Laser Flash Photolysis Experiments Research Articles

Photophysical properties of a β-Carboline Rhenium (I) complex. Solvent effects on excited states and their redox reactivity

The photochemical and photophysical properties of a Re(I) tricarbonyl complex, ClRe(CO)3(nHo)2, where nHo = 9H-pyrido[3,4-b] indole (norharmane), were investigated in solution phase by a combination of steady state emission spectroscopy, laser flash photolysis (LFP) and pulse radiolysis (PR) techniques. These results allowed us to identify and study the reactivity of the β-carboline (nHo) Rhenium(I) complex main excited states. The absorption spectrum as well as the steady-state and time-resolved luminescence of the complex exhibits a marked dependence with the solvent properties. These experimentally observed results were corroborated by quantum chemical calculations, TD-DFT. The most important electronic transitions present in the spectrum in all solvents are MLLCTRe(CO)3→nHo1, nHo2 along with a mixture of ILnHo and LLCTCl→nHo transitions. The relationship between the dipole moment and the polarity of the solvent was rationalized in terms of the electron density inside and outside the complex. While the luminescence of the complex is mainly attributed to the emitting 1ILnHo state, in LFP experiments a MLCT excited state was also detected. The species generated in either reductive or oxidative conditions in LFP experiments were compared with those obtained in PR. Also, the quenching rate constant (kq) of the excited state with MV+2 was calculated. The excited state of the complex can efficiently generate singlet oxygen in acetonitrile yielding a ΦΔ = 0.25 ± 0.02. Optoacoustic measurements showed that, after photonic excitation, almost all the absorbed energy by the complex is released to the medium as prompt heat. The investigated photophysical and photochemical properties of ClRe(CO)3(nHo)2 are of significant importance in relation to the use of this β-carboline Rhenium(I) complex in several biomedical fields, such as photodynamic therapy and photoactivated chemotherapy as well as new alternative therapies such as regional hyperthermia.

Aqueous photodegradation of the benzophenone fungicide metrafenone: Carbon-bromine bond cleavage mechanism

Metrafenone (MF), as a new type of benzophenone fungicide, has been widely used in agriculture and is persistent in the environment. Understanding its photochemical fate is essential for the comprehensive evaluation of its ecological risk. In the present work, we reported a detailed study on the photochemical transformation of MF in aqueous solution under irradiation (at λ > 290 nm using a high pressure mercury lamp). MF was relatively photo-reactive showing a low polychromatic quantum yield of photolysis (1.06 × 10−4, 20 µM) counterbalanced by a significant light absorption above 290 nm. A series of photoproducts were identified by high resolution mass spectrometry (HR-MS) analysis, and three different pathways, including oxidation of the methyl group, debromination and replacement of bromine by hydroxyl group were proposed. Among them, debromination was identified as the dominating process that could be achieved via homolytic C-Br bond cleavage from singlet and triplet MF, as confirmed by laser flash photolysis (LFP) experiments and density functional theory (DFT) calculations. Toxicity assessment revealed that photochemical degradation reduced the ecotoxicity of MF efficiently. Nitrate ions and humic acid promoted the MF photolysis, while bicarbonate exhibited no effect. Results obtained in this work would increase our understanding on the environmental fate of MF in sunlit surface waters.

Insights into the hydrated electron generation from UV/aniline: Mechanism and quantum efficiency

The insight into the neglected reduction process accompanied by UV-based oxidation process may provide new ideas for the development of advanced oxidation and reduction technologies. In this study, aniline was comprehensively investigated as an unexpected indicator of hydrated electron (eaq−) under UV irradiation. Monochloroacetic acid (MCAA) was selected as the probe of eaq− and the balance of chloride ions indicated the reduction of MCAA. Further, laser flash photolysis experiments demonstrated the generation of eaq− in the UV/aniline process and the half-life period of formed eaq− was demonstrated to be 0.13 μs. The photolysis of aniline along with the decay of the excited state of aniline was responsible for the eaq− generation. Besides, the hydrogen atom (H) generated from the photolysis can subsequently reacted with OH− to generate eaq−. The photolysis pathways of aniline were proposed by the results of GC-MS. Aniline was abstracted of H in solution to the formation of aniline radical (PhNH) or form aminophenol in three different isomers (orto-, meta- and para-aminophenol). Moreover, UV/aniline showed a higher reducing capacity of MCAA compared with other organic electron donors and sustained a highly reducing ability in a wide pH. And the calculation results of quantum efficiency (Φ) showed that excessive aniline was not conducive to the elevation of Φ. This study introduced a novel pathway of eaq− generation during the photolysis of aniline and provided a new perspective for eaq−-based advanced reduction processes.

