Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e^- _aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λ_max) at 330 and 590 nm. The transient species (pK_a = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e^{- aq} at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λ_max at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.     

Reactions of flavosemiquinone radicals in the presence of metal ions

The rate constants of disproportionation of flavosemiquinone radicals were obtained by pulsed spectroscopy. The yield of the flavosemiquinone radical increased when Mohr’s salt was introduced in the aqueous solutions of riboflavin. The spectral kinetic characteristics of complexes of flavosemiquinone radical anions with Zn²⁺ and Cd²⁺ ions were determined.     

DITHIONITE TREATMENT OF FLAVINS: SPECTRAL EVIDENCE FOR COVALENT ADDUCT FORMATION and EFFECT ON in vitro BACTERIAL BIOLUMINESCENCE

Intrigued by the apparent requirement of dithionite for FMN reduction (as opposed to photoreduction or catalytic hydrogenation) in the H2O2-initiated bacterial bioluminescence reaction, we chose 5-ethyl-3-methyllumiflavinium cation I as a model to investigate possible flavin adduct formation by treatment with dithionite or (bi)sulfite. In the range of pH 5-8, the reaction of dithionite with 5-ethyl-3-methyllumiflavinium cation, which is in equilibrium with the 5-ethyl-4a-hydroxy-3-methyl-4a, 5-dihydrolumiflavin pseudobase II (X = OH), is not limited to the formation of flavosemiquinone and dihydroflavin following two one-electron steps. Several parallel and sequential reactions may take place involving the intermediacy of covalent flavin adducts. Addition of (bi)sulfite gave a 4a-sulfiteflavin adduct II (X = SO3-). Consistent with the S2O4(2-) in equilibrium with 2 SO2-. equilibrium, the reaction of dithionite and II (X = OH; SO3-) gave rise to two flavin adducts in competitive nucleophilic displacements: a 4a-sulfoxylate-flavin radical (II, X = SO2.) and a 4a-dithioniteflavin adduct (II, X = S2O4-), respectively. On increasing the (S2O4(2-), SO2.-)/flavin ratio under N2, the formation of the 4a-sulfoxylate-flavin radical became predominant. The II (X = SO2.) so formed was in equilibrium with the flavosemiquinone and bisulfate and can be trapped by reacting with hydroxylamine. In the initial presence of oxygen, II (X = SO2.) was highly reactive toward O2, giving a fast oxidation to II (X = SO3-) and effectively suppressing the formation of the flavosemiquinone.(ABSTRACT TRUNCATED AT 250 WORDS)     

1,2-和1,4-萘醌248 nm激光光解的瞬态吸收光谱研究

利用纳秒级激光光解动态吸收光谱装置,研究了1,2-和1,4-萘醌中性水溶液的瞬态吸收光谱.发现1,2-萘醌及1,4-萘醌被光电离后形成的阳离子自由基在380nm均有最大吸收,但1,4-萘醌阳离子自由基在衰变过程中又形成了两种新的活性粒子,它们的最大吸收分别位于410和580nm,分析表明:410nm属于1,4-萘醌脱氢自由基的吸收,而580nm很可能归属由于电子转移而形成的瞬态产物.进一步研究发现,1,2-萘醌在中性水溶液中能被248nm激光单光子电离.     

Laser spectroscopic excitation function and reaction threshold studies of the H + DCl --> HCl + D gas-phase isotope exchange reaction

The dynamics of the gas-phase hydrogen atom exchange reaction H + DCl --> HCl + D were studied using the pulsed laser photolysis/laser induced fluorescence "pump-and-probe" method. Laser photolysis of H2S at 222 nm was used to generate nonequilibrium distributions of translationally excited hydrogen atoms at high dilution in a flowing moderator gas (Ar)/reagent (DCl) mixture. H and D atoms were detected with sub-Doppler resolution via Lyman-alpha laser induced fluorescence spectroscopy, which allowed the measurement of the line shapes of the moderated H atom Doppler profiles as well as the concentration of the D atoms produced in the H + DCl --> HCl + D reaction. From the measured H atom Doppler profiles, the time evolution of the initially generated nascent nonequilibrium H atom speed distribution toward its room-temperature thermal equilibrium form was determined. In this way, the excitation function and the reaction threshold (E0 = 0.65 +/- 0.13 eV) for the H + DCl --> HCl + D reaction could be determined from the measured nonequilibrium D atom formation rates and single collision absolute reaction cross-section values of 0.12 +/- 0.04 A2 and 0.45 +/- 0.11 A2 measured at reagent collision energies of 1.0 and 1.4 eV, respectively.     

