Si-De Yao

Riboflavin (VB2) photosensitized oxidation of 2′-deoxyguanosine-5′-monophosphate (dGMP) in aqueous solution: a transient intermediates study

The use of time-resolved 337 nm and 248 nm laser flash photolysis with transient absorbance detection has shown direct evidence of electron transfer from dGMP to the triplet state of riboflavin (3RF*). dGMP was used as a DNA model system in order to study the damaging potential of photoexcited riboflavin. The evidence obtained was that: (1) formation of radical anion of riboflavin (RF•−/RFH•) matched the decay of 3RF*; (2) the decay of 3RF* was pseudo-first-order with the concentrations of dGMP, the bimolecular reaction rate constant was determined to be 6.6 × 108 dm3 mol−1 s−1; (3) after the complete decay of 3RF*, the transient absorption spectra of the deprotonated radical cation of dGMP [dGMP(-H)•] was observed in aerated condition for the first time; (4) the addition of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) to the experimental system resulted in the formation of the TMPD radical cation via scavenging dGMP radical cation; and (5) the free energy change (ΔG) from dGMP to 3RF* was calculated to be −43.7 kJ mol−1 with the Rehm–Weller equation, which indicates that photo-oxidation of dGMP attacked by triplet state of riboflavin is thermodynamically possible. Electron-transfer from dGMP to the oxidized radicals of riboflavin is found, which may be another pathway of DNA damage in vivo and in vitro. The direct observation of oxidized guanine radical has provided unambiguously direct initial proof for a photosensitization process of dGMP, with riboflavin as photosensitizer.

Generation and photosensitization properties of the oxidized radical of riboflavin: a laser flash photolysis study

Abstract Using 248 nm laser flash photolysis, it has been demonstrated that the neutral oxidized radical of riboflavin (RF(–H) ) resulting from rapid deprotonation of the radical cation (RF  + ) can be generated via monophotonic ionization when it is directly excited with a 248 nm laser and via oxidation by the SO 4 − radical in aqueous solution. The RF(–H) or RF  + reacts with both purine nucleotides and pyrimidine nucleotides via electron transfer to give the radical cations of nucleotides. These results suggest that the reactions of oxidized riboflavin radical might be involved in DNA photosensitization in vivo and in vitro.

Antioxidative properties of hydroxycinnamic acid derivatives and a phenylpropanoid glycoside. A pulse radiolysis study

Abstract Spectral and redox properties of the phenoxyl radicals from hydroxycinnamic acid derivatives and one selected component of phenylpropanoid glycosides, verbascoside, were studied using pulse radiolysis techniques. On the basis of the pH dependence of phenoxyl radical absorptions, the p K a values for deprotonation of sinapic acid radical and ferulic acid radical are 4.9 and 5.2. The rate constants of one electron oxidation of those antioxidants by azide radical and bromide radical ion were determined at pH 7. The redox potentials of those antioxidants were determined as 0.59–0.71 V vs NHE at pH 7 with reference standard 4-methoxyphenol and resorcinol.

Studies on the fullerol of C60 in aqueous solution with laser photolysis and pulse radiolysis

Abstract Using time-resolved technique of laser photolysis, it has been demonstrated that fullerol of C 60 in aqueous solution, unlike some other fullerene derivatives (such as C 60 [C(COOEt) 2 ] m , C 60 (C 4 H 6 O), C 60 (C 3 H 7 N)), does not result in excited triplet state but in ionization via monophotonic process under UV light. Studies on the reaction of fullerol of C 60 with the primary irradiated products of water: ·OH, ·H and e aq − have been carried out by pulse radiolysis. Rate constants for the reactions with ·OH, ·H radical and e aq − have been determined by following the build-up kinetics of the species or the decay of hydrated electron observed at 650 nm respectively.

Fast repair of oxidizing oh radical adduct of dGMP by hydroxycinnamic acid derivatives. A pulse radiolytic study

Abstract Using the technique of pulse radiolysis, it has been demonstrated that the interaction of the oxidizing OH radical adduct of 2′-deoxyguanosine-5′-monophosphate (dGMP) with hydroxycinnamic acid derivatives proceeds via an electron transfer reaction. From buildup kinetics of radical species the rate constants of electron transfer from hydroxycinnamic acid derivatives to the oxidizing OH radical adduct of dGMP have been determined to be k ≈ (2–3) x 10 9 dm −3 mol −1 s −1 . The yields for oxidizing OH radical adduct of dGMP are determined to be G ≈ 2.8.

