Investigation of effects produced by 26 various phenol and diphenol derivatives, including industrial and natural antioxidants (ionol, bis-phenol 2246, alpha-tocopherol), on final product yields of radiation-induced free-radical processes involving peroxyl, alkyl, alpha-hydroxyalkyl and alpha,beta-dihydroxyalkyl radicals has been performed. Ionol and bis-phenol 2246 have been shown to be more effective than alpha-tocopherol or diphenol derivatives in suppressing hydrocarbon oxidation processes. At the same time, alpha-tocopherol and its water-soluble analogues, as well as diphenol-based substances, are more effective than phenol derivatives in regulating various homolytic processes involving carbon-centered radicals. This fact can be accounted for by taking into consideration the contribution to formation of the final product set and the respective yields made by semiquinone radicals and compounds with quinoid structure arising in the course of homolytic transformations in systems containing diphenol derivatives.
In the course of radiation-induced free-radical transformations of hexane and ethanol, pyrocatechol and hydroquinone derivatives, as well as their respective quinones, are more effective than phenol and resorcinol derivatives in controlling reactions that involve alkyl and hydroxyalkyl radicals. The opposite result takes place in the inhibition by phenols of hexane oxidation in which the key role belongs to the peroxyl radicals generated from the starting compounds.Phenol, di-and polyphenol derivatives are widely known industrial [1] and natural [2] antioxidants, which brings interest in their properties. Considerable attention has been given to correlations between the structure of phenolic compounds and their antioxidant activity. It is commonly accepted that the ability of phenols to inhibit oxidation of organic compounds is determined by their high reactivity toward peroxyl (ROO . ) and other oxygen-centered radicals [1, 2].The main concept we develop in our studies is that processes involving ROO . and other reactive oxygen species are not the only ones responsible for damage of biosystems and industrial materials. Specifically, it was shown that fragmentation of carbon-centered radicals can cause destruction and modification of lipids [3,4], peptides [5], carbohydrates [6], and other biologically important molecules. Peroxide oxidation of synthetic and natural objects, too, begins with generation of carbon-centered radicals from the starting materials. Reactions of carbon-centered radicals with phenolic compounds have scarcely been studied [1,7]. In this connection we have performed a comparative assessment of the reactivity of phenol, di-, and polyphenol derivatives toward oxygen-and carbon-centered radicals [8,9].In the present work we studied the effect of sterically congested phenolic compounds on radiochemical transformations of hexane and ethanol. The resulting data allow us to judge about the reactivity of the compounds studied toward oxygen-and carboncentered organic radicals.It is known that radiolysis of hexane involve reactions leading to hexyl radicals [10] whose recombination gives dodecanes [schemes (1)3(6)].
Effects of curcumin and related compounds on product formation in radiolysis of aerated and deaerated ethanol were studied. Ab initio calculations of enthalpy values relating to O-H bond dissociation and H-atom addition to > C = O bonds of the compounds under study have been performed. The obtained data allowed the conclusion that the presence of a 7-carbon chain containing conjugated > C = C < and > C = O bonds in the structures of curcumin and its analogues makes these compounds capable of inhibiting the reactions involving α-hydroxyl-containing carbon-centered radicals. This finding broadens the existing views concerning radical-regulating properties of curcuminoids, and it should be taken into account when practical use of these compounds is envisaged.
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