Abnormal solvent effects on hydrogen atom abstraction. 2. Resolution of the curcumin antioxidant controversy. The role of sequential proton loss electron transfer

Download
  1. Get@NRC: Abnormal solvent effects on hydrogen atom abstraction. 2. Resolution of the curcumin antioxidant controversy. The role of sequential proton loss electron transfer (Opens in a new window)
DOIResolve DOI: http://doi.org/10.1021/jo049254j
AuthorSearch for: ; Search for:
TypeArticle
Journal titleJournal of Organic Chemistry
ISSN0022-3263
Volume69
Issue18
Pages58885896; # of pages: 9
SubjectHydrogen atom abstraction; Non hydroxylic solvents; Phenoxide anions; Acetic acid; Antioxidants; Aromatic compounds; Electron transitions; Ethanol; Hydrogen; Methanol; Negative ions; Organic solvents; 1,1 diphenyl 2 picrylhydrazyl; acetic acid ethyl ester; alcohol; antioxidant; curcumin; dioxane; hydrogen; methanol; radical; solvent; article; chemical reaction; chemical structure; electron transport; hydrogen bond; ionization; molecular interaction; Antioxidants; Curcumin; Electron Transport; Models, Chemical; Molecular Structure; Oxidation-Reduction; Picrates; Protons
AbstractThe 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.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC)
Peer reviewedYes
NPARC number21276790
Export citationExport as RIS
Report a correctionReport a correction
Record identifiere101d14b-402c-4d0e-8fd4-a45527d6c23e
Record created2015-10-13
Record modified2016-05-09
Bookmark and share
  • Share this page with Facebook (Opens in a new window)
  • Share this page with Twitter (Opens in a new window)
  • Share this page with Google+ (Opens in a new window)
  • Share this page with Delicious (Opens in a new window)