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Quantitative Treatment of the Kinetics of Free‐Radical‐Mediated Damage. Protection by Free‐Radical Scavengers
Author(s) -
Pino Eduardo,
Lissi Eduardo A.
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20011219)84:12<3677::aid-hlca3677>3.0.co;2-x
Subject(s) - chemistry , trolox , radical , scavenger , free radical scavenger , radical substitution , kinetics , free radical addition , photochemistry , free radical reaction , organic chemistry , antioxidant , dpph , physics , quantum mechanics
Equations are derived to quantitatively describe the effect of a free‐radical scavenger upon the rate of a radical‐mediated process that senses the steady‐state free‐radical concentration. The dependence of the ratio R °/ R (where R ° is the rate of the process in the absence of additive) upon the additive concentration depends upon the type of reaction that determines the free‐radical lifetime. Normal Stern‐Volmer ‐like behavior is expected only when the lifetime of the radical in the absence of free‐radical scavengers is determined by the concentration of the substance employed as the reporter of the free‐radical concentration. These predictions are tested in a system comprised of 2,2′‐azobis[2‐methylpropanimidamide dihydrochloride) as the free‐radical source, c ‐phycocyanin as the reporter molecule, and Trolox (=3,4‐dihydro‐6‐hydroxy‐2,5,7,8‐tetramethyl‐2 H ‐1‐benzopyran‐2‐carboxylic acid) tryptophan and 4‐methoxyphenol as peroxyl‐radical scavengers. The data obtained with Trolox show that it behaves as a nearly ideal free‐radical scavenger. On the other hand, the data obtained with tryptophan and 4‐methoxyphenol as scavengers show, when plotted according to the Stern‐Volmer equation, a strong downward curvature. These results are explained in terms of c ‐phycocyanin bleaching by scavenger‐derived free radicals.