Photoinduced electron transfer from electron donor to bis-carbocyanine dye in excited triplet state

1. Download : Download high-res image (117KB)
2. Download : Download full-size image

     

Intermolecular vs. intramolecular photoinduced electron transfer from nucleotides in DNA to acridinium ion derivatives in relation with DNA cleavage

Photoirradiation of various 10-methylacridinium ions (AcrR⁺, R = H, ⁱPr, and Ph) intercalated in DNA results in ultrafast intramolecular electron transfer, followed by rapid back electron transfer between AcrR⁺ and nucleotides in DNA. The electron-transfer dynamics in DNA were monitored by femtosecond time-resolved transient absorption spectroscopy. Both acridinyl radical and nucleotide radical cations, formed in the photoinduced electron transfer in DNA, were successfully detected in an aqueous solution. These transient absorption spectra were assigned by the comparison with those of DNA nucleotide radical cations, which were obtained by the intermolecular electron-transfer oxidation of nucleotides with the electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr–Mes⁺) produced upon photoexcitation of Acr⁺–Mes. Photoinduced cleavage of DNA with various acridinium ions (AcrR⁺, R = H, ⁱPr, Ph, and Mes) has also been examined by agarose gel electrophoresis, which indicates that the rapid intramolecular back electron transfer between acridinyl radical and nucleotide radical cation in DNA suppresses the DNA cleavage as compared with the intermolecular electron-transfer oxidation of nucleotides with Acr–Mes⁺.     

Photoinduced electron transfer reactions in the 10-methylacridinium cation–benzyltrimethylsilane system: steady-state and flash photolysis studies

The mechanism of the photoinduced reaction of the lowest excited singlet state of the 10-methylacridinium (AcrMe⁺) cation with benzyltrimethylsilane (BTMSi) in acetonitrile has been investigated by means of steady-state and time-resolved methods. A variety of stable products was found after irradiation (365 nm) of the reaction mixture under aerobic and oxygen-free conditions. The stable products were identified and analyzed using UV–Vis spectrophotometry, high performance liquid chromatography (HPLC), and mass spectrometry (MS). Based on Stern–Volmer plots of the AcrMe⁺ fluorescence quenching by BTMSi (using fluorescence intensity and lifetime measurements), the rate constants were determined to be k _q = 1.24 (± 0.02) × 10¹⁰ M⁻¹ s⁻¹ and k _q = 1.23 (± 0.02) × 10¹⁰ M⁻¹ s⁻¹, i.e., close to the diffusion-controlled limit in acetonitrile, indicating the dynamic quenching mechanism. The quenching process was shown to occur via an electron-transfer reaction leading to the formation of acridinyl radicals (AcrMe^•) and C₆H₅CH₂Si(CH₃)₃ ^•+ radical cations. Based on stationary and flash photolysis experiments, a detailed mechanism of the secondary reactions is proposed and discussed. The AcrMe^• radical was shown to decay by two processes. The fast decay, observed on the nanosecond timescale, was attributed to the back-electron transfer occurring within the initial radical ion pair. The slow decay on the microsecond timescale was explained by recombination reactions of radicals which escaped from the radical pair, including benzyl radicals formed via C–Si bond cleavage in the C₆H₅CH₂Si(CH₃)₃ ^•+ radical cation.     

Generation and reactions of 3-alkylidene-1-pyrazoline radical cations by photoinduced electron transfer

The behavior of the 3-alkylidene-1-pyrazoline radical cations generated by photoinduced electron transfer reactions was examined. The nitrogen-retained radical cations have been detected using laser flash photolysis. The photochemical products indicate that E/Z isomerization, intramolecular cyclization, and solvent addition (acetonitrile) occurred.     

Study on electron transfer of fluorescent probe lumichrome and nucleic acid by laser flash photolysis

Laser flash photolysis (LFP) of lumichrome (LC) has been studied upon laser excitation at 355 nm. The transient absorption spectra of LC in aqueous solution displayed two bands at 380 nm and 540–700 nm for the triplet ³LC, and a wide band at 430–530 nm for the radical anion LC^.. The triplet-state lifetime was measured up to 12 μs. It was found that the self-quenching of ³LC by the ground state of LC occurred to produce LC^. during LFP. The transient spectra of LC in the presence of electron-donating triphenylamine (TPA), nucleosides and nucleic acids showed that the absorbance of ³LC was strongly quenched by these substrates and the formation of LC^. was enhanced in a great extent simultaneously. The bimolecular quenching kinetics were studied and the rate constants of ³LC by a series of quenchers, including TPA, LC, nucleosides and nucleic acids were determined to be 10⁸–10⁹ M⁻¹ s⁻¹. The photo-induced electron-transfer mechanism for these processes involving the triplet ³LC was proposed.     

