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To elucidate the nitrogen dioxide radical-scavenging-activity difference of three hydroxycinnamic acid derivatives (HCAs), i.e., caffeic acid (= 3-(3,4-dihydroxyphenyl)prop-2-enoic acid; 1), ferulic acid (=3-(4-hydroxy-3-methoxyphenyl)prop-2-enoic acid; 2), and sinapic acid (= 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoic acid: 3), a combined density-functional-theory (DFT) method, labeled as B3LYP/6-31G(d)//AM1, was employed to calculate adiabatic ionization potentials (IPs) for parent molecules and anions derived by proton dissociation. It was found that not the IPs of the parent HCAs but those of the anions account well for the experimentally observed higher activity of caffeic acid as compared to those of the other two HCAs, suggesting that the antioxidant anion should be taken into consideration in the selection or rational design of novel antioxidants; this, however, was neglected in previous studies.

The adsorption of hypocrellin B (HB) on the surface of TiO2 colloidal particles and the electron transfer process from its singlet excited state to the conduction band of TiO2 were examined by absorption, fluorescence quenching. fluorescence lifetime measurements, laser flash photolysis and pulse radiolysis techniques. Adsorption of HB onto the surface of colloidal TiO2 particle led to red-shifted absorption, and the apparent association constant (K-app) was 3145 (mol/l)(-1) from fluorescence quenching. Exciting HB with visible light could induce electron transfer from its singlet excited state into conduction band of TiO2 (K-et = 4.93 x 10(9) s(-1)). Laser flash photolysis and pulse radiolysis studies demonstrated the formation of radical cation of HB. (C) 2001 Elsevier Science Ltd. All rights reserved.

The radical cations of naturally occurring furanochromones visnagin (VI) and khellin (KH) have been generated and identified for the first time by use of laser flash photolysis and pulse radiolysis techniques. The lifetimes of VI(.+) and KH(.+) are determined as approximately 6 and approximately 35 micros under these conditions, respectively. Direct 308-nm excitation of VI in aqueous buffer at physiological pH results in monophotonic photoionization to generate VI(.+), with a quantum yield of 0.075, which is much higher than that of 8-methoxypsoralen and KH under identical conditions. Though VI(.+) is a more powerful oxidant than KH(.+), both of them react with guanosine mononucleotide (k=1.2x10(9) and 3.8x10(7) dm(3) mol(-1) s(-1), respectively) via electron transfer to give the guanine radical cation. Furthermore, selective oxidation of guanine in single and double strand DNA by VI(.+) was also observed. These novel findings suggest that electron transfer reactions involving furanochromone radical cations may be of considerable importance in furanochromone photochemotherapy.

The photodegradation kinetics for some cyanine and merocyanine dyes was studied by UV-visible spectroscopy and the results show that the fading process follows quasi-first-order or zero-order kinetics in acetonitrile. In other experiments, the principal photodegradation products of the dyes were identified with the aid of GC/MS, and it was found that those cyanine dyes holding a positive charge had higher photostability than the corresponding merocyanines. The relationship between photostability and chemical structure was established using PM3 and AMI MO calculations. Experimental results in this regard suggest that the species associated with the photodegradation of cyanine dyes may be the semioxidized dye free radical cation, Dye(dagger). The transient absorption spectra of Dye(dagger) were determined by nanosecond pump and probe spectroscopy. (C) 2001 Elsevier Science Ltd. All rights reserved.

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.

The triplet properties of pheophorbide a (PPa) have been investigated in benzene using nanosecond laser flash photolysis. The absorption maximum at 460 nm, which decays with a lifetime of 9.1 mu s, has been assigned to the triplet-triplet (T-T) absorption of PPa. The extinction coefficient of this absorption is 75,900 dm(3) mol(-1) cm(-1) and the triplet quantum yield is 0.65+/-0.03 (versus benzophenone). (C) 2000 Elsevier Science Ltd. All rights reserved.

The reactions of etoposide (VP 16, 4′-demethyl-epipodophyllotoxin ethylidene- β - d -glucoside) and 4′-demethylepipodophyllotoxin (DMEP) with the primary radiolytic products of water, such as e − aq , H and OH/O − radicals, and the secondary radical (SO − 4 ) in aqueous solution were studied by use of the techniques of pulse radiolysis, respectively. The absorption spectra of reaction products with e − aq , H and OH/O − and SO − 4 radicals were observed, and the rate constants of them were determined by following the build-up kinetics of radicals produced or the decay of hydrated electron observed at 600 nm, respectively.