Reactivity and DNA Damage by Independently Generated 2′-Deoxycytidin-N4-yl Radical

Oxidative stress produces a variety of radicals in DNA, including pyrimidine nucleobase radicals. The nitrogen-centered DNA radical 2'-deoxycytidin-N4-yl radical (dC·) plays a role in DNA damage mediated by one electron oxidants, such as HOCl and ionizing radiation. However, the reactivity of dC· is not well understood. To reduce this knowledge gap, we photochemically generated dC· from a nitrophenyl oxime nucleoside and within chemically synthesized oligonucleotides from the same precursor. dC· formation is confirmed by transient UV-absorption spectroscopy in laser flash photolysis (LFP) experiments. LFP and duplex DNA cleavage experiments indicate that dC· oxidizes dG. Transient formation of the dG radical cation (dG+•) is observed in LFP experiments. Oxidation of the opposing dG in DNA results in hole transfer when the opposing dG is part of a dGGG sequence. The sequence dependence is attributed to a competition between rapid proton transfer from dG+• to the opposing dC anion formed and hole transfer. Enhanced hole transfer when less acidic O6-methyl-2'-deoxyguanosine is opposite dC· supports this proposal. dC· produces tandem lesions in sequences containing thymidine at the 5'-position by abstracting a hydrogen atom from the thymine methyl group. The corresponding thymidine peroxyl radical completes tandem lesion formation by reacting with the 5'-adjacent nucleotide. As dC· is reduced to dC, its role in the process is traceless and is only detectable because of the ability to independently generate it from a stable precursor. These experiments reveal that dC· oxidizes neighboring nucleotides, resulting in deleterious tandem lesions and hole transfer in appropriate sequences.

Unravelling the Photoprotection Capacity of Resveratrol on Histidine Oxidation

Exposure to sun radiation causes great oxidative stress and activates a numerous of defense mechanisms in living systems, such as the synthesis of antioxidants. Resveratrol (RSV), a naturally occurring stilbene molecule, has antioxidant properties and is synthesized in large amounts when plants are under high oxidative stress. Likewise, under UV and visible radiation, biomolecules are oxidized, losing their physiological properties and, therefore, avoiding the harmful effects of solar radiation is crucial in order to preserve the functionality of cellular components. In proteins, one essential component that is often susceptible to degradation is the amino acid histidine (His), which can be modified via several oxidizing mechanisms. In this article, we evaluate the photoprotection capacity of RSV in photosensitized oxidation of His, which is initiated with a one-electron transfer reaction, yielding the His radical cation (His•+). The photoprotective properties of RSV are evaluated using kinetics analysis during steady-state irradiation and laser flash photolysis experiments. The experimental results reveal that the presence of RSV in the solution causes an evident decrease of the His consumption initial rates as a result of a reaction between His•+ and RSV that recovers the amino acid. In addition, we conclude that during its antioxidant action, RSV is consumed being a sacrificial antioxidant.

Free radical polymerization of acrylates bearing acetylene for preparation of clickable polymers

It is still a challenge to directly, without any control means, synthesize polymers bearing acetylene groups by normal free radical polymerization (FRP) of acetylenyl vinyl monomers, such as propargyl (meth)acrylates, because the reactive acetylene groups result in chain branching and gelation. Some propargyl acrylates with alkyl substitutes, however, can afford linear and soluble polymers. To clarify the substitute effects the FRP behaviours of a series of acrylates bearing acetylene groups with or without substitutes are studied. Homopolymerizations of acetylenyl acrylates without or with mono-substitute on propargyl led to branched polymers at early reaction stage, and became gelled after 0.5 h with monomer conversion over 12% and gel content over 50%. The acetylenyl acrylates with di-substitutes on propargyl can afford linear and soluble homo- and copolymers with‾Mn up to over 10,000 and with 45–75% monomer conversion, except for monomer 2-(2-methylbut-3-yn-2-yloxy)ethyl acrylate (DMPOEA) which has di-substitutes and a spacer connecting to the propargyl. Reactivities of vinyl and acetylene groups were found within 20 times difference by laser flash photolysis (LFP) experiments. The substitute effects of acetylenyl acrylates on FRP are explained by the intramolecular cycloaddition between CC and CC to form lactone from the s-cis conformers of the di-alkyl substituted acetylenyl acrylates. Propagating radicals can transform to stable allyl radicals by α-H atom transfer and thus the chain propagations stop. Both real-time FT-IR monitoring of photo-polymerization and high resolution 13C NMR spectra of the polymer confirmed the existence of lactone units on polymer chains. The hypothesis is also accordance with facts obsorved in the FRP. Finally postpolymerization modification through acetylene groups on polymer chains by photo thiol-yne click reaction is exemplified.