Pulse radiolysis of 4,4'-bipyridyl aqueous solutions at elevated temperatures: spectral changes and reaction kinetics up to 400 degrees C

The spectral changes as well as the reaction kinetics of the transient species of 4,4'-bipyridyl (4,4'-bpy) have been experimentally investigated by pulse radiolysis techniques up to 400 degrees C. The results show that the transient species such as OH adduct 4,4'-bpyOH*, monoprotonated electron adduct 4,4'-bpyH*, and doubly protonated electron adduct 4,4'-bpyH2+* have 15-20 nm blue shifts from room temperature to 400 degrees C. For a deaerated neutral solution of 4,4'-bpy in the presence of tert-butyl alcohol, ethanol, or NaCOOH, the doubly protonated electron adduct is the main transient species at room temperature. But at temperatures > 350 degrees C, a monoprotonated form, the N-hydro radical 4,4'-bpyH*, becomes predominant. Interestingly, at room temperature, CO2-* could not efficiently react with 4,4'-bpy, but the reaction was accelerated with increasing temperature; at 350 degrees C, this reaction completed within 2 mus. Using an alkaline solution (pH = 11.5) of 4,4'-bpy in the presence of tert-butyl alcohol, we studied the N-hydro radical 4,4'-bpyH* from room temperature to 400 degrees C at 25 MPa. An estimation of the temperature-dependent G(e(aq)-) at 25 MPa agrees with our previous result with methyl viologen as a scavenger.     

Lifetimes and reaction pathways of guanine radical cations and neutral guanine radicals in an oligonucleotide in aqueous solutions

The exposure of guanine in the oligonucleotide 5'-d(TCGCT) to one-electron oxidants leads initially to the formation of the guanine radical cation G(?+), its deptotonation product G(-H)(?), and, ultimately, various two- and four-electron oxidation products via pathways that depend on the oxidants and reaction conditions. We utilized single or successive multiple laser pulses (308 nm, 1 Hz rate) to generate the oxidants CO(3)(?-) and SO(4)(?-) (via the photolysis of S(2)O(8)(2-) in aqueous solutions in the presence and absence of bicarbonate, respectively) at concentrations/pulse that were ～20-fold lower than the concentration of 5'-d(TCGCT). Time-resolved absorption spectroscopy measurements following single-pulse excitation show that the G(?+) radical (pK(a) = 3.9) can be observed only at low pH and is hydrated within 3 ms at pH 2.5, thus forming the two-electron oxidation product 8-oxo-7,8-dihydroguanosine (8-oxoG). At neutral pH, and single pulse excitation, the principal reactive intermediate is G(-H)(?), which, at best, reacts only slowly with H(2)O and lives for ～70 ms in the absence of oxidants/other radicals to form base sequence-dependent intrastrand cross-links via the nucleophilic addition of N3-thymidine to C8-guanine (5'-G*CT* and 5'-T*CG*). Alternatively, G(-H)(?) can be oxidized further by reaction with CO(3)(?-), generating the two-electron oxidation products 8-oxoG (C8 addition) and 5-carboxamido-5-formamido-2-iminohydantoin (2Ih, by C5 addition). The four-electron oxidation products, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp), appear only after a second (or more) laser pulse. The levels of all products, except 8-oxoG, which remains at a low constant value, increase with the number of laser pulses.     

Time-resolved spectroscopic studies of B(12) coenzymes: a comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5'-deoxyadenosylcobalamin.

An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented. Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV gamma-band absorption, and 520 nm in the strong visible alphabeta-band absorption. These data are compared with results reported earlier for the B(12) coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying alphabeta absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recombination in aqueous solution fall within a factor of 2 between 0.76 and 1.4 ns(-1). Intrinsic cage escape occurs on time scales ranging from 相似文献   

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t₄-PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr₄). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ_exc=670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q_x-band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.     

