A. V. Trofimov

Antioxidant Activity of Chalcones: The Chemiluminescence Determination of the Reactivity and the Quantum Chemical Calculation of the Energies and Structures of Reagents and Intermediates

Six antioxidants from the class of chalcones (ArOH), compounds from which flavonoids are obtained in nature, were studied. The antiradical activity of chalcones and a number of related compounds was determined by a chemiluminescence method using the scavenging of peroxide radicals ROO· + ArOH → ROOH + OAr· (with the rate constant k7) in a model reaction of diphenylmethane (RH) oxidation. The structures and energies of the reagents and intermediates were determined by semiempirical quantum chemical (PM3, PM6) calculations. 3,4-Dihydroxychalcone and caffeic acid, which have a catechol structure, that is, two neighboring OH groups in phenyl ring B, exhibited high antioxidant activity (k7 ≈ 107 l mol−1 s−1); this is consistent with the lowest bond strengths D(ArO-H) of 79.2 and 76.6 kcal/mol, respectively. The abstraction of a hydrogen atom by the ROO· radical is the main reaction path of these compounds; however, the low stoichiometric coefficients of inhibition (f = 0.3–0.7) suggest a contribution of secondary and/or side reactions of ArOH and OAr·. In the other chalcones, the ArO-H bond is stronger (D(ArO-H) = 83–88 kcal/mol) and the antioxidant activity is lower (k7 = 104–105 l mol−1 s−1).

Facile chemiluminescence assay for antioxidative properties of vegetable lipids: fundamentals and illustrative examples

The general approach disclosed herein opens the new possibilities of exploiting the oxidation processes followed by chemiluminescence (CL) emission for the assessment of an antioxidant potential of natural lipid materials and enables determination of the amount and strength of lipid-borne antioxidants in one experiment. The reliability of the analytical procedure is completely unaffected by an inevitable entering of oxidizable lipid portions into the probe chemiluminescent mixture, which is exemplarily illustrated for the case of vegetable oils which served as sources of antioxidant-containing lipids. As a matter of fact, the difference in the effective radical-scavenging rate constants, determined for the antioxidative constituents of the sunflower and corn oils, perfectly matches the distinction of their qualitative tocopherol contents. In addition to the antiradical activity of lipid samples, the antioxidant potential of the latter may be modified by their influence on hydroperoxide stability, as it has been also demonstrated in the present work.

Back electron transfer in electron-exchange chemiluminescence of oxyaryl-substituted spiroadamantyl dioxetane, an analog of firefly bioluminescence

The viscosity dependence of the excitation yield of the singlet oxybenzoate anion, an emitter of electron-exchange chemiluminescence of oxyaryl-substituted spiroadamantyl dioxetane, a chemical analog of firefly bioluminescence, was studied. At 299, K the excitation yield increases from 8 to 23% as the content of diphenylmethane in its mixture with benzene increases to 97%. This effect was quantitatively interpreted in terms of a probabilistic model of the solvent-cage effect, assuming that the chemiexcitation results from the back electron transfer between the products of chemically initiated decomposition of the starting reagent.

Chemiluminescence quantum yield in permanganate reduction reactions in acid medium

Chemiluminescence quantum yields for the reactions of permanganate with oxalic, tartaric, and citric acids; hydrazine; KBr; and FeSO4 in aqueous solutions of sulfuric acid have been measured. The maximum quantum yield reaches 1.2 × 10−5 einstein/mol with the chemiexcitation yield being 2%. Hence, the relatively low chemiluminescence quantum yield is due to a low yield of light emission by chemiexcited particles, rather than the low chemiexcitation yield.

Dioxetane formation in the phenol-inhibited oxidation of hydrocarbons

The structures and energies of possible products of the reaction between the phenoxy radical OAr• and dioxygen are calculated using the PM6 semiempirical method. The possible adducts are the peroxyl radical OArOO• and dioxetane radicals. The latter are the cyclization products resulting from the addition of the terminal atom of OArOO• to a carbon atom of the Ar ring. This reaction is nearly thermoneutral and is, therefore, likely.