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.

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.

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.

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.

Podophyllotoxin and its derivatives are very light (UV) sensitive, and can be readily photo-ionized to produce some transient species, such as radical cation, radical anion and superoxide anion radical. The laser flash photolysis of podophyllotoxin and its derivatives has been performed for the first time. The mechanism of photo-ionization has been confirmed and illustrated. In addition, the related reaction rate constants and molar extinction coefficients of the transient species have been determined.

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.

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.

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.

Photoionization of 2′-deoxyguanosine 5′-monophosphoric acid (dGMP), polyadenylic-guanylic acid (poly[A,G]) and single-stranded DNA (ssDNA) in alkaline aqueous solution by 248 nm laser pulses was investigated. Unlike the reactions in neutral solution, the ionization at pH > 9 was found to be monophotonic. The transition from mono- to biphotonic ionization with increasing pH was shown to be associated with deprotonation of guanine and the corresponding decrease of ionization potential. In the case of Poly[A,G] and ssDNA, photoionization occurs at deprotonated guanine moieties, thus giving guanyl radicals. The quantum yields of formation of hydrated electrons ( Φ e −) were determined at room temperature with KI solutions used as a reference.

The interactions of triplet acetone with polyadenylic acid (Poly[A]), polyguanylic acid (Poly[G]), polyadenylic-guanylic acid (Poly[A,G]) and single-stranded DNA (ssDNA) were investigated in neutral aqueous solution using KrF (248 nm) laser flash photolysis. The transient absorption spectra and kinetics of DNA and polynucleotides obtained under acetone sensitization demonstrated that the predominant transient species was guanine radical. These novel findings have offered time-resolved evidence for photochemical modification of DNA and Poly[A,G] at guanine moiety.

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.

The effect of 248 nm laser light on alkaline aqueous solutions of guanine (Gua), guanosine (Guo), 2′-deoxyguanosine 5′-monophosphoric acid (dGMP), and polyguanylic acid (5′) (poly[G]) was investigated. Under these conditions monophotonic ionization was suggested to occur as inferred from the linear dependences of the concentration of ejected electrons. The quantum yields of formation of hydrated electrons ( Φ e − ) are therefore independent of the laser pulse intensities ( I L ), and were determined with KI solution used as a reference. On the basis of pH titration, it was confirmed that photoionization occurs on deprotonated Gua and its derivatives. As shown by comparison with our previous results obtained from the acetone-sensitized photolysis, the ionized Gua, Guo and dGMP anion, i.e. the neutral radicals, deprotonate in alkaline solution, yielding radical anions that are the same as those from acetone-sensitized excitation.

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.

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.

The photochromic reactions of five indolinospirooxazine derivatives ( II, III, V and VII) were examined using nanosecond laser flash photolysis techniques. Photolysis of II–V leads to the formation of long-lived photomerocyanines (PMCs), whereas the only detectable product in the photolysis of CII is a short-lived, charge-separated, twisted species (CT intermediate). The results show that the substituent in the 2′ position of the oxazine ring has a decisive role on the photochromic reaction products and all of the substituents influence the absorption maxima of the photoproducts of indolinospirooxazines. A proposed potential energy surface model has been established to explain these phenomena. Moreover, the characteristics of the absorption spectra of the PMCs and CT intermediate are discussed.

The photochromic process of (1,4-diphenyl-2-methyl-3-pyrryl)-ethylidene (isopropylidene) succinic anhydride (I) has been examined with nanosecond laser flash photolysis techniques in cyclohexane and acetonitrile respectively. A new transient species was found in the nanosecond time domain. The presence of oxygen was shown to be the important factor in elucidating the mechanism of photocyclization of compound I. The maximum absorption wavelength and lifetime of the transient species are solvent dependent. Both the excited singlet state and the excited triplet state are involved in the photocyclization process.

