Weizhen 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.

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.

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.

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.

MONOPHOTONIC IONIZATION OF ETOPOSIDE IN AQUEOUS SOLUTION BY 248 NM LASER LIGHT : IDENTIFICATION OF TRANSIENT INTERMEDIATES

Abstract Using 248 nm laser flash photolysis, it has been demonstrated that etoposide (VP16, 4′-demethyl-epipodophyllotoxin ethylidene-β-D-glucoside) in aqueous solution is ionized via a monophotonic process giving a hydrated electron and neutral radical that results from rapid deprotonation of the radical cation of etoposide. The quantum yield of photoionization is determined to be 0.17 at room temperature with KI solution used as a reference. On the basis of comparison of the result with that obtained from pulse radiolysis, the reaction mechanism has been unambiguously identified.

Comparative studies of triplet states of thymine components by acetone sensitization and direct excitation in aqueous solution at room temperature

The transient absorption spectra and the kinetic parameters of decay of the triplet states of thymine and its nucleoside, nucleotide, were obtained from acetone sensitization and direct excitation in aqueous solution at room temperature. The kinetic parameters from two methods are equivalent, thus time-resolved acetone sensitization method was verified by the laser photolysis experiment. Furthermore, advantages of acetone sensitization method were clarified by the characteristics of triplet acetone and enhanced yields of triplet states of thymine components.

Generation of C60 radical cation and radical adduct of dichloro- or monochloromethyl radical to C60: Pulse radiolysis and laser photolysis of C60 in polar chloromethane

Abstract The radical cation of C 60 is generated either via hole transfer from the radical cation of polar chlorinated methane to C 60 or via electron transfer from the triplet of C 60 to electrophilic polar chlorinated methane. The radical adducts CHCl 2 and CH 2 Cl of C 60 were produced via a similar mechanism to the generation of the radical adduct [C 60 -CCl 3 ] in CCl 4 .

Radical adduct of trichloromethyl to C70: A laser flash photolysis of C70 in carbon tetrachloride

Abstract The addition reaction of trichloromethyl radical to C 70 has been investigated by use of laser flash photolysis. The difference absorption spectrum of radical adduct, [C 70 CCl 3 ], exhibits broad absorption band in the visible region. The rate constant for reaction of trichloromethyl radical with C 70 is determined.