Enlisting electrochemistry to reveal melanin's redox-related properties

Abstract

Melanin has been surprisingly difficult to characterize using either bottom-up studies focused on molecular structure or top-down studies focused on functional properties. We have been developing electrochemical methods to understand the redox-activities of melanin. These studies show that melanins from various sources: (i) are reversibly redox-active; (ii) have redox potentials in the mid-physiological range; and (iii) react with a broad range of electron-donors (i.e., reductants) and acceptors (i.e., oxidants). Spectroelectrochemically-based operando methods have shown that when melanin is exchanging electrons, it also undergoes changes in its redox state. The observation that melanin can exist in two (or possibly more) oxidized or reduced states helps to explain some of its context-dependent behaviors. For instance, when melanin is in a reduced state, it has donate-able electrons that can quench an oxidative radical or partially-reduce O₂ to generate reactive oxygen species (ROS). Further, melanin can promote redox-cycling when it is located in metabolically-active contexts that are characterized by steep O₂-gradients because short diffusion distances separate aerobic from anaerobic conditions. We suggest that future studies may enable a fuller understanding of how melanin's redox activities contribute to its observed electrical conductivities (ionic and/or electrical), and if melanin's redox-capacitor properties confer a biological benefit (e.g., for energy harvesting).