丙酮三重态与含芳香氨基酸肽的物理化学过程

用激光闪光光解瞬态吸收光谱研究了水溶液中含芳香氨基酸残基肽的光敏化反应过程.结果表明,在丙酮存在的含色氨酸残基肽(Trp-Gly,n-f-Met-Trp,Trp-Phe)体系的光解,丙酮三重态与Trp分别通过三重态-三重态(T-T)激发能转移和电子转移生成Trp激发三重态和N中心自由基(Trp/N^·);丙酮三重态仅与含酪氨酸残基肽(Phe-Tyr)通过电子转移生成Tyr酚氧自由基(Tyr/O^·).在色氨酰酪氨酸(Trp-Tyr)与丙酮的光解体系中,观察到分子内的电子转移,即由Trp/N^·-Tyr→Trp-Tyr/O^·自由基的生成过程     

4-硝基喹啉氧化物激发三重态与甲硫氨酸二肽之间的电子转移

4 硝基喹啉氧化物 ( 4ＮＱＯ)在紫外光的作用下 ,发生电子跃迁生成较高量子产额的激发三重态 ,这一性质在光化学研究中可作为光敏剂 ,同时又是一种典型的致癌物 .在水溶液中 ,具有与ＤＮＡ通过电荷转移作用连接到碱基上的性质 ,这一性质可能与其致癌作用有关[1～ 4].由于ＤＮＡ具有的遗传功能通过染色体来实现 ,染色体是ＤＮＡ与蛋白质形成的配合物 ,这种蛋白质结合在ＤＮＡ上 ,同ＤＮＡ碱基一样参与化学反应诱导的致癌过程 .因此 ,研究光引发 4ＮＱＯ与氨基酸及相关肽的反应对于了解光诱导 4ＮＱＯ致癌作用的分子机制有重要意义 .Ｓｅｋｉ…     

Electron-transfer fluorescence quenching processes in coaggregates between excited N-alkylcarbazoles as electron donors and 2, 4-dinitrophenyl carboxylates or pentafluorophenyl carboxylates as acceptors

Electron-transfer processes facilitated by hydrophobic-lipophilic interaction ( HLI) between excited N-alkylcar-bazoles (1-n, n=4, 8, 12, 16) as electron donors and 2,4-dintrophenyl carboxylates (2-n, n = 4, 8, 12, 16) or pentafluorophenyl carboxylates (3-n, n = 4, 8, 12, 16) as electron acceptors have been investigated by means of fluorescence spectroscopy in aqueous or aquiorgano binary mixtures. The fluorescence quenching of -n * by 2-n or -n indicates that preassociation precedes the electron transfer. The extent of HLI-driven coaggregation of the acceptor and the donor may be assessed from the B value of the equation I0/I = A B [Q]. The chain-length effect and possibly also a chain-fold-ability effect, as well as the solvent aggregating power (SAgP) effect have been observed. Comparison of the quenching constants (B) for 1-n * /2-n combinations and 1-n * / 3-n combinations shows that the order of increasing R values for the quenching processes is 3-n < 2-n.     

N-羟乙基-1,8-萘二酰亚胺探针与核苷间电子转移的激光闪光光解研究

核酸与核酸前体参与的电子转移(ET)作用能够直接或间接导致核酸主链和碱基侧链的断裂,因此对核酸碱基光动态损害机理的深入研究具有重要的理论和实际意义．其中,核酸荧光探针逐渐成为研究生物分子的主要技术之一,借助于时间分辨的瞬态吸收光谱技术,检测荧光探针激发态物种及其与核酸之间发生电子转移作用而产生的活性中间体,能够深入了解光断裂反应的最初步骤,揭示核酸断裂电子转移反应的微观机理．     

Laser-induced modification of DNA and Poly[A,G] at guanine moiety using acetone as photosensitizer

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.     

