Proton-coupled electron transfer in hallachrome, a natural 1,2 anthraquinone: Linking electrochemical properties to biological activity

Hallachrome is a 1,2-anthraquinone secreted by marine polychaete worms whose toxicity and antimicrobial properties have been reported; the origin of this biological activity, however, remains elusive. Voltammetric studies reveal reversible redox behavior in the pH range 1–10, consistent with a two-electron, two-proton (2e−/2H+) mechanism. The electron-donating substituents in hallachrome (hydroxyl and methyl groups) stabilize the protonated hydroquinone form and result in a substantial cathodic shift of 0.4–0.5 V compared to unsubstituted 1,2-benzoquinone. The electrochemical analysis reveals that hallachrome (redox potential −0.12 V vs. SHE at pH 7) might be capable, from a thermodynamic point of view, of oxidizing key cellular antioxidants, including NADH (−0.32 V), NADPH (−0.32 V), and glutathione (−0.24 V). These findings provide fundamental insights into the structure-activity relationships governing quinone electrochemistry and establish a foundation for understanding hallachrome's biological activity.