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Absolute Rate Constants for the Quenching of Reactive Excited States by Melanin and Related 5,6‐Dihydroxyindole Metabolites: Implications for Their Antioxidant Activity
Author(s) -
Zhang Xiangyang,
Erb Carl,
Flammer Josef,
Nau Werner M.
Publication year - 2000
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2000)0710524arcftq2.0.co2
Subject(s) - chemistry , photochemistry , flash photolysis , reaction rate constant , quenching (fluorescence) , benzophenone , excited state , ultrafast laser spectroscopy , absorption spectroscopy , kinetics , spectroscopy , fluorescence , physics , quantum mechanics , nuclear physics
The triplet‐excited state of benzophenone and the singlet‐excited state of 2,3‐diazabicyclo[2.2.2]oct‐2‐ene (Fluorazophore‐P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6‐dihydroxyindole (DHI) and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA). The excited states were generated by the laser‐flash photolysis technique and their reaction kinetics was examined by time‐resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O‐centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1 – 8.4 × 10 9 M −1 s −1 ) and DHICA (3.3–5.5 × 10 9 M −1 s −1 ) were near the diffusion‐controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin, assessed through the quenching rate constant of Fluorazophore‐P and normalized to the number of monomer units, was more than one order of magnitude lower (2.7 × 10 8 M −1 s −1 ) than that of its precursors. The trend of the quenching rate constants, i.e. DHI > DHICA ∼α‐tocopherol > melanin, along with the preferential solubility of DHICA in aqueous environments, serves to account for several experimental results from biochemical studies on the inhibition of lipid peroxidation by these natural antioxidants.