Kinetics of curcumin oxidation by 2,2-diphenyl-1-picrylhydrazyl (DPPH˙): an interesting case of separated coupled proton-electron transfer
| Autor: | Vessela D. Kancheva, Mario C. Foti, Adriana Slavova-Kazakova, Concetta Rocco |
|---|---|
| Rok vydání: | 2016 |
| Předmět: |
Curcumin
Proton Kinetics Ether Electrons dpph 010402 general chemistry Photochemistry 01 natural sciences Biochemistry Medicinal chemistry Chemical kinetics Electron Transport chemistry.chemical_compound Electron transfer Reaction rate constant Picrates Moiety Physical and Theoretical Chemistry Molecular Structure 010405 organic chemistry Chemistry Organic Chemistry Biphenyl Compounds radicals 0104 chemical sciences Solvent Solvents Protons Oxidation-Reduction |
| Zdroj: | Organic & biomolecular chemistry 14 (2016): 8331–8337. doi:10.1039/c6ob01439a info:cnr-pdr/source/autori:Foti, Mario C.; Stavova-Kazakova, Adriana; Rocco, Concetta; Kancheva, Vessela D./titolo:Kinetics of curcumin oxidation by 2,2-diphenyl-1-picrylhydrazyl (DPPH center dot): an interesting case of separated coupled proton-electron transfer/doi:10.1039%2Fc6ob01439a/rivista:Organic & biomolecular chemistry/anno:2016/pagina_da:8331/pagina_a:8337/intervallo_pagine:8331–8337/volume:14 |
| ISSN: | 1477-0539 |
| DOI: | 10.1039/c6ob01439a |
| Popis: | The decay of dpph(center dot) in absolute ethanol at 25 degrees C and in the presence of curcumin (1), 4-methylcurcumin (3), 4,4-dimethylcurcumin (4) or curcumin 4'-methyl ether (5) follows bi-exponential kinetics. These unusual reaction kinetics are compatible with a two-step process in which an intermediate accumulates in a reversible first step followed by an irreversible process. As in other similar cases (Foti et al., Org. Lett., 2011, 13, 4826-4829), we have hypothesised that the intermediate is a pi-stacked complex, formed between one curcumin anion (in the case of 1, 3 and 5 the enolate anion) and the picryl moiety of dpph(center dot), in which an intra-complex electron transfer from the (enolate) anion takes place. By comparing the kinetics of curcumin 4',4 ''-dimethyl ether (2) (no phenolic OH), (5) (one phenolic OH) and (1) (two phenolic OHs), we have deduced that the electron transfer process must be accompanied by a simultaneous proton transfer from the phenolic OHs to the bulk solvent (separated coupled proton-electron transfer). The rate constants k(alpha) for the forward reaction of 2, 5 and 1 with dpph(center dot) are in fact similar to 0.7.5 x 10(3) and 1.8 x 10(4) M-1 s(-1), respectively, in a clear dependence on the number of phenolic OHs. |
| Databáze: | OpenAIRE |
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