Determination of the Reaction Rate Constants of Antioxidants with the Hydroxyl Radical by a Rapid-Flow ESR Method

Abstract

The reaction rate constants of antioxidants with the hydroxyl radical (HO•) were determined by a rapid-flow ESR method. ESR spectra of the antioxidant radical formed by a reaction with HO• generated from the Ti3+ + H2O2 system were measured. When an antioxidant and ethanol were mixed with HO•, a superposed spectrum of the 1-hydroxyethyl radical and antioxidant radical was obtained. The intensity ratio of the signals of these radicals was calculated from the doubly integrated curve, and then the ratio of the reaction-rate constant of the antioxidant with HO• to that of ethanol was obtained. The ratios of pyrogallol, gallic acid, catechol, phloroglucinol, resorcinol, and methanol were 19, 17, 11, 1.5, 1.2, and 0.56, respectively. The dissociation energies of the bonds in antioxidant molecules were obtained by MO calculations, which demonstrated that phenoxy, hydroxymethyl, and 1-hydroxylethyl radicals were formed by the reactions with HO•, as expected from their spectra. The relationship between the relative activation energies obtained from the rate constants and the bond-dissociation energies showed that the Evans–Polanyi equation holds in polyphenol series, but the line was shifted from that of alcohols. This suggested that the structures of the transition state of alcohols were stabilized by a polar effect.

Electron spin resonance

“…The relative activation energies obtained from their reaction rate constants and computed O-H bond dissociation energies shows that the Evans Polyani relationship is valid for the series albeit that there is a departure from the linear plot for the alcohols which suggested that their transition states were stabilised by a polar effect. 86 Previously, we had reported that relative rates for tocopherols and other sterically hindered phenols, in H-atom transfer reactions to oxygen centred radicals, correlated well with the computed free energies, obtained from calculations of both enthalpies and entropies for the O-H dissociation step. 87 The ''b-carotene'' dilemma refers to the seemingly unpredictable behaviour of the material, which while well demonstrated to be an efficient radical scavenger, when implemented as a dietary supplement in smokers, it caused an elevation in rates of lung cancer from the controls, with the result that the study was brought to a halt well ahead of its originally scheduled end.…”

Electron spin resonance (some chemical applications)

“…It is thought that the structures of the transition states for the alcohols are stabilised by a polar effect. 66 The O-H bond dissociation enthalpies (BDEs) of a series of thirteen oximes (R 2 CQNOH) have been reported. Experimental anchor points used to validate the results of theoretical calculations include: (1) the O-H BDEs of Bu t 2 CQNOH, Bu t Pr i CQNOH and Bu t (1-adamantyl)CQNOH (determined previously from the heat released in the reaction between Bu t 2 CQNO d and PhNHNHPh in benzene), all of which were decreased by 1.7 kcal mol À1 to reflect a revision to the heat of formation of (E)azobenzene (which, in fact, has significant implications for other BDEs) and to correct for the heat of hydrogen bonding of Bu t 2 CQNOH to benzene (0.43 kcal mol À1 ); (2) the measured rates of thermolysis of six R 2 CQNOCH 2 Ph derivatives at 423 or 443 K, which were used to derive O-H BDEs for the corresponding R 2 CQNOH molecules.…”