A LASER FLASH PHOTOLYSIS AND PULSE RADIOLYSIS STUDY OF AMILORIDE IN AQUEOUS AND ALCOHOLIC SOLUTION

Abstract Laser flash photolysis and pulse radiolysis have been carried out on the diuretic drug amiloride. The primary photochemical processes in aqueous solution were found to be photoionisation and triplet formation. Photoionisation was found to occur by a biphotonic process for 265 nm excitation but by a monophotonic process for excitation at 353 nm. The spectral properties of the resulting cation radical of amiloride were determined by pulse radiolysis using one electron oxidation by the radical anion Br₂·^¯ Photoexcitation of amiloride in isopropanol did not result in photoionisation but instead a semireduced radical of amiloride was observed. The spectral properties of the semireduced radical of amiloride were determined using one electron reduction by the CO₂·^¯ radical.     

Photoionization versus photoheterolysis of all-trans-retinol. The effects of solvent and laser radiation intensity

The time-resolved formation of the retinyl carbocation from all-trans-retinol and all-trans-retinol acetate was studied by use of picosecond flash photolysis. From both precursors, the retinyl cation is produced by heterolytic C-O bond cleavage in solvents of medium polarity (acetonitrile, tetrahydrofuran, propanol with Reichardt polarity parameter ET(N) approximately 0.5) and high polarity (EtOH, MeOH, TFE, HFIP, ET(N) > 0.6) during the laser pulse (< or =5 ps) where its lifetime is >10 ns. The absorption maximum of the cation at early times (t < 100 ps) is at lambda = 590-600 nm; it shifts to shorter wavelengths (Deltalambda = 5-10 nm) within 1-10 ns. This spectral shift is suggested to be due to contact ion pair --> solvent-separated ion pair --> free-ion transformation. The quantum yield of cation formation phi(cat) is independent of excitation wavelength (213, 266 or 355 nm). Photoheterolysis proceeds via a one-quantum process. In chlorinated solvents, i.e. n-BuCl, 1,2-dichloroethane, chloroform or CCl(4), formation of the retinol radical cation (which is characterized by a peak at 610 nm and further absorption maxima at approximately 840 and approximately 940 nm) by intermolecular electron transfer to the solvent molecules was detected. The radical cation lifetime in all these solvents is 1.5-2 ns, except for CCl(4) where it is 0.25 ns. The formation of the radical cation or cation was not detected in the low polarity solvents: cyclohexane, hexane, dioxane and p-xylene. However, in solvents of medium and high polarity, at high radiation intensities the radical cation may form in addition to the cation (as a result of two-quantum ionization). DFT calculations confirm our experimental results. The rate of retinol S(1) depopulation (k = 0.3-1 x 10(9) s(-1)) is almost independent of the solvent polarity in the range from cyclohexane to methanol. In highly polar solvents (ET(N) > 0.9) the rate increases to (0.5-5) x 10(10) s(-1).     

活性碳纤维阴极电芬顿反应降解微囊藻毒素研究

以具有高比表面积的活性碳纤维作为阴极,通过电芬顿反应降解水中微囊藻毒素(MCRR,MCLR)的电化学方法系统考察了电流密度、pH值和Fe2+浓度等因素对微囊藻毒素降解效果的影响.实验结果表明,在Fe2+浓度为1.0mmol/L和电流密度为6.6mA/cm2条件下,电化学处理60min,MCRR(8.81mg/L)去除率为75%,MCLR(6.36mg/L)去除率为94%.证明过氧化氢可以通过电化学还原在活性碳纤维阴极表面高效产生,微囊藻毒素可被高效降解去除.     

