| Abstract
| - 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-acceptingabilities as the alcohols. The reactions of all three substrates are orders of magnitude faster in thealcohols, but these high rates can be suppressed to values essentially equal to those in the twonon-hydroxylic solvents by the addition of acetic acid. The fast reactions in alcohols are attributedto the reaction of dpph• with the CU, DHZ, and IEanions (see J. Org. Chem. 2003, 68, 3433), aprocess which we herein name sequential proton loss electron transfer (SPLET). The most acidicgroup in CU is the central keto−enol moiety. Following CU's ionization to a monoanion, ET fromthe [−(O)CCHC(O)−]- moiety to dpph• yields the neutral [−(O)CCHC(O)−]• radical moiety whichwill be strongly electron withdrawing. Consequently, a phenolic proton is quickly lost into the alcoholsolvent. The phenoxide anion so formed undergoes charge migration to produce a neutral phenoxylradical and the keto−enol anion, i.e., the same product as would be formed by a hydrogen atomtransfer (HAT) from the phenolic group of the CU monoanion. The SPLET process cannot occur ina nonionizing solvent. The controversy as to whether the central keto−enol moiety or the peripheralphenolic hydroxyl groups of CU are involved in its radical trapping (antioxidant) activity is thereforeresolved. In ionizing solvents, electron-deficient radicals will react with CU by a rapid SPLETprocess but in nonionizing solvents, or in the presence of acid, they will react by a slower HATprocess involving one of the phenolic hydroxyl groups.
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