Nian-Yun Lin

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.

Reactive intermediates in laser photolysis of guanosine

The photochemical reactions of aqueous solution guanosine with acetone as photosensitizer have been investigated by laser flash photolysis. From detailed kinetic analysis, the reaction mechanism has been derived: dehydrogenated guanosine radical G(-H)* and triplet guanosine 3G* were produced via electron transfer and triplet-triplet energy transfer respectively. The half-life time of 3G* has been determined to be 7.4 μs, and the quenching rate constant of Mn2+ on it was obtained to be 1.9 × 108 M-1 s-1. Based on pH titration, the pKa value of 3G* was also obtained to be 8.7.

Interaction of phenolic antioxidants and hydroxyl radicals

Abstract Based on pulse radiolysis of aqueous solutions of four phenolic antioxidants including green tea polyphenols, quercetin, caffeic acid and sinapic acid the rate constants for reactions of OH and the antioxidants were determined. And green tea polyphenols and quercetin are the strongest antioxidants.

Generation of the C60 radical cation via electron transfer reaction. A pulse radiolysis study

Abstract Using pulse radiolysis techniques the C 60 radical cation has been generated for the first time via electron transfer from the triplet state of C 60 to CCl 4 or CCl 3 CF 3 . The radiation-induced electron transfer rate constant was determined to be 7.8 × 10 6 M −1 s −1 . A novel transient absorption peak at 650 nm of the C 60 cation was observed from pulse radiolysis of benzene containing C 60 and CCl 4 or CCl 3 CF 3 . The extinction coefficient of the C 60 cation at 650 nm was estimated to be ⩾ 4.6 × 10 3 M −1 cm −1 .

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.