Quenching dynamics study on photoinduced excited triplet duroquinone by TEMPO in 1,2-propandiol

Chemically induced dynamic electron polarization (CIDEP) spectrum and transient absorptive spectrum are recorded in photolysis of duroquinone (DQ) in 1,2-propanodiol (PG). Durosemiquinone neutral radical DQH^• and PG ketyl radical \textCH₃[(\textC)\dot]\textOHCH₂\textOH{\text{CH}}_{3}{{{\dot{\text{C}}}\text{OHCH}}}_{2}{\text{OH}} are produced through hydrogen transfer reaction from PG to ³DQ*. When stable radical TEMPO is added to DQ/PG solution, photolysis results in CIDEP on TEMPO, which can be interpreted as a quartet precursor radical-triplet pair mechanism (QP-RTPM). There is competition between PG and TEMPO to quench ³DQ*. The CIDEP intensity of DQH^• decreases with the increase of TEMPO concentration. The quenching dynamics in photolysis of DQ/TEMPO/PG system is analyzed in detail. Based on the dynamics analysis and the measurement of the lifetime of ³DQ* by its transient absorbance decay, the quenching rate constant of ³DQ* by TEMPO in PG is obtained as 1.34 × 10⁷ L mol⁻¹ s⁻¹. This quenching rate constant is closely diffusion-controlled.     

蒽醌在乙二醇/Triton X胶束与乙二醇/三乙胺溶液中光还原的时间分辨电子自旋共振研究

用时间分辨电子自旋共振研究了乙二醇/Triton X 胶束与乙二醇/三乙胺溶液中蒽醌(AQ)的光还原. 在蒽醌/乙二醇/Triton X/H₂O胶束中, 获得蒽半醌自由基AQH^•的很强的TR-ESR信号, 并检测到一定强度的负离子基AQ^•－ TR-ESR信号. 在蒽醌/乙二醇/三乙胺/H₂O体系中同时有较强的AQH^•与AQ^•－的TR-ESR信号. 分析并讨论了蒽醌在两种体系中的还原过程. 根据CIDEP的强度与Triton X及三乙胺浓度的关系, 推求了三线态³AQ^*对自由基AQH^•的反应速率常数. 根据化学诱导动态电子极化(CIDEP)信号随时间的变化, 计算了AQH^•的CIDEP弛豫时间.     

Effect of electron-withdrawing power of the substituted group on?OH radical reaction with substituted aryl sulphides: A pulse radiolysis study

In neutral aqueous solution of (phenylthio)acetic acid, hydroxyl radical is observed to react with a bimolecular rate constant of 7.2 × 10^-1 dm³ mol⁻s⁻ and the transient absorption bands are assigned to^•OH radical addition to benzene and sulphur with a rough estimated values of 50 and 40% respectively. The reaction of the^•OH radical with diphenyl sulphide (k = 4.3 × 10⁸ dm³ mol⁻¹ s⁻¹) is observed to take place with formation of solute radical cation, OH-adduct at sulphur and benzene with estimated values of about 12, 28 and 60% respectively. The transient absorption bands observed on reaction of^•OH radical, in neutral aqueous solution of 4-(methylthio)phenyl acetic acid, are assigned to solute radical cation (λ_max = 550 and 730 nm), OH-adduct at sulphur (λ_max = 360 nm) and addition at benzene ring (λ_max = 320 nm). The fraction of^•OH radical reacting to form solute radical cation is observed to depend on the electron-withdrawing power of substituted group. In acidic solutions, depending on the concentration of acid and electron-withdrawing power, solute radical cation is the only transient species formed on reaction of^•OH radical with the sulphides studied.     

Photoinitiated quenching reactions of excited triplet duroquinone by vitamin C in homogeneous and AOT (SDS) micelle solutions

Time-resolved EPR has been used to study the photoinitiated reactions of the excited triplet of duroquinone (³DQ^*) quenched by the antioxidant vitamin C (VC) in homogeneous solutions of ethylene glycol/water (EG/H₂O) and micelle solutions of aerosol OT (AOT) and sodium dodecyl sulphate (SDS). During the photolysis reactions of DQ and VC in homogeneous solutions of EG/H₂O, ³DQ^* abstracts hydrogen atoms from the solvent EG and the antioxidant VC. The rate constant for the quenching of ³DQ^* by VC is 4.90 × 10⁷ L mol^?1 s^?1, close to being diffusion-controlled. In AOT and SDS micelle solutions, rate constants for the quenching of ³DQ^* by VC are 3.28 × 10⁷ and 3.15 × 10⁷ L mol^?1 s^?1, respectively. Lipid-soluble ³DQ* and water-soluble VC need to diffuse to the w/o interface to react, which reduces the reaction rate between ³DQ* and VC. The charge repulsion interaction between the anionic shell of AOT (SDS) micelles and the VC monoanions AsH^? also slows the reaction.     

