PMID- 10818782
OWN - NLM
STAT- MEDLINE
DCOM- 20000605
LR  - 20190818
IS  - 0031-8655 (Print)
IS  - 0031-8655 (Linking)
VI  - 71
IP  - 5
DP  - 2000 May
TI  - Absolute rate constants for the quenching of reactive excited states by melanin 
      and related 5,6-dihydroxyindole metabolites: implications for their antioxidant 
      activity.
PG  - 524-33
AB  - 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 x 10(9) M-1 s-1) and DHICA (3.3-5.5 x 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 x 10(8) 
      M-1 s-1) than that of its precursors. The trend of the quenching rate constants, 
      i.e. DHI > DHICA approximately alpha-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.
FAU - Zhang, X
AU  - Zhang X
AD  - Departement Chemie, Universitat Basel, Switzerland.
FAU - Erb, C
AU  - Erb C
FAU - Flammer, J
AU  - Flammer J
FAU - Nau, W M
AU  - Nau WM
LA  - eng
PT  - Journal Article
PT  - Research Support, Non-U.S. Gov't
PL  - United States
TA  - Photochem Photobiol
JT  - Photochemistry and photobiology
JID - 0376425
RN  - 0 (Antioxidants)
RN  - 0 (Free Radicals)
RN  - 0 (Indoles)
RN  - 0 (Melanins)
RN  - Z3OC8499KG (5,6-dihydroxyindole)
SB  - IM
MH  - Antioxidants/chemistry/*pharmacology
MH  - Free Radicals
MH  - Hydrolysis
MH  - Indoles/chemistry/*pharmacology
MH  - Kinetics
MH  - Melanins/chemistry/metabolism/*pharmacology
MH  - Photolysis
EDAT- 2000/05/20 09:00
MHDA- 2000/06/10 09:00
CRDT- 2000/05/20 09:00
PHST- 2000/05/20 09:00 [pubmed]
PHST- 2000/06/10 09:00 [medline]
PHST- 2000/05/20 09:00 [entrez]
AID - 10.1562/0031-8655(2000)071<0524:arcftq>2.0.co;2 [doi]
PST - ppublish
SO  - Photochem Photobiol. 2000 May;71(5):524-33. doi: 
      10.1562/0031-8655(2000)071<0524:arcftq>2.0.co;2.