Solvent Effect on the Photoinduced Electron Transfer Reaction Between Thioxanthen-9-one and Diphenylamine

Photoinduced chemical reaction between thioxanthen-9-one (TX) and diphenylamine (DPA) were investigated by the nanosecond laser flash photolysis. With photolysis at 355 nm, the triplet TX (³TX^*) is produced via a Franck-Condon excitation and intersystem crossing. In the transient absorption spectra of the reduction of ³TX^* by DPA in pure and aqueous CH₃CN, four bands are clearly observed and assigned to absorption of ³TX^*, TXH^·, TX^·- and DPA^·+, respectively. With the increase of solvent polarity, the blue-shift was observed for all absorption bands of the intermediates. With the aid of dynamic decay curves, an electron transfer followed by a protonation process is confirmed for the reduction of ³TX^* by DPA. The quenching rate constants of ³TX^* by DPA very slightly decrease from 9.7×10⁹ L/(mol·s) in pure CH₃CN, to 8.7×10⁹ L/(mol·s) in CH₃CN:H₂O (9:1), 8.0×10⁹ L/(mol·s) in CH₃CN:H₂O (4:1) and 7.5×10⁹ L/(mol·s) in CH₃CN:H₂O (1:1). Therefore water plays a minor role in the title reaction, and moreover no obvious medium effect of solvent polarity is observed for the electron transfer between ³TX^* and DPA, indicating that the ³nπ^* and ³ππ^* states of TX have the approximate ability to attract an electron from DPA.     

Kinetics and mechanism of sensitized photooxidation of tetramethylammonium salt of 2-(phenylthio)acetic acid in solution: Steady-state and flash photolysis studies

The mechanism for the photo-induced oxidation of the tetramethylammonium salt of 2-(phenylthio)acetic acid was elucidated. The photosensitizer was the benzophenone triplet in acetonitrile solutions. Time-resolved absorption spectra and kinetics were used to follow the intermediates which included the triplet of benzophenone, the ketyl radical of benzophenone, and an ion pair consisting of a radical anion of benzophenone and a tetramethylammonium cation. Rate constants for the growth and decay of the transients were determined along with the quantum yields of the transients. The intermediacy of other radicals was inferred by the products observed following steady-state photolysis. Quantum yields were also determined for photoproducts resulting from the steady-state irradiation. The mechanism was proposed that rationalized the quantitative observations. Of particular note was how the nature of the counter ion effected the secondary reactions of the radicals and the radical ions.     

H-Atom Transfer Reaction of Photoinduced Excited Triplet Duroquinone with Tryptophan and Tyrosine in Acetonitrile-Water and Ethylene Glycol-Water Homogeneous Solutions

Laser flash photolysis was used to investigate the photoinduced reactions of excited triplet bioquinone molecule duroquinone (DQ) with tryptophan (Trp) and tyrosine (Tyr) in acetonitrile-water (MeCN-H\begin{document}$_2$\end{document}O) and ethylene glycol-water (EG-H\begin{document}$_2$\end{document}O) solutions. The reaction mechanisms were analyzed and the reaction rate constants were measured based on Stern-Volmer equation. The H-atom transfer reaction from Trp (Tyr) to \begin{document}$^3$\end{document}DQ\begin{document}$^*$\end{document} is dominant after the formation of \begin{document}$^3$\end{document}DQ\begin{document}$^*$\end{document} during the laser photolysis. For DQ and Trp in MeCN-H\begin{document}$_2$\end{document}O and EG-H\begin{document}$_2$\end{document}O solutions, \begin{document}$^3$\end{document}DQ\begin{document}$^*$\end{document} captures H-atom from Trp to generate duroquinone neutral radical DQH\begin{document}$^\bullet$\end{document}, carbon-centered tryptophan neutral radical Trp\begin{document}$^\bullet$\end{document}/NH and nitrogen-centered tryptophan neutral radical Trp/N\begin{document}$^\bullet$\end{document}. For DQ and Tyr in MeCN-H\begin{document}$_2$\end{document}O and EG-H\begin{document}$_2$\end{document}O solutions, \begin{document}$^3$\end{document}DQ\begin{document}$^*$\end{document} captures H-atom from Tyr to generate duroquinone neutral radical DQH\begin{document}$^\bullet$\end{document} and tyrosine neutral radical Tyr/O\begin{document}$^\bullet$\end{document}. The H-atom transfer reaction rate constant of \begin{document}$^3$\end{document}DQ\begin{document}$^*$\end{document} with Trp (Tyr) is on the level of 10\begin{document}$^9$\end{document} L\begin{document}$\cdot$\end{document}mol\begin{document}$^{-1}$\end{document}\begin{document}$\cdot$\end{document}s\begin{document}$^{-1}$\end{document}, nearly controlled by diffusion. The reaction rate constant of \begin{document}$^3$\end{document}DQ\begin{document}$^*$\end{document} with Trp (Tyr) in MeCN/H\begin{document}$_2$\end{document}O solution is larger than that in EG/H\begin{document}$_2$\end{document}O solution, which agrees with Stokes-Einstein relationship qualitatively.     