Chemiluminescence in reactions of manganese(III) reduction by lactic acid: Two-electron mechanism of chemiexcitation

Chemiluminescence is generated during reduction of manganese(III) ions with lactic acid to have a quantum yield as high as 0.1, a record-breaking chemi-excitation value for inorganic emitters. Kinetic features of the chemiluminescence and a nonradical mechanism of lactic acid oxidation lead to the conclusion that chemiexcitation results from two-electron reduction of manganese(IV) ions in the complex with lactic acid.

Hydrogen Peroxide Derived from Cigarette Smoke: “Pardon Impossible, to be Sent to Siberia”?

Hydrogen peroxide (H2O2) is one of the most important constituents in the metabolic chain transforming cigarette smoke in the human organism. The present analytical review provides a survey of the state-of-art in the study of H2O2 chemistry and biochemistry (both as a reactive and signaling species) in the context of cigarette smoking, taking into account both endogenous and exogenous (environmental, dietary, etc.) sources of H2O2. Particular attention is given to the methodological problems of monitoring smoke-associated H2O2. Our analysis is of prime interest for understanding the various mechanisms of smoke-induced oxidative stress and for developing rational approaches for diminishing the risk factors for human health associated with cigarette smoking.

Phenomenological features of chemiluminescence in manganese(III) reduction reactions

Chemiluminescence is generated in the reduction reactions of Mn(III) in sulfuric acid solutions; however, it is absent during the reduction of Mn(III) with iron(II) sulfate or potassium bromide. The dependences of the light yield and the kinetic parameters of chemiluminescence upon the concentration of reagents have been determined for the reactions of Mn(III) reduction with malonic acid. The rate of Mn(III) consumption during the light pathway of the reaction (i.e., reaction that results in chemiexcitation) depends on the squared Mn(III) concentration. The light pathway loses in competition with the dark pathway. The chemiexcitation of Mn2+ follows either one- or two-electron mechanism, but the complex character of the reaction does not allow making selection between these two routes.

Role of chemiexcited particles in permanganate reduction by citric acid: Investigation with spectrophotometric and chemiluminescence methods

Potassium permanganate reduction by citric acid in the presence of sulfuric acid includes stages of chemigeneration of electronically excited Mn(II) and emission of photons by it. The electronic absorption, reactant concentration, and chemiluminescence kinetics have been investigated. It has been shown that the chemiexcited species Mn(II) (chemiluminescence emitter) acts as a permanganate reduction catalyst.

Extremely Efficient Antioxidants Evolved into Environment from Aging Polymeric Materials: Chemiluminescent Monitoring

The paper describes a novel phenomenon: the evolution of very efficient acceptors of peroxyl radicals from aging polymeric materials into the gaseous phase. A chemiluminescence method for detection and determination of these “technogenic” antioxidants and for evaluation of their kinetic parameters is outlined and discussed. The method is based on the competition between the self-reaction of peroxyl radicals giving rise to light and scavenging the peroxyls by the antioxidants resulting in quenching. The rate constant for the reaction of peroxyls with the technogenic antioxidants is very high, approximately 109 l/(mol·s), which is close to the diffusion-limited rate and may mean that these species are free radicals (probably with bulky substituents at a partly delocalized electron center) as the reactions of peroxyls with molecules are known to be much slower. Acceptors have been detected in the ambient air: the probable sources are polymeric fabrics, parts, waste, etc. in the environment. The concentration of technogenic antioxidants may reach 10−8 – 10−7 mol/l in the air. It decreases on breathing: the antioxidants penetrate from the air into tissues and are able to suppress the free-radical oxidation of biological membrane lipids, thereby correcting the excitability level of the central nervous system and modifying behavior of experimental animals.