Side Yao

Study on the photo-generation of superoxide radicals in Photosystem II with EPR spin trapping techniques

Direct EPR evidence of the photo-generation of superoxide radicals (O2−.) was obtained by using a novel spin trapping probe in spinach Photosystem II (PS II) membrane fragments. The production of O2−. was detected by following the formation of 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO) superoxide adducts (DEPMPO-OOH). The inhibition of O2−. formation by 3-(3,4-dichlorophenyl) -1,1-dimethylurea (DCMU) and the 77 K fluorescence spectrum indicated that O2−. were generated from PS II, not from PS I. The inhibition of O2−. formation by DCMU also suggested that O2−. were generated from the QBbinding site, not at a site prior to DCMU blockage. The extrinsic proteins and Mn are very important to eliminate O2−., showing that the oxygen-evolving system is involved in O2−. removal rather than production.

Photooxidative degradation mechanism of model compounds of poly(p-phenylenevinylenes) (PPVs)

Abstract Seven model compounds of poly( p -phenylenevinylenes) [M-PPVs] were designed to perform the photooxidative degradation experiments. The degradation rates of M-PPVs depend on the nature, number, and position of substituents. The transient species including singlet oxygen, superoxide radical anion, M-PPV triplet state, M-PPV radical cation and anion in M-PPV photodegradation were observed and characterized systematically with ESR spin trapping, pulse radiolysis, and laser flash photolysis techniques. By using rose bengal, methylene blue, fullerene, tetraphenyl porphyrin, 9,10-dicyanoanthracene, or biacetyl as sensitizer in dye-sensitized M-PPV photodegradation experiments, we found that singlet oxygen does not play the principal role as previous reported. Results of triplet–triplet energy transfer from biacetyl to M-PPVs and M-PPV triplet state quenching by 1,4-diazabicyclo[2.2.2] octane (DABCO) suggested that the reaction between M-PPV triplet state and oxygen is really the key step in the processes of photodegradation of the conjugated structure.

Electron transfer in colloidal TiO2 semiconductors sensitized by hypocrellin A

The electron transfer from singlet states of hypocrellin A (HA) to colloidal TiO2 nanometer-sized particles has been examined by absorption, fluorescence quenching, fluorescence lifetime measurements, laser flash photolysis and pulse radiolysis techniques. Adsorption of HA onto the surface of TiO2 particles extended its absorption spectrum further into the visible region, and the apparent association constant (K-app) for the association between HA and colloidal TiO2 was 3600 (mol/l)(-1) determined by fluorescence quenching method. Fluorescence lifetime measurement was used to elucidate the process of electron transfer from the singlet state of HA to conduction band of TiO2 (K-et = 4.26 X 10(9) s(-1)). Laser flash photolysis and pulse radiolysis studies demonstrated formation of the radical cation of HA

Fast repair of deoxynucleotide radical cations by phenylpropanoid glycosides (PPGs) and their analogs.

The repair effects on deoxynucleotide radical cations of phenylpropanoid glycosides (PPGs) and their analogs, isolated from a Chinese folk medicinal herb, were studied using the pulse radiolysis technique. The radical cations of deoxynucleotides were formed by the reaction of SO4*- with deoxynucleotides. On pulse irradiation of a nitrogen saturated deoxynucleotide aqueous solution containing 20 mM K2S2O8, 200 mM t-BuOH and one of the PPGs or their analogs, the transient absorption spectra of the radical cations of nucleotide decayed with the formation of those of the radical cation of PPGs or their analogs within several tens of microseconds after electron pulse irradiation. The result indicates that deoxynucleotide radical cations can be repaired by PPGs or their analogs. The rate constants of the repair reactions were determined to be 0.48-1.1 x 10(9), 0.64-1.80 x 10(9) and 2.12-4.4 x 10(9) M(-1) s(-1) for dAMP, dGMP and dCMP radical cations respectively. It is obvious that the rate constants of the repair reaction depend on the number of phenolic hydroxyl groups contained in the PPGs and their analogs. A deeper understanding of this new repair mechanism will undoubtedly help researchers design strategies to prevent and/or intervene more effective in free radical related diseases.

Fast repair of deoxythymidine radical anions by two polyphenols: rutin and quercetin

The effects of rutin and quercetin on the repair of the deoxythemindine radical anion (dT*) were studied using the technique of pulse radiolysis. The radical anion of dT was formed by the reaction of hydrated electron with dT. After pulse irradiation of nitrogen-saturated aqueous solutions containing dT, 0.2M t-BuOH and either rutin or quercetin, the initially formed dT*(-), detected spectrophotometrically, rapidly decayed with the concurrent formation of the radical anion of rutin or quercetin. The results indicated that dT*(-) can be rapidly repaired by rutin or quercetin. The rate constants of the repair reactions were determined to be 3.1 and 4.1 x 10(9)M(-1)s(-1) for rutin and quercetin, respectively. With substitution by glycosyl groups at C(3)-OH bond being neighbor to C(4) keto group, which is the active site for electron transfer, rutin has a lower repair reaction rate constant toward dT*(-) than quercetin. Together with findings from our previous studies, the present results demonstrated that nonenzymatic fast repair may be a universal form of repair involving phenolic antioxidants.

