title: Kinetics of curcumin oxidation by 2,2-diphenyl-1-picrylhydrazyl (DPPH˙): an interesting case of separated coupled proton-electron transfer.
abstract: The decay of dpph˙ in absolute ethanol at 25 °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 π-stacked complex, formed between one curcumin anion (in the case of 1, 3 and 5 the enolate anion) and the picryl moiety of dpph˙, 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 kα for the forward reaction of 2, 5 and 1 with dpph˙ are in fact ∼0, 7.5 × 10(3) and 1.8 × 10(4) M(-1) s(-1), respectively, in a clear dependence on the number of phenolic OHs.
authors.affiliation: Istituto di Chimica Biomolecolare del CNR, Via P. Gaifami 18, I 95126 Catania, Italy. mario.foti@cnr.it.
authors.forename: Mario C
authors.initials: MC
authors.lastname: Foti
authors.forename: Adriana
authors.initials: A
authors.lastname: Slavova-Kazakova
authors.forename: Concetta
authors.initials: C
authors.lastname: Rocco
authors.forename: Vessela D
authors.initials: VD
authors.lastname: Kancheva
mesh_terms: D001713:Biphenyl Compounds / Q000737:chemistry*; D003474:Curcumin / Q000737:chemistry*; D004579:Electron Transport; D004583:Electrons; D007700:Kinetics; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010851:Picrates / Q000737:chemistry*; D011522:Protons; D012997:Solvents
chemical_list: D001713:Biphenyl Compounds; D010851:Picrates; D011522:Protons; D012997:Solvents; C004931:1,1-diphenyl-2-picrylhydrazyl; D003474:Curcumin