Quantitative Treatment of the Kinetics of Free-Radical-Mediated Damage. Protection by Free-Radical Scavengers

Abstract

Dedicated to Professor Andre¬ M. Braun on the occasion of his 60th birthday 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 R8/R (where R8 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-2H-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.

Interaction and reactivity of urocanic acid towards peroxyl radicals

“…Values of R°/R, plotted as a function of trans-urocanic acid concentration, are shown in Figure 5B. Comparison of these values with those obtained employing Trolox under similar conditions 31 indicate that the latter is nearly 400 times more efficient than trans-urocanic acid. This means that, with regard to trapping of peroxyl radicals, trans-urocanic acid is only a moderate free radical scavenger.…”

5-Hydroxy-6-methyluracil, an Efficient Scavenger of Peroxyl Radical in Water

“…The absorption of quercetin in the presence of the excess of radical initiator decays by pure zero-order law until the appearance of the absorption band of the chalcone. Assuming a competition between inhibitors for peroxy radicals, the following equation can be derived [11]: Using the equation in the case of the competition between the inhibitors we calculate that the rate constant of interaction of ROO • with HMU in a water solution is five times lower than with quercetin ( Table 2). The rate constant for the reaction of quercetin with ROO • radicals is 10 8 -10 7 M -1 s -1 [12].…”

Reaction of 5-Aminosalicylic Acid with Peroxyl Radicals: Protection and Recovery by Ascorbic Acid and Amino Acids

“…This ability was evidenced, firstly, by showing the capacity of 5-ASA to protect c-Pc against AAPH-induced loss of fluorescence (12). The activity of 5-ASA was similar to that displayed by Trolox, a water-soluble vitamin E analog, priorly proven to trap peroxyl radicals very efficiently (15). The above results were complemented by experiments in which the reactivity of 5-ASA against peroxyl radicals was assessed by the loss of 5-ASA-associated fluorescence.…”

“…Results from the latter studies indicate that 5-ASA would trap peroxyl radicals very efficiently, and that the consumption of 5-ASA would follow zero-order kinetics. Under such conditions, most of the AAPH-derived radicals would be trapped by 5-ASA (15). The high reactivity of 5-ASA toward peroxyl radicals has been ascribed by previous workers to the presence of the amino group in its structure (see Fig.…”