title: Examination of the chemical behavior of the quercetin radical cation towards some bases.
abstract: It has been generally accepted that, due to high ionization potential values, single electron transfer followed by proton transfer (SET-PT) is not a plausible mechanism of antioxidant action in flavonoids. In this paper the SET-PT mechanism of quercetin (Q) was examined by revealing possible reaction paths of the once formed quercetin radical cation (Q(+)˙) at the M0-52X/6311+G(d,p) level of theory. The deprotonation of Q(+)˙ was simulated by examining its chemical behavior in the presence of three bases: methylamine (representative of neutral bases), the MeS anion (CH3S(-)) and the hydroxide anion (representative of anionic bases). It was found that Q(+)˙ will spontaneously be transformed into Q in the presence of bases whose HOMO energies are higher than the SOMO energy of Q(+)˙ in a given medium, implying that Q cannot undergo the SET-PT mechanism in such an environment. In the reaction with the MeS anion in both gaseous and aqueous phases and the hydroxide anion in the gaseous phase Q(+)˙ accepts an electron from the base, and so-formed Q undergoes the hydrogen atom transfer mechanism. On the other hand, SET-PT is a plausible mechanism of Q in the presence of bases whose HOMO energies are lower than the SOMO energy of Q(+)˙ in a given medium. In such cases Q(+)˙ spontaneously donates a proton to the base, with energetic stabilization of the system. Our investigation showed that Q conforms to the SET-PT mechanism in the presence of methylamine, in both gaseous and aqueous phases, and in the presence of the hydroxide anion, in the aqueous solution.
authors.affiliation: Department of Chemical-Technological Sciences, State University of Novi Pazar, Novi Pazar, Serbia. zmarkovic@np.ac.rs
authors.forename: Zoran
authors.initials: Z
authors.lastname: Marković
authors.forename: Dragan
authors.initials: D
authors.lastname: Amić
authors.forename: Dejan
authors.initials: D
authors.lastname: Milenković
authors.forename: Jasmina M
authors.initials: JM
authors.lastname: Dimitrić-Marković
authors.forename: Svetlana
authors.initials: S
authors.lastname: Marković
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D006878:Hydroxides / Q000737:chemistry*; D008744:Methylamines / Q000737:chemistry*; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D010084:Oxidation-Reduction; D011794:Quercetin / Q000737:chemistry*
chemical_list: D000975:Antioxidants; D002412:Cations; D005609:Free Radicals; D006878:Hydroxides; D008744:Methylamines; C031356:hydroxide ion; D011794:Quercetin; C027451:methylamine