Laser flash photolysis study was carried out for the first time in the binary mixture of room-temperature ionic liquid (RTIL) and traditional organic solvent. Photochemical properties of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and its binary mixed solutions with MeCN were investigated by laser photolysis technique at an excitation wavelength of 355 nm, using anthraquinone (AQ) as a probe molecule. It was indicated that [bmim] [PF6] itself participates in the reaction with 3AQ*. Moreover, along with the change in the ratio of RTIL to organic solvent, the decay reaction rate constant of 3AQ* changes regularly and a critical point was observed at chiRTIL = 0.06. Before the critical point, the decay rate constant increases rapidly with increasing chiRTIL; however, it decreases obviously with increasing chiRTIL after the critical point. The kinetic data are given, and the possible reaction mechanisms in mixed solutions are also speculated.

Choosing suitable gene carrier is very important in gene therapy. Recently, polyethylenimine (PEI), a new polycation compound, is particularly attractive due to its high transduction efficiency and low toxicity. This study was to synthesize a series of PEI nanogels of different particle diameters by photochemistry, investigate the relation between the transfection efficiency and particle diameter, and screen ideal gene carrier.PEI nanogels were synthesized by photochemistry. The particle diameter was detected by photo correlation spectroscopy (PCS). The spherical morphology of the nanogels was characterized by scanning electron microscopy (SEM), and confirmed by atomic force microscopy (AFM). Using PEI/DNA complex as a plasmid vector, the enhanced green fluorescence protein (EGFP) gene was transferred into Bel7402 and A549 cells. Gene expression was quantitatively evaluated by fluorescent microscopy and flow cytometry (FCM).The diameter of synthesized PEI nanogels was in the range of 80-200 nm, and most of them were globular. The delivery rate reached the maximum when using 4 microg PEI (86.9 nm) to transfer 2 microg EGFP: the delivery rates were (32.75+/-1.01)% for Bel7402 cells and (29.81+/-1.84)% for A549 cells when detected by fluorescent microscopy, and were (32.40+/-1.41)% for Bel7402 cells and (30.00+/-1.86)% for A549 cell when detected by FCM. There was no significant difference between PEI and LipofectamineTM 2000 in the transfection efficiency (P0.05).PEI nanogels synthesized by photochemistry are effective nonviral vectors for gene delivery into human tumor cells in vitro. The transfection efficiency of 86.9 nm PEI is the highest.

In the present paper the photo-ionization and properties of excited state of benzoic acid and its methyl substitutes were studied using 248 nm laser flash photolysis. The quantum yields of hydrated electrons produced from photo-ionization, the lifetime of excited triplet, and the self quenching rate constants were also determined. It is shown that there isn't significant effect of different position of methyl residue at benzene ring on its excited triplet. The resonant excitation of triplet of benzoic acid was explored by the use of two-steps laser flash photolysis. The possible upper excited state and radical involved in the processes were guessed.

The reactions of *OH and e(aq)-, with AO7 in aqueous solutions were studied by nano-second pulse radiolysis techniques. The transient absorption spectra were attributed and the second order reaction rate constants were determined by following the buildup kinetics of the transient absorptions. In combination with the analysis of the UV-Vis absorption changes of AO7 after gamma-radiolysis, the underlying reaction mechanisms were elucidated.

Free radical reactions of Martynoside (MAR), a phenylpropanoid glycoside, with a variety of oxidants were studied in the aqueous solution by laser photolysis and pulse radiolysis techniques. The pK(a) value of MAR in aqueous solution was measured from the pH dependent changes of the UV absorption at 384 run with value of pK(a) = 9.2. The phenoxyl radical of MAR which exhibits maximum absorption at 360 nm was generated by one-electron transfer to N-3(.) or Br-2(.-). Other important properties of phenoxyl radical such as extinction coefficient, formation and decay rate constants were also determined. The reaction rate constant of O-2(.-) with MAR, k = 8.5 x 10(4) dm(3) . mol(-1) . s(-1), was measured by the method of competition kinetics. By measuring time-resolved luminescence emission at 1270 nm, the quenching rate constant of singlet oxygen by MAR was obtained to be 3.3 x 10(6) dm(3) . mol(-1) . s(-1). Reduction potential of the MAR couple (MAR(.) /MAR), determined using rutin as reference compound, gave a value E=0.66V vs. NHE. The antioxidative properties of MAR were compared with those of some well-known antioxidants.