HYDRATED ELECTRON FORMATION IN NANOSECOND and PICOSECOND LASER FLASH PHOTOLYSIS OF HEMATOPORPHYRIN IN AQUEOUS SOLUTION

Nanosecond (lambda exc = 266, 355 and 532 nm) and picosecond (lambda exc = 355 nm) laser flash photolysis of hematoporphyrin (Hp) was performed in neutral (pH 7.4) and alkaline (pH 12) aqueous solution, as well as in the presence of 0.1% Triton X-100. The dependence of the yield of photoproduced hydrated electrons (e-aq) on laser pulse energy was studied over a wide range of energies (0.2 to greater than 1000 mJ cm-2). The results show that e-aq are predominantly formed in a two-photon process at lambda exc = 266 and 355 nm. One-photon quantum yields are higher at lambda exc = 266 nm than at lambda exc = 355 nm. Both one-photon and two-photon pathways are less efficient at higher Hp concentration, reflecting the influence of Hp self-aggregation. Two-photon e-aq formation is more efficient when 30 ps pulses are used for excitation, as compared to 10 ns pulses. No e-aq could be detected at lambda exc = 532 nm. Nanosecond pulse-induced transient spectra obtained at pH 7.4 are also discussed.     

Relationships Between Fluorescence Properties of Benzoquinolines and Physicochemical Changes in the Sol–Gel–Xerogel Transitions of Silicon Alkoxide Systems

Fluorescence and excitation spectra of 3,4-, 5,6- and 7,8-benzoquinolines (BQs) dispersed in the individual sol–gel–xerogel transitions systems of silicon alkoxide have been observed as a function of the reaction time. The fluorescence spectra of excited-state species (neutral, ion-pair and protonated species) of each BQ have been obtained clearly. In the starting sol–gel systems, the fluorescence mainly originated from the neutral species of each BQ. As hydrolysis of the silicon alkoxide proceeded, an interaction between the resulting silanol group and the neutral species led to a formation of the ion-pair species in their ground state. Upon excitation, a part of the ion pair relaxed to the protonated species during the fluorescence lifetime, and the resulting excited species emitted a corresponding fluorescence. As the sol–gel reaction proceeded further, the geometrical relaxation was gradually prevented with an increase in rigidity around the BQ molecules, so that a fluorescence tended to be observed from an unrelaxed conformer of the ion pair. During the gel to xerogel transition, a part of the BQ molecules exists in relatively large spaces of pores, where water plays an important role, and slowly came to exhibit the fluorescence from the protonated species because of concentrated water in the spaces. Contributions of the three excited-state species to the total fluorescence spectra have been estimated by spectral curve fitting. Consequently, the sol–gel reaction has been resolved into four physicochemical reaction stages, which govern the fluorescence behaviors of BQs. Differences in the fluorescence behaviors among the BQ isomers reflect the acid-dissociation constants in the excited states.     

S-benzoyl O-ethyl xanthate as a new photoinitiator: Photopolymerization and laser flash photolysis studies

A new sulfur-containing photoinitiator, S-benzoyl O-ethyl xanthate ( 2 ) has been prepared and used for the photopolymerization of methyl methacrylate (MMA). The photoinitiation property of 2 has been examined by conventional polymerization methods and nanosecond laser flash photolysis studies. Upon 308 nm laser pulse excitation, 2 gave rise to transients with absorption maxima at 350 and 650 nm, assigned to the benzoyl radical ( 3 ) and (ethoxythiocarbonyl)thiyl radical ( 4 ), respectively, on the basis of their quenching by nitroxy radicals and spectral similarity to analogous species, reported in the literature. © 1993 John Wiley & Sons, Inc.     

Photochemistry of iron(iii)-lactic acid complex in aqueous solutions

Photochemistry of a 1: 1 Fe^III-lactic acid complex, [Fe(Lact)]⁺, in aqueous solutions was studied by stationary photolysis, nanosecond laser flash photolysis (355 nm, 6 ns), and femtosecond pump-probe spectroscopy (400 nm, 200 fs). The quantum yield of photolysis of [Fe(Lact)]⁺ upon excitation at 355 nm is 0.4 and 0.22 in the deoxygenated and air-saturated solutions, respectively. Weak transient absorption in the range 500–750 nm was observed in the nanosecond experiments. It was assigned to a [Fe^II...-O-CH(Me)-COO^·]⁺ radical complex. The spectral properties of the ligand-to-metal charge transfer excited state and the characteristic time of formation of the radical complex (1.5 ps) were determined in the femtosecond spectroscopy experiments. A reaction mechanism was proposed, which involves inner-sphere electron transfer in the excited complex with the formation of a radical complex [Fe^II...-O-CH(Me)-COO^·]⁺ and its subsequent transformation to the end product of the photochemical reaction.     