Reaction of hydroxyl radicals with S-nitrosothiols: Formation of thiyl radical (RS?) as the intermediate

The decomposition studies of S-nitrosothiols (RSNO) are important due to their potential role in vivo in connection with the storage and transport of nitric oxide (^•NO) within the body. Reactions of hydroxyl radicals (^•OH) with a number of RSNOs (S-nitroso derivatives of N-acetyl-dl-penicillamine, l-cysteinemethylester, N-acetylcysteamine, and dl-penicillamine) in aqueous medium at neutral and acidic pH have been reported in the present study. Radiation chemical technique (steady state and pulse radiolysis) has been utilized for the determination of the reaction rate constants, the end product analyses, and the transient intermediate species. The rate constants for the reaction of ^•OH with the selected RSNOs were determined using a competition kinetic method with 2′-deoxy-d-ribose as the competitor. All the rate constants were found to be of the order of diffusion controlled (10¹⁰ M⁻¹ s⁻¹). The degradation yield of RSNOs was found to be quantitative (i.e., G(–RSNO) ≈ G(^•OH)) at neutral and acidic pH. The major products of decomposition were the respective disulfide (RSSR) and nitrite (NO₂ ⁻). A good material balance is also obtained between the degradation yield and the formation of the products (i.e., G(–RSNO) ≈ G(RSSR) + G(NO₂ ⁻)). The major transient intermediate was the thiyl radical (RS^•). Its intermediacy was confirmed by making use of the electron transfer reaction of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS²⁻) to RS^•, which results in the formation of ABTS^•− having a transient absorption spectrum with λ_max at 410 nm. Based on these results, a generalized reaction mechanism is deduced for the reaction of ^•OH with RSNO.     

Formation of p-terphenyl excited states in the ionic liquid methyltributylammonium bis[(trifluoromethyl)sulfonyl]imide: pulse radiolysis study

Solutions of 80 mM benzophenone (BP) and up to 14 mM p-terphenyl (TP) in the ionic liquid methyltributylammonium bis[(trifluoromethyl)sulfonyl]imide (R₄NNTf₂) have been investigated by nanosecond pulse radiolysis. The resulting transient absorption spectra of pulse-irradiated argon saturated solutions correspond to the formation of several intermediates derived from BP and TP: benzophenone radical anion [(C₆H₅)₂CO]^•− (BP^•−) converted after ~20 μs into ketyl radicals (C₆H₅)₂C^•OH (BP^•H), a hydrogen adduct to the phenyl ring of benzophenone C₆H₅COC₆H₆^•, p-terphenyl triplet excited state ³TP*, and traces of TP radical ions. ³TP* was formed in two steps, the first immediately during the pulse and the second in pseudo-first order process with a second order reaction rate constant calculated from TP concentration dependence: k = ~2 × 10⁸ dm³ mol⁻¹ s⁻¹.     

Transient species and its properties of melatonin

Reactive oxygen species (ROS) are generated dur- ing radiation, respiratory burst, normal metabolic processes and so on. There are enzymatic and non-enzymatic antioxidants such as superoxide dis- mutase (SOD), glutathione peroxidase (GSH-Px), vi- tamin E (VE) and carotenoids that can either inhibit or repair the ROS-induced damage. ROS is essential to maintain physiological homeostasis. However, exces- sive ROS give rise to oxidative damage to proteins, lipids and DNA which related t…     

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.     

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.     

Use of Raman spectroscopy for the identification of radical-mediated damages in human serum albumin

Damages induced by free radicals on human serum albumin (HSA), the most prominent protein in plasma, were investigated by Raman spectroscopy. HSA underwent oxidative and reductive radical stress. Gamma-irradiation was used to simulate the endogenous formation of reactive radical species such as hydrogen atoms (^•H), solvated electrons (e_aq⁻) and hydroxyl radicals (^•OH). Raman spectroscopy was shown to be a useful tool in identifying conformational changes of the protein structure and specific damages occurring at sensitive amino acid sites. In particular, the analysis of the S–S stretching region suggested the radical species caused modifications in the 17 disulphide bridges of HSA. The concomitant action of e_aq⁻ and ^•H atoms caused the formation of cyclic disulphide bridges, showing how cystine pairs act as efficient interceptors of reducing species, by direct scavenging and electron transfer reactions within the protein. This conclusion was further confirmed by the modifications visible in the Raman bands due to Phe and Tyr residues. As regards to protein folding, both oxidative and reductive radical stresses were able to cause a loss in α-helix content, although the latter remains the most abundant secondary structure component. β-turns motifs significantly increased as a consequence of the synergic action of e_aq⁻ and ^•H atoms, whereas a larger increase in the β-sheet content was found following the exposure to ^•OH and/or ^•H attack.     

