Phenolcarboxylic acid antioxidants do not act
in vivo as radical-scavengers in isolation, but rather together with GSH (glutathione), a coantioxidant, they constitute an intricate antioxidant network. Caffeic acid,
p-coumaric acid, ferulic acid and chlorogenic acid with or without 2-mercaptoethanol (ME), as a
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Phenolcarboxylic acid antioxidants do not act
in vivo as radical-scavengers in isolation, but rather together with GSH (glutathione), a coantioxidant, they constitute an intricate antioxidant network. Caffeic acid,
p-coumaric acid, ferulic acid and chlorogenic acid with or without 2-mercaptoethanol (ME), as a substitute for GSH, was investigated by the induction period (IP) method for polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN, a source of alkyl radicals, R
.) and benzoyl peroxide (BPO, a source of peroxy radicals, PhCOO
.) using differential scanning calorimetry (DSC). Upon PhCOO
. radical scavenging, the stoichiometric factors (
n, number of free radical trapped by one mole of antioxidant) for caffeic acid, ferulic acid,
p-coumaric acid and chlorogenic acid were 2.4, 1.8, 1.7 and 0.9, whereas upon R
. radical scavenging, the corresponding values were 1.3, 1.2, 1.0 and 0.8, respectively. Antioxidants with
n values close to 2 suggest the stepwise formation of semiquinone radicals and quinones. By contrast, those with
n values close to 1 suggest the formation of dimers after single-electron oxidation, possibly due to recombination of corresponding aryloxy radicals. The ratio of the rate constant of inhibition to that of propagation (k
inh/k
p) declined in the order chlorogenic acid >
p-coumaric acid > ferulic acid > caffeic acid. The ratio of the observed IP for the phenolcarboxylic acid/2-mercapto-ethanol (ME) mixture (1:1 molar ratio) (A) to the calculated IP (the simple sum of phenol acid antioxidant and ME) (B) was investigated. Upon R
. scavenging, the caffeic acid or
p-coumaric acid/ME mixture was A/B > 1, particularly the former was 1.2, suggesting a synergic effect. By contrast, upon PhCOO
. scavenging, the corresponding mixture was A/B < 1, particularly the latter was 0.7, suggesting an antagonistic effect. Upon both radicals scavenging, the A/B for the ferulic acid or chlorogenic acid/ME mixture was approximately 1. The reported beneficial antioxidant, anti-inflammatory and anticancer effects of caffeic acid and
p-coumaric acid may be related to their prooxidant-antioxidant balance in the presence of GSH.
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