title: Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block.
abstract: Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.
authors.affiliation: Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna , Währinger Str. 17, A-1090 Wien, Austria.
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Nogueira
authors.affiliation: Department of Chemical Physics, Lund University , Box 124, 22100 Lund, Sweden.
authors.forename: Alice
authors.initials: A
authors.lastname: Corani
authors.affiliation: Department of Chemical Physics, Lund University , Box 124, 22100 Lund, Sweden.
authors.forename: Amal
authors.initials: A
authors.lastname: El Nahhas
authors.affiliation: Department of Chemistry and Sciences, University of Naples Federico II , Via Cintia, 80126 Naples, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Pezzella
authors.affiliation: Department of Chemistry and Sciences, University of Naples Federico II , Via Cintia, 80126 Naples, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: d'Ischia
authors.affiliation: Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna , Währinger Str. 17, A-1090 Wien, Austria.
authors.forename: Leticia
authors.identifier: http://orcid.org/0000-0001-5112-794X
authors.initials: L
authors.lastname: González
authors.affiliation: Department of Chemical Physics, Lund University , Box 124, 22100 Lund, Sweden.
authors.forename: Villy
authors.initials: V
authors.lastname: Sundström