Electron transfer from triplet thymine and thymidine to lipoic acid

Electron transfer from the triplet excited state of thymine or thymidine to the disulphide compound lipoic acid (RSSR) was studied using KrF laser flash photolysis (248 nm, 20 ns). The electron transfer reaction rate constants, measured at 310 nm, were determined to be 1.3×10¹⁰ M⁻¹ s⁻¹ and 6.9×10⁹ M⁻¹ s⁻¹ for thymine and thymidine respectively. The transient absorbance at 400 nm in the presence of the quencher is attributed to the anion radical of lipoic acid.     

Photoinduced Electron and H-atom Transfer Reactions of Xanthone by Laser Flash Photolysis

The property of the lowest excited triplet states of xanthone in acetonitrile was investigated using time-resolved laser °ash photolysis at 355 nm. The transient absorption spectra and the quenching rate constants(kq) of the excited xanthone with several amines were determined. Good correlation between lgkq and the driving force of the reactions suggests the electron transfer mechanism, except aniline and 3-nitroaniline (3-NO2-A) which showed energy transfer mechanism. With the appearance of ketyl radical, hydrogen atom transfer also happened between xanthone and dimethyl-p-toluidine, 3,5,N,N-tetramethylaniline, N,N-dimethylaniline, and triethylamine. Therefore, both electron transfer and H-atom transfer occured in these systems. Great discrepancies of kq values were discovered in H-atom abstraction reactions for alcohols and phenols, which can be explained by di?erent abstraction mechanisms. The quenching rate constants between xanthone and alcohols correlate well with the ?-C?H bonding energy of alcohols.     

Photochemical Hydrogen Abstraction and Electron Transfer Reactions of Tetrachlorobenzoquinone with Pyrimidine Nucleobases

Pentachlorophenol, a widespread environmental pollutant that is possibly carcinogenic to humans, is metabolically oxidized to tetrachloroquinone (TCBQ) which can result in DNA damage. We have investigated the photochemical reaction dynamics of TCBQ with two pyrimidine type nucleobases (thymine and uracil) upon UVA (355 nm) excitation using the technique of nanosecond time-resolved laser flash photolysis. It has been found that 355 nm excitation populates TCBQ molecules to their triplet state ³TCBQ^*, which are highly reactive towards thymine or uracil and undergo two parallel reactions, the hydrogen abstraction and electron transfer, leading to the observed photoproducts of TCBQH· and TCBQ·^- in transient absorption spectra. The concomitantly produced nucleobase radicals and radical cations are expected to induce a series of oxidative or strand cleavage damage to DNA afterwards. By characterizing the photochemical hydrogen abstraction and electron transfer reactions, our results provide potentially important molecular reaction mechanisms for understanding the carcinogenic effects of pentachlorophenol and its metabolites TCBQ.     

Laser flash photolysis study of electron transfer processes of adsorbed anthracene on titania–silica surfaces

We have studied the photophysics of anthracene adsorbed to photocatalytic silica–titania mixed oxide systems prepared by two different methods: a sol–gel synthesis and an impregnation route. The observed photophysics depend upon the method of synthesis, with the sol–gel prepared samples resulting in enhanced radical cation formation via static excited singlet state quenching. This mechanism, whilst operative, does not give rise to long-lived charge separation in the case of the impregnated samples. These results are discussed in terms of differing sample morphologies.