ABSTRACT
Electron-transfer disproportionation of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPHË) occurred in the presence of Sc(3+) acting as a strong Lewis acid in deaerated acetonitrile. In contrast, in the case of weaker Lewis acids than Sc(3+), such as Mg(2+) and Li(+), external protons from acetic acid were required for the disproportionation of DPPHË to occur.
Subject(s)
Biphenyl Compounds/chemistry , Mesylates/chemistry , Picrates/chemistry , Reactive Oxygen Species/chemistry , Scandium/chemistry , Acetic Acid/chemistry , Acetonitriles/chemistry , Catalysis , Lewis Acids/chemistry , Magnesium Compounds/chemistry , Perchlorates/chemistryABSTRACT
Superoxide ion (O2Ë-) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)(3)(3+)], which can be detected in solution by ESR spectroscopy. Electron transfer from O2Ë- -Sc(HMPA)(3)(3+) complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)(3)(3+) to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)(3)(3+) complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)(3)(3+) depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)(3)(3+)-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)(3)(3+)-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)(3)(3+) to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)(3)(3+). The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)(3)(3+) occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)(3)(3+) depending the number of Sc(HMPA)(3)(3+) (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.
Subject(s)
Benzoquinones/chemistry , Scandium/chemistry , Superoxides/chemistry , Coordination Complexes/chemistry , Electron Spin Resonance Spectroscopy , Electron Transport , Ions/chemistry , KineticsABSTRACT
UV-B photoirradiation of a neurotransmitter (serotonin) and aromatic amino acids (tryptophan and tyrosine) with oxygen results in DNA cleavage by generation of reactive oxygen species (ROS) as demonstrated by agarose gel electrophoresis with pBR 322 DNA, ESR and laser flash photolysis measurements.
Subject(s)
DNA Cleavage/radiation effects , Oxygen/metabolism , Reactive Oxygen Species/metabolism , Serotonin/metabolism , Tryptophan/metabolism , Tyrosine/metabolism , Photochemical Processes , Ultraviolet RaysABSTRACT
Neurotransmitters such as catecholamines (dopamine, L-dopa, epinephrine, norepinephrine) have phenol structure and scavenge reactive oxygen species (ROS) by hydrogen atom transfer (HAT) to ROS. Radical scavenging reactivity of neurotransmitters with galvinoxyl radical (GO*) and cumyloxyl radical (RO*) in acetonitrile at 298 K was determined by the UV-vis spectral change. The UV-vis spectral change for HAT from catecholamine neurotransmitters to GO* was measured by a photodiode array spectrophotometer, whereas HAT to much more reactive cumylperoxyl radical, which was produced by photoirradiation of dicumyl peroxide, was measured by laser flash photolysis. The second-order rate constants (k(GO)) were determined from the slopes of linear plots of the pseudo-first-order rate constants vs concentrations of neurotransmitters. The k(GO) value of hydrogen transfer from dopamine to GO* was determined to be 23 M(-1) s(-1), which is the largest among examined catecholamine neurotransmitters. This value is comparable to the value of a well-known antioxidant: (+)-catechine (27 M(-1) s(-1)). The k(GO) value of hydrogen transfer from dopamine to GO* increased in the presence of Mg(2+) with increasing concentration of Mg(2+). Such enhancement of the radical scavenging reactivity may result from the metal ion-promoted electron transfer from dopamine to the galvinoxyl radical. Inhibition of DNA cleavage with neurotransmitters was also examined using agarose gel electrophoresis of an aqueous solution containing pBR322 DNA, NADH, and catecholamine neurotransmitters under photoirradiation. DNA cleavage was significantly inhibited by the presence of catecholamine neurotransmitters that can scavenge hydroperoxyl radicals produced under photoirradiation of an aerated aqueous solution of NADH. The inhibition effect of dopamine on DNA cleavage was enhanced by the presence of Mg(2+) because of the enhancement of the radical scavenging reactivity.
Subject(s)
Catecholamines/chemistry , DNA Cleavage/drug effects , Free Radical Scavengers/chemistry , Neurotransmitter Agents/chemistry , Benzyl Compounds/chemistry , Benzyl Compounds/metabolism , Catecholamines/pharmacology , Free Radical Scavengers/pharmacology , Kinetics , Neurotransmitter Agents/pharmacology , Photolysis , Reactive Oxygen Species/metabolismABSTRACT
More than two-fold augmentation in the radical-scavenging activity of artepillin C could be achieved via altering the O-H bond dissociation enthalpy of artepillin C by means of structural modifications.
Subject(s)
Free Radical Scavengers/chemistry , Phenylpropionates/chemistry , Benzene Derivatives/chemistry , Free Radical Scavengers/radiation effects , Molecular Structure , Peroxides/chemistry , Peroxides/radiation effects , Phenylpropionates/radiation effects , Photochemistry , Stereoisomerism , Thermodynamics , Time FactorsABSTRACT
The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH.) or galvinoxyl radical (GO.) by a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO4)2 in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.