E. M. Pliss

Substituted p-hydroquinones as inhibitors of lipid peroxidation.

The technique based on monitoring oxygen consumption was applied to study 12 alkyl- and methoxy-substituted p-hydroquinones (QH(2)) as a chain-breaking antioxidant during the oxidation of styrene and methyl linoleate (ML) in bulk as well as ML oxidation in micellar solution of sodium dodecyl sulfate (SDS) at 37 degrees C. The antioxidant activities of QH(2) were characterized by two parameters: the rate constant k(1) for reaction of QH(2) with the peroxy radical LO(2)*: QH(2)+LO(2)*-->QH*+LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH(2). In the case of styrene and ML oxidation in bulk, f values never exceed two; for the majority of QH(2), f was found to be significantly less than two due to the interaction of QH* with molecular oxygen. In the absence of superoxide dismutase (SOD), all the studied QH(2) displayed a very moderate if any antioxidant capability during ML oxidation in SDS micelles. When 20U/ml SOD was added, the majority of QH(2) showed a pronounced ability to inhibit ML oxidation, f parameter being ca. one. The features of QH(2) as an antioxidant in aqueous environment are suggested to associate with the reactivity of semiquinone (Q*(-)). Q*(-) reacts readily with molecular oxygen with formation of superoxide (O(2)*(-)); further reactions of O(2)*(-) result in fast depleting QH(2) and chain propagation. The addition of SOD results in purging a reaction mixture from O(2)*(-) and, as a corollary, in depressing undesirable reactions with the participation of O(2)*(-). With all the oxidation models, QH(2) were found to be very reactive to LO(2)*. The rate constants k(1) decreased progressively when going from the oxidation of styrene to ML oxidation in bulk and further to ML oxidation in SDS micelles.

The kinetics and mechanism of reactions of aliphatic stable nitroxyl radicals with alkyl and peroxide radicals during styrene oxidation

A kinetic study (EPR, microvolumometry), quantum-chemical analysis (DFT B3LYP/6-31G*), and kinetic simulation of the antioxidant activity of aliphatic stable nitroxyl radicals during styrene oxidation was performed. The key reactions constituting the detailed mechanism of the process were analyzed. It was shown that the inhibiting action of nitroxyl radicals was caused by their reaction not only with alkyl radicals but also with substrate peroxy radicals, which resulted in the regeneration of nitroxyl radicals in chain-termination steps.

Kinetic study and simulation of methyl linoleate oxidation in micelles

The results of experiments and computer simulations on the radical-chain oxidation of methyl linoleate in micelles are reported. The reaction rate constants included in the developed mathematical model are estimated. The ratio of the rates of first-order and bimolecular chain termination is examined. It is demonstrated that the rate constant of bimolecular chain termination in micelles is lower than that in the homogeneous phase.

Stable nitroxyl radicals and hydroxylamines as inhibitors of methyl linoleate oxidation in micelles

The kinetics of methyl linoleate oxidation in micelles inhibited by nitroxyl radicals and hydroxylamines was studied. The regeneration of nitroxyl radicals in this process was found. The activity of the studied inhibitors increases with increasing their lipophilicity. This effect was explained by increasing the distribution coefficient of the antioxidants in a micelles–water system. The reaction of nitroxyl radicals with the hydroperoxide radical formed upon the oxidation of methyl linoleate in micelles is proposed to be the key reaction in nitroxyl radical regeneration.

Five-membered cyclic nitroxyl radicals as inhibitors of the oxidation of methyl linoleate in micelles

The kinetics of methyl linoleate oxidation in micelles inhibited by five-membered cyclic nitroxyl radicals and the corresponding hydroxylamines was studied. The partial regeneration of nitroxyl radicals in this process was found. The activity of the studied inhibitors increases with an increase in their lipophilicity.

Multiple chain termination by aromatic nitroxides in oxidation of styrene

The antioxidant activity of aromatic nitroxides in the oxidation of styrene was studied using a complex of kinetic methods in combination with quantum chemical calculations and kinetic modeling. During oxidation of styrene and its saturated analog, ethylbenzene, aromatic nitroxides terminate oxidation chains via the reaction with both alkyl and peroxyl radicals. The mechanism of the process was proposed, which explains multiple chain termination by the interaction of peroxyl and nitroxide radicals revealed in the investigation of styrene oxidation.

The mechanism of inhibited oxidation of norbornene series bicycloolefins

The kinetics of oxidation (323 K) of bicycloolefins from the norbornene series: norbornene, vinylnorbornene, ethylidenenorbornene, and norbornadiene, inhibited by phenols (PhOH), aromatic amines (AmH), transition metal compounds (Men), or stable nitroxyl radicals of the piperidine series (>NO·), in chlorobenzene solutions have been studied. It has been found that the phenols are single-action inhibitors, whereas the amines, the transition metals, and the nitroxyl radicals repeatedly terminate oxidation chains. The results are explained in terms of phenomenology of the bicycloolefin oxidation process associated with the fact that peroxide radicals attack different reaction centers (π- and α-CH bonds), thereby resulting in the formation of different types of chain carrier radicals: low-molecular-weight hydroperoxide and polyperoxide radicals.

Effect of microheterogeneity on the kinetics of oxidation of methyl linoleate in micelles

The kinetic results of the radical chain oxidation of methyl linoleate (LH) in micelles were given. The oxidation rate depends on the average number of substrate molecules in the micelle and is almost independent of the micelle concentration under the given conditions. The chain propagation and termination were assumed to predominantly occur inside the micelle. Possible reasons for linear chain termination during the oxidation of polyunsaturated fatty esters in microheterogeneous systems were discussed.

Quantum-chemical analysis of the disproportionation of nitroxyl and peroxyl radicals in oxidizing organic compounds

The enthalpy change of disproportionation reaction of nitroxyl and peroxyl radicals was calculated by the DFT B3LYP/cc-pVDZ method. The regeneration of the corresponding hydroxylamine during the oxidation of unsaturated compounds inhibited by nitroxyl radicals is shown to be associated with the disproportionation of the nitroxyl and peroxyl radicals.

Superoxide radicals in the kinetics of nitroxide-inhibited oxidation of methyl linoleate in micelles

The kinetics of the oxidation of methyl linoleate in micelles inhibited by stable nitroxides under the conditions of initiation by superoxide radicals was studied. Multiple breakdown of the oxidation chains was observed in this process. The antioxidant activity of nitroxides increased when the reduction potential of the oxoammonium cation/nitroxide pair decreased and their lipophilicity increased.