Anti- and pro-oxidant properties of sodium 2-mercaptoethane sulphonate (Mesna)

The ^•OH and the NO₂ ^• radicals generated pulse radiolytically in N₂O-saturated aqueous solution at pH 8–8.5 oxidize Mesna to form the corresponding thiyl radicals which on reaction with thiolate ions form an RSSR^{• −} type of transient with λ_max = 420 nm. The rate constants for the formation of these transients were determined. In the absence of O₂ at pH=6, the RS^• radicals formed show an absorption maximum at 360 nm and an ε=200±50 dm³ mol⁻¹ cm⁻¹. The rate constant k (^•OH+RSH) was 6×10⁹ dm³ mol⁻¹ s⁻¹ as determined from competition kinetics. In the presence of O₂ the Mesna thiyl radical was seen to rapidly add oxygen to form an RSOO^• type of species with λ_max = 535 nm, ε=700±50 dm³ mol⁻¹ cm⁻¹ and k (RS^•+O₂)=1.3×10⁸ dm³ mol⁻¹ s⁻¹. Both the RS^• and the RSOO^• radicals formed by the oxidation of Mesna were able to abstract H-atoms from ascorbate ions and k(RS^• +AH⁻)=~k(RSOO^•+AH⁻)=~6−7×10⁸ dm³ mol⁻¹ s^−1-. Moderately strong oxidants like CCl₃OO^• and the (CH₃)₃CO^• radicals, having a reduction potential of +1.4−1.6 V vs NHE were unable to oxidize Mesna. The results thus reflect on the pro- and anti-oxidant properties of Mesna.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.     

Synthesis and properties of paramagnetic derivatives of linear and fused polyaromatic compounds

Paramagnetic derivatives of tolan, anthracene, and terphenyl of the A−Sp−R^• type, where A is an aromatic group, R^• is a stable radical center, and Sp is a spacer, were synthesized. The electronic absorption and luminescence spectra of these compounds were examined. The introduction of the paramagnetic dihydroimidazole fragment causes a decrease (10–500-fold) in the quantum yield of luminescence compared to emission of the individual aromatic luminophore. The presence of a radical center (R^•) in the (alkane)^•+/−•A−Sp−R^• radical-ionic pair leads to the disappearance or fast (nanoseconds) damping of the magnetic effect of luminescence arising upon recombination of these pairs.     

Formation of excited uranyl in oxidation of tetravalent uranium with oxygen in HClO<Subscript>4</Subscript> aqueous solutions: IV. Enhancement of the chemiluminescent reaction in the presence of Fe<Superscript>2+</Superscript>

Experimental data that support the hypothesis on the determining role of ^•OH radicals in the emergence of luminescence during the oxidation of U(IV) with atmospheric oxygen in aqueous HClO₄ solutions have been obtained using the H₂O₂-FeSO₄ system as a source of OH radicals. It has been found that brighter chemiluminescence (CL) is observed in the presence of 10⁻⁵ mol/l Fe²⁺ in a 5 × 10⁻⁴ mol/l U(IV) solution in 0.1 mol/l HClO₄ compared with the FeSO₄-free solution. The CL yield in the presence of Fe²⁺ (η_CL = 3.9 × 10⁻⁸) is 2.8 times that in the solution without iron (η_CL = 1.4 × 10⁻⁸). These results can be regarded as a further piece of evidence for the idea that the elementary event of the formation of a CL emitter—electronically excited uranyl ion *(UO₂²⁺)—in radical chain U(IV) oxidation reactions is electron transfer from the uranoyl ion (UO₂⁺) to the oxidant, the ^•OH radical. Thus, one of the main prerequisites for light emission during U(IV) oxidation reactions is a high generation efficiency of ^•OH radicals and their easy access to the uranoyl UO₂⁺ ion.     

Viscosity effect on the CIDEP of laser photolysis of duroquinone in hydrogen-donor solvents

Chemically induced dynamic electron polarization (CIDEP) is one of the important research fields of dynamic spin chemistry. In the present work, with ethylene glycol, 1,2-propanol and their mixtures with different ratios in volume as solvents and duroquinone as photosensitive molecule, the influence of solvent viscosity on the CIDEP of photolyzed duroquinone radicals has been studied experimentally. In each solvent, duroquinone neutral radicals are observed. Duroquinone neutral radicals are generated through hydrogen transfer reaction from hydrogen-donor solvent molecules to excited duroquinone triplet. The CIDEP mechanism is mainly triplet mechanism. The CIDEP intensity of duroquinone neutral radicals decreases with the increase in the solvent viscosity, and the variation was rapid in low-viscosity range and slow in high-viscosity range.     

