π‐Electron Donor with Free‐Radical Properties that can be Tuned by Ionic Liquids

“…Non‐nitrogenous fungal melanin mimics devoid of catechol functions are thus proposed as innovative prototypes of melanin‐based functional materials with exceptionally robust π‐electron properties and radical scavenging ability suitable for applications in biomedicine and materials science. In addition, these results may provide an improved background to inquire into the fate of PAH‐derived carbon‐containing species in potential astrochemical and prebiotic environments …”

Characterization and Fate of Hydrogen-Bonded Free-Radical Intermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 1,8-Naphthalenediol

“…Further elucidation of this chemistry may provide a useful background to inquire into the structural basis of the peculiar redox properties of DHN-based fungal melanin. 4…”