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 (kinh
) 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.