Diffuse-reflectance laser flash photolysis of 16-(1-pyrene)-hexadecanoic acid adsorbed on silica

The absorption and fluorescence of 16-(1-pyrene)-hexadecanoicacid adsorbed on silica have been investigated. Time-resolved transient diffuse reflectance spectra were recorded following pulsed nanosecond laser excitation at 355 nm of pyrene, 1-methylpyrene and 16-(1-pyrene)-hexadecanoicacid adsorbed on silica. In addition to a rapidly decaying transient, absorbing at 420 nm assigned as the triplet state, and of the radical cation, absorbing at 460 nm, another long living transient species absorbing at 420 nm was observed for 16-(1-pyrene)-hexadecanoic acid. The decay is reversible but complete recovery takes several hours. Although no definitive assignment could be made for this transient several possibilities are discussed. The radical cations of the investigated molecules are formed by a biphotonic process. The non-exponential decay of the radical cations could be analyzed in the framework of a Gaussian distribution of free energy barriers.     

PHOTOCHEMISTRY OF FLAVINS—I. CONVENTIONAL AND LASER FLASH PHOTOLYSIS STUDY OF ALLOXAZINE

Abstract— The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λ_max 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with Λ_max 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH₂*) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines.     

The mechanism of matrix to analyte proton transfer in clusters of 2,5-dihydroxybenzoic acid and the tripeptide VPL

Intracluster proton transfer from the matrix-assisted laser desorption/ionization matrix 2,5-dihydroxybenzoic acid (DHB) to the peptide valyl-prolyl-leucine has been investigated as a function of excitation laser wavelength and power. Ionization laser power studies at 308 nm indicate that cluster ionization occurs with a two-photon dependence, whereas matrix-to-analyte proton transfer and cluster dissociation requires an additional photon. At 266 nm, two-photon absorption leads to both cluster ionization and cluster dissociation/proton transfer. A consideration of these results clearly indicates that analyte protonation occurs following ionization of the cluster to produce a radical cation matrix/analyte cluster. Mass spectral features also indicate that mixed DHB/peptide cluster ionization can occur via two-photon ionization at wavelengths as long as 355 nm. These results suggest a reduction in the ionization potential of larger mixed DHB/peptide clusters of greater than 1 eV. The reduced ionization potential seen in these clusters suggests that radical cation initiated proton transfer remains a viable mechanism for analyte protonation in matrix-assisted laser desorption/ionization at these longer wavelengths.     

Coumarin 314 free radical cation: formation, properties, and reactivity toward phenolic antioxidants

We have explored the photogeneration of the coumarin 314 radical cation by using nanosecond laser excitation at wavelengths longer than 400 nm in benzene, acetonitrile, dichloromethane, and aqueous media. In addition, time-resolved absorption spectroscopy measurements allowed detection of the triplet excited state of coumarin 314 (C(314)) with a maximum absorption at 550 nm in benzene. The triplet excited state has a lifetime of 90 μs in benzene. It is readily quenched by oxygen (k(q) = 5.0 × 10(9) M(-1) s(-1)). From triplet-triplet energy transfer quenching experiments, it is shown that the energy of this triplet excited state is higher than 35 kcal/mol, in accord with the relatively large singlet oxygen quantum yield (Φ(Δ) = 0.25). However, in aqueous media, the coumarin triplet was no longer observed, and instead of that, a long-lived (160 μs in air-equilibrated solutions) free radical cation with a maximum absorbance at 370 nm was detected. The free radical cation generation, which has a quantum yield of 0.2, occurs by electron photoejection. Moreover, density functional theory (DFT) calculations indicate that at least 40% of the electronic density is placed on the nitrogen atom in aqueous media, which explains its lack of reactivity toward oxygen. On the other hand, rate constant values close to the diffusion rate limit in water (>10(9) M(-1) s(-1)) were found for the quenching of the C(314) free radical cation by phenolic antioxidants. The results have been interpreted by an electron-transfer reaction between the phenolic antioxidant and the radical cation where ion pair formation could be involved.