Abnormal Solvent Effects on Hydrogen Atom Abstraction. 2. Resolution of the Curcumin Antioxidant Controversy. The Role of Sequential Proton Loss Electron Transfer
| Autor: | K. U. Ingold, Grzegorz Litwinienko |
|---|---|
| Rok vydání: | 2004 |
| Předmět: |
chemical reaction
antioxidant Curcumin dioxane chemical DPPH Radical Ethyl acetate electron transitions negative ions Hydrogen atom abstraction Photochemistry non hydroxylic solvents solvent Medicinal chemistry Antioxidants Electron Transport models Acetic acid chemistry.chemical_compound Electron transfer Picrates ionization hydrogen atom abstraction Moiety organic solvents acetic acid ethyl ester methanol radical hydrogen bond Molecular Structure alcohol aromatic compounds Biphenyl Compounds Organic Chemistry phenoxide anions acetic acid 1 1 diphenyl 2 picrylhydrazyl Models Chemical chemistry hydrogen molecular interaction chemical structure ethanol Protons Solvent effects Oxidation-Reduction |
| Zdroj: | The Journal of Organic Chemistry. 69:5888-5896 |
| ISSN: | 1520-6904 0022-3263 |
| Popis: | The rates of reaction of 1,1-diphenyl-2-picrylhydrazyl (dpph*) radicals with curcumin (CU, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione), dehydrozingerone (DHZ, "half-curcumin"), and isoeugenol (IE) have been measured in methanol and ethanol and in two non-hydroxylic solvents, dioxane and ethyl acetate, which have about the same hydrogen-bond-accepting abilities as the alcohols. The reactions of all three substrates are orders of magnitude faster in the alcohols, but these high rates can be suppressed to values essentially equal to those in the two non-hydroxylic solvents by the addition of acetic acid. The fast reactions in alcohols are attributed to the reaction of dpph* with the CU, DHZ, and IE anions (see J. Org. Chem. 2003, 68, 3433), a process which we herein name sequential proton loss electron transfer (SPLET). The most acidic group in CU is the central keto-enol moiety. Following CU's ionization to a monoanion, ET from the [-(O)CCHC(O)-](-) moiety to dpph* yields the neutral [-(O)CCHC(O)-]* radical moiety which will be strongly electron withdrawing. Consequently, a phenolic proton is quickly lost into the alcohol solvent. The phenoxide anion so formed undergoes charge migration to produce a neutral phenoxyl radical and the keto-enol anion, i.e., the same product as would be formed by a hydrogen atom transfer (HAT) from the phenolic group of the CU monoanion. The SPLET process cannot occur in a nonionizing solvent. The controversy as to whether the central keto-enol moiety or the peripheral phenolic hydroxyl groups of CU are involved in its radical trapping (antioxidant) activity is therefore resolved. In ionizing solvents, electron-deficient radicals will react with CU by a rapid SPLET process but in nonionizing solvents, or in the presence of acid, they will react by a slower HAT process involving one of the phenolic hydroxyl groups. |
| Databáze: | OpenAIRE |
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