Photodynamics of Melanin Radicals: Contribution to Photoprotection by Melanin

• Published in: Photochemistry and photobiology Vol. 99; no. 2; pp. 866 - 868
• Main Author: Sarna, Tadeusz
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01.03.2023
• Subjects: Absorption; Absorption spectra; Deactivation; Dihydroxyphenylalanine; Free Radicals; Melanin; Melanins - metabolism; Photochemistry; Photoexcitation; Photoionization; Radicals; Recombination
• ISSN: 0031-8655; 1751-1097; 1751-1097
• DOI: 10.1111/php.13753

The mechanism of very efficient relaxation of the melanin‐photoexcited states, responsible for the photoprotective action of the pigment, remains a subject for intense investigation. The most recent study by C. Grieco, F. Kohl, and B. Kohler, entitled “Ultrafast radical photogeneration pathways in eumelanin,” addresses key issues of melanin photophysics and photochemistry. By using femtosecond broad‐band pump probe‐transient absorption measurements, the researchers were able to identify the absorption spectrum of DOPA melanin radicals for the first time and proposed two distinct mechanisms of radical formation—photoionization and photoinduced charge separation. The observed photodynamic of melanin radicals suggests a new paradigm in which the ultrafast excited state deactivation is due to the efficient recombination of melanin radicals created promptly by photoexcitation. Photoexcitation of melanin with photons of different energies leads to the ultrafast formation of melanin radicals via ionization (1) and charge separation (2) mechanisms (scheme). Although most excited states form melanin radicals possibly in <1 ps, few radicals persist beyond the nanosecond time scale due to very efficient radical recombination (figure). The results suggest a new paradigm in which the ubiquitous ultrafast excited state deactivation seen in melanins comes about through the recombination of charges or neutral radicals that are created promptly by photoexcitation.