Triplet state mechanism for electron transfer oxidation of DNA

The interaction of anthraquinone-2-sulfonate with nucleotides and DNA in acetonitrile and acetonitrile water solvent mixture have been studied using KrF laser photolysis aimed at elucidation of the reaction mechanism. Laser spectroscopy directly demonstrates that the initial species from interaction of anthraquinone-2-sulfonate with nucleotides are radical cations of nucleotides and radical anion of anthraquinone-2-sulfonate. In addition, formation of ion pair from interaction of any of nucleotides with anthraquinone-2-sulfonate is synchronous with decay of triplet anthraquinone-2-sulfonate, which has provided dynamic evidence for initiation of electron transfer from DNA bases to triplet anthraquinone-2-sulfonate. Moreover, direct observation of stabilized DNA guanyl radical cation from interaction of anthraquinone-2-sulfonate with DNA has provided initial evidence for selective cleavage of DNA at guanine moiety. The solvent-separated ion pairs in acetonitrile have evidently dissociated into free ions, thereby enabling independent study of the behavior of guanyl radical cations and radical anion of anthraquinone-2-sulfonate.

Photophysical and photochemical processes of riboflavin (vitamin B2) by means of the transient absorption spectra in aqueous solution

Using time-resolved techniques of 337 and 248 nm laser flash photolysis, the photo physical and photochemical processes of riboflavin (RF, vitamin B2) were studied in detail in aqueous solution. The excited triplet state of riboflavin (3RF*) was produced with 337 nm laser, while under 248 nm irradiation, both3RF* and hydrated electron (eaq) formed from photoionization could be detected. Photobiological implications have been inferred on the basis of reactivity of3RF* including energy transfer, electron transfer and hydrogen abstraction. The RF·+ was generated by oxidation of SO4·-radical with the aim of confirming the results of photolysis.

Kinetic observation of rapid electron transfer between pyrimidine electron adducts and sensitizers of riboflavin, flavin adenine dinucleotide (FAD) and chloranil: a pulse radiolysis study

Abstract Rapid electron transfer from electron adducts of pyrimidine nucleotides dCMP, TMP and UMP to sensitizers of riboflavin (RF), flavin adenine dinucleotide (FAD) and chloranil (CHL) were studied using the techniques of pulse radiolysis. Both spectroscopic and kinetic analyses showed that the transient absorption spectra of pyrimidine electron adducts formed at first, and then were replaced by those of radical anions of RF, FAD and CHL after several microseconds of electron beam pulse, which indicated that electron transfer from electron adducts of pyrimidines to RF, FAD and CHL occurred. From buildup kinetics of sensitizer radical anions, a series of rate constants for electron transfer are determined respectively. RF and FAD can capture electron from pyrimidine electron adducts induced in irradiated cell, which would reduce possible repair of damaged targets due to the inhibition of charge recombination of radical ion pair. Thereby RF and FAD might play the role of radio-sensitizer in DNA damage process.

Studies on reactions of oxidizing sulfur–sulfur three-electron-bond complexes and reducing α-amino radicals derived from OH reaction with methionine in aqueous solution

The technique of pulse radiolysis with spectrophotometric detection has been used to investigate the possibility of electron transfer reactions between oxidizing sulfur-sulfur three-electron-bond complexes (Met2/S thereforeS+), or reducing alpha-amino radicals (CH3SCH2CH2CH.NH2) derived from reaction of methionine with OH radicals and hydroxycinnamic acid (HCA) derivatives, riboflavin (RF) or flavin adenine dinucleotide (FAD), respectively. The HCA derivatives, such as caffeic acid, ferulic acid, sinapic acid and chlorogenic acid, widely distributed phenolic acids in fruit and vegetables, have been identified as good antioxidants previously can rapidly and efficiently repair oxidizing three-electron-bond complexes via electron transfer. RF and FAD can oxidize reducing alpha-amino radicals derived from methionine. The electron transfer rate constants approximately 10(9) dm3 x mol(-1)x s(-1) were determined by following the build-up kinetics of species produced.

Photooxidation of 2′-deoxyguanosine 5′-monophosphate (dGMP) by flavin adenine dinucleotide (FAD) via electron transfer: a laser photolysis study

Abstract The study of the photosensitization mechanism of dGMP by flavin adenine dinucleotide (FAD) as sensitizer was carried out by time-resolved 308 and 248 nm laser flash photolysis. Direct evidence of electron transfer from dGMP to either triplet state of FAD ( 3 FAD * ) or oxidized FAD radical (FAD +/FAD(−H) ) was obtained. A comparison of nucleotide-free and -bound riboflavin suggests that the nucleotide environment does not markedly influence sensitizer reactivity.