Interaction of hydroxycinnamic acid derivatives with the Cl3COO radical: A pulse radiolysis study

The electron transfer reactions between the trichloromethylperoxyl radical (Cl3COO*) and hydroxycinnamic acid derivatives, including chlorogenic acid, sinapic acid, caffeic acid, ferulic acid and 3,4-(methylenedioxy)cinnamic acid, have been studied by pulse radiolysis. The hydroxycinnamic acid derivatives, especially sinapic acid, are identified as good antioxidants for reduction of Cl3COO* via electron transfer reactions. From buildup kinetic analysis of phenoxyl radical, the rate constant for reaction of Cl3COO* with sinapic acid has been determined to be 8.2x10(7) dm3 mol(-1) s(-1), while the rate constants of electron transfer from other hydroxycinnamic acid derivatives to Cl3COO* were obtained to be about 2x10(7) dm3 mol(-1) s(-1). The reaction of 3,4-(methylenedioxy) cinnamic acid with Cl3COO* was investigated as an evidence for the electron transfer mechanism.

Characterization of reactive intermediates in laser photolysis of guanine and its derivatives using acetone as photosensitizer : the pH dependence

Abstract The pH effects on photochemical reactions of guanine (Gua), guanosine (Guo) and 2′-deoxyguanosine-5′-monophosphoric acid (dGMP) using acetone as a photosensitizer have been investigated by 248 nm laser flash photolysis. Laser-induced transient species have been characterized according to kinetic analysis and quenching experiments by Mn 2+ and O 2 . In acid aqueous solutions, the intermediates recorded are assigned to the excited triplet states and the dehydrogenated radicals of Gua, Guo and dGMP. The excited triplet states produced via triplet-triplet excitation transfer and the dehydrogenated radicals arising from electron transfer reactions have been identified for the first time. In alkaline aqueous solutions, transient absorption spectra of dehydrogenated radical anions have been observed, but no triplet states of Gua, Guo and dGMP could be detected. The kinetics of the formation and decay of the exciplex arising from interaction of 3 A ∗ and Gua[−H + ] − and its derivatives have been offered for the first time. However, in neutral aqueous solution the intermediates recorded are assigned to the excited triplet state and dehydrogenated radical of dGMP arising from energy transfer and an electron transfer reaction. The sites of dehydrogenation have been suggested. Futhermore, four sets of kinetic parameters of the triplet decay of Gua and its derivatives have also been determined.

A study on the laser flash photolysis of phenothiazine and its N-alkyl derivatives

Abstract Photophysical and photochemical properties of phenothiazine and N -alkylphenothiazines (alkyl = methyl, ethyl, isobutyl, phenyl, benzyl) in acetonitrile were studied by means of nanosecond laser flash photolysis. Transient absorption measurements showed that the excited triplet state, radical cation, neutral radical, dimeric radical cation and dimeric dication were formed during the photolysis. The kinetics of the formation and conversion of the transient species was investigated. The deprotonation of the phenothiazine radical cation (PTH + ) to give the neutral radical PT + was prevented by the presence of tetracyanoethylene (TCNE). The mechanism of the interaction of phenothiazines and TCNE is discussed.

Photodynamic action of actinomycin D: an EPR spin trapping study

Actinomycin D is one of the most widely studied anticancer antibiotic that binds to both double-stranded and single-stranded DNA, and this binding greatly enhances the DNA photosensitization. By use of electron paramagnetic resonance spin trapping techniques, both superoxide radical anion and the radical anion of actinomycin D were identified as important intermediates in the photodynamic process. A mechanism of electron transfer from a DNA base to excited actinomycin D was proposed. These novel findings may shed new light on future application of this drug in photodynamic therapy or cleavage of DNA in unique and controllable ways.

A kinetic study on the interaction of deprotonated purine radical cations with amino acids and model peptides

By use of pulse radiolysis techniques, the radical cations of purine nucleotides have been successfully produced by the SO4- ion oxidation. Time-resolved spectroscopic evidence is provided that the one-electron-oxidized radicals of dAMP and dGMP can be efficiently repaired by aromatic amino acids (including tyrosine and tryptophan) via electron transfer reaction. As a model peptide, Arg-Tyr-AcOH was also investigated with regard to its interaction with deprotonated purine radical cations. The rate constants of the electron transfer reactions were determined to be (1 approximately 5) x 10(8) dm(3) mol(-1) s(-1). These results suggest that the aromatic amino acids in DNA-associated proteins may play some role in electron transfer reactions through DNA.