Gas-phase fragmentation of long-lived cysteine radical cations formed via no loss from protonated S-nitrosocysteine

In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine (Cys^•+), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: •COOH ≫ CH₂S > •CH₂SH-H₂S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic Cys^•+ with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of CH₂S, •COOH, •CH₂SH, and H₂S.     

The sorption of Eu(III) on calcareous soil: effects of pH,ionic strength,temperature, foreign ions and humic acid

The sorption of Eu(III) on calcareous soil as a function of pH, humic acid (HA), temperature and foreign ions was investigated under ambient conditions. Eu(III) sorption on soil was strongly pH dependent in the observed pH range. The effect of ionic strength was significant at pH < 7, and not obvious at pH > 8. The type of salt cation used had no visible influence on Eu(III) uptake on soil, however at low pH values, the influence of anions was following the order: Cl⁻ ≈ NO₃ ⁻ > ClO₄ ⁻. In the presence of HA, the sorption edge obviously shifted about two pH units to the lower pH, whilst in range of pH 6–7, the sorption of Eu(III) decreased with increasing pH because a considerable amount of Eu(III) was present as humate complexes in aqueous phase, then increased again at pH > 11. The results indicated that the sorption of Eu(III) on soil mainly formed outer-sphere complexes and/or ion exchange below pH ~7; whereas inner-sphere complexes and precipitation of Eu(OH)₃(s) may play main role above pH ~8.     

Intra- and intermolecular quenching of carbazole photoluminescence by imidazolidine radicals

Mechanisms of carbazole photoluminescence quenching by the free and chemically bound nitroxyl radicals in the model bound system “carbazole (CBZ)—imidazolidine nitroxyl radical R^•” were investigated and the photophysical properties of the system were studied and compared with those of free CBZ and R^• in solution. The quantum yield and lifetime of fluorescence from the local singlet excited state of the carbazole moiety in the bound CBZ—R^• system is three orders of magnitude lower than in free CBZ. The lifetime of the local triplet excited state of the carbazole moiety in the bound system is shorter than 50 ns. The rate constants for intermolecular quenching of the singlet and triplet excited states of free CBZ by R^• in acetonitrile were found to be (1.4±0.1)·10¹⁰ and (1.5±0.2)·10⁹ L mol⁻¹ s⁻¹, respectively. The most plausible mechanisms of both free and covalently bound carbazole luminescence quenching by nitroxyl radicals are exchange energy transfer and acceleration of internal conversion due to electron exchange.     

Time-resolved FT-EPR study of photoreduction of duroquinone by triethylamine in methanol

Using time resolved Fourier transform EPR spectroscopy the photoreduction of duroquinone by triethylamine in methanol solution was investigated. It is found that the spin-polarized (CIDEP) duroquinone triplet deactivates by electron transfer from triethylamine generating duroquinone radical anion and amine radical cation, and by hydrogen transfer from the solvent generating durosemiquinone radical and hydroxymethyl radical, respectively. All radicals are observed at different conditions and are spin-polarized by triplet mechanism and partially by ST₀ radical pair mechanism. The time dependence of FT-EPR intensities of radical cation and radical anion on the amine concentration is investigated in the range of 1 to 100 mM triethylamine. The contribution of the triplet mechanism to the spin polarization of radicals changes with different triethylamine concentrations. The durosemiquinone radical is found to be transformed into duroquinone radical anion in the presence of triethylamine in the solution. CIDNP experiments indicate that the hydrogen back transfer between the durosemiquinone radical and hydroxymethyl radical pair has a significant influence on the time behaviour of duroquinone radical anion. The intensity of triethylamine radical cation is found to be decreased with the increase of triethylamine concentration, which is interpreted that the triethylamine radical cation is deprotonated by the amine. Based on the FT-EPR results, a new complete mechanism is proposed.     

Pulse radiolysis study of 2-Mercapto-benzothiazole-a corrosion inhibitor

Pulse radiolysis of 2-Mercaptobenzothiazole (2-MBT) has been undertaken in aqueous solution. The semi-oxidized species formed at pH 4.5 due to the reaction of OH^•, Br₂ ^•− and N₃ ^• and at pH 10.5 with OH^• yielded a spectrum with λ_max = 348 and 595 nm. These semi-oxidized species were able to oxidize phenothiazine drugs (E^o⋟0.8 V). Reducing species such as e_aq ⁻, CO₂ ^•− and H^• atoms react with 2-MBT resulting in the formation of a transient having λ_max = 350 nm and reducing in nature. Kinetic and spectroscopic data of interest are reported.