[reaction: see structure] The antioxidant activity of curcumin (1, 7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) was determined by inhibition of controlled initiation of styrene oxidation. Synthetic nonphenolic curcuminoids exhibited no antioxidant activity; therefore, curcumin is a classical phenolic chain-breaking antioxidant, donating H atoms from the phenolic groups not the CH(2) group as has been suggested (Jovanovic et al. J. Am. Chem. Soc. 1999, 121, 9677). The antioxidant activities of o-methoxyphenols are decreased in hydrogen bond accepting media.

The H-atom donating activities of 2,6-di-tert-butyl-4-methylphenol (BHT), 2,6-di-tert-butyl-4-methoxyphenol (DBHA), 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC), and 3,5-di-tert-butylcatechol (DTBC) toward the nitrogen-centered 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical were measured by stopped flow methods in hexane, 1-propanol, tert-butyl alcohol, and acetone. Decreases in these activities on transferring from hexane to the hydrogen bond accepting (HBA) solvents, the kinetic solvent effect (KSE), are attributed to hydrogen bonding from the phenolic group. Steric hindrance accounts for a lower decrease observed for the highly hindered BHT and DBHA compared to PMHC. The catechol, DTBC, a very active H-atom donor to DPPH in hexane, showed a dramatic loss of activity in HBA solvents, especially acetone. Higher H-atom donating activities of BHT, DBHA, and PMHC were observed toward the oxygen-centered radical of 2,6-di-tert-butyl-4-(4‘-methoxyphenyl)phenoxyl (DBMP), and the decreases in activity in the HBA solvents paralleled those found with DPPH. Thus the KSE was found to be independent of the nature of the abstracting radical for DPPH and DBMP. The inhibition of the oxygen uptake (IOU) method was used to determine the antioxidant activities (k inh) of α-tocopherol, PMHC, catechol, and DTBC during free radical autoxidation of styrene and mixtures of styrene and tert-butyl alcohol. The k inh of α-tocopherol and PMHC dropped to one-tenth of the values with increasing tert-butyl alcohol content due to the HBA activity of the alcohol compared to styrene.

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=489bf6ea-c7ae-4f4f-a366-b0ff338d365a http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=489bf6ea-c7ae-4f4f-a366-b0ff338d365a 1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-toctylphenyl)-1-picrylhydrazyl radical (DOPPH• , or as antioxidants during inhibited styrene autoxidation, k ArOH/ROO• , initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-ditert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k ArOH/ROO• ) 6.0 × 10, of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O-H bond dissociation enthalpies compared to those of phenols, (∆BDEs), of 1-6 predict a HAT order of reactivity of 2 < 1 < 3 ≈ 4 < 5 < 6 in general agreement with kinetic results. Calculations on the diols show that intramolecular H-bonding stabilizes the radicals formed on H-atom transfer more than it does the parent diols, and this effect contributes to the increased HAT activity of 5 and 6 compared to the activities of the catechols. For example, the increased stabilization due to the intramolecular H-bond of 5 radical over 5 parent of 8.6 kcal/mol was about double that of 2 radical over 2 parent of 4.6 kcal/mol. Linear free energy plots of log k ArOH/DOPPH• and log k ArOH/ROO• versus ∆BDEs for compounds 1-6 along with available literature values for nonsterically hindered monophenols placed the compounds on common scales. The derived EvansPolanyi constants from the plots for the two reactions, R DOPPH • ) 0.48 >R ROO • ) 0.32, gave the expected order, since the ROO • reaction is more exothermic than the DOPPH • reaction. Compound 6 is sufficiently reactive to react directly with oxygen, and it lies off the log k ArOH/ROO• versus ∆BDE plot.

No abstract