Competition Between Cα -S and Cα -Cβ Bond Cleavage in β-Hydroxysulfoxides Cation Radicals Generated by Photoinduced Electron Transfer.

A kinetic and product study of the 3‐cyano‐N‐methyl‐quinolinium photoinduced monoelectronic oxidation of a series of β‐hydroxysulfoxides has been carried out to investigate the competition between Cα‐S and Cα‐Cβ bond cleavage within the corresponding cation radicals. Laser flash photolysis experiments unequivocally established the formation of sulfoxide cation radicals showing their absorption band (λ max ≈ 520 nm) and that of 3‐CN‐NMQ• (λ max ≈ 390 nm). Steady‐state photolysis experiments suggest that, in contrast to what previously observed for alkyl phenyl sulfoxide cation radicals that exclusively undergo Cα‐S bond cleavage, the presence of a β‐hydroxy group makes, in some cases, the Cα‐Cβ scission competitive. The factors governing this competition seem to depend on the relative stability of the fragments formed from the two bond scissions. Substitution of the β‐OH group with ‐OMe did not dramatically change the reactivity pattern of the cation radicals thus suggesting that the observed favorable effect of the hydroxy group on the Cα‐Cβ bond cleavage mainly resides on its capability to stabilize the carbocation formed upon this scission.

Mechanistic Insight into the Light-Triggered CuAAC Reaction: Does Any of the Photocatalyst Go?

The attainment of transition-metal catalysis and photoredox catalysis has represented a great challenge over the last years. Herein, we have been able to merge both catalytic processes into what we have called "the light-triggered CuAAC reaction". Particularly, the CuAAC reaction reveals opposite outcomes depending on the nature of the photocatalyst (eosin Y disodium salt and riboflavin tetraacetate) and additives (DABCO, Et3N, and NaN3) employed. To get a better insight into the operating processes, steady-state, time-resolved emission, and laser flash photolysis experiments have been performed to determine reactivity and kinetic data. These results, in agreement with thermodynamic estimations based on reported data, support the proposed mechanisms. While for eosin Y (EY), Cu(II) was reduced by its triplet excited state; for riboflavin tetraacetate (RFTA), mainly triplet excited RFTA state photoreductions by electron donors as additives are mandatory, affording RFTA•- (from DABCO and NaN3) or RFTAH• (from Et3N). Subsequently, these species are responsible for the reduction of Cu(II). For both photocatalysts, photogenerated Cu(I) finally renders 1,2,3-triazole as the final product. The determined kinetic rate constants allowed postulating plausible mechanisms in both cases, bringing to light the importance of kinetic studies to achieve a strong understanding of photoredox processes.

Photohomolysis and Photoheterolysis in Aryl Sulfonates and Aryl Phosphates.

The photochemical behaviour of selected aryl sulfonates and phosphates (ArOX) in polar and nonpolar media has been investigated by laser flash photolysis (LFP) experiments. Two main pathways have been identified, namely the photohomolysis of the ArO-X bond or the photoheterolysis of the Ar-OX bond depending on the nature of the leaving group (OX) and on the nature of the substituents on the aromatic ring. In nonpolar solvents the esters are quite photostable due to an efficient triplet deactivation. In polar solvents, the homolytic fragmentation of the ArO-S bond from the exited singlets was found in aryl sulfonates bearing moderately electron-donating groups as well as electron-withdrawing groups. In electron-rich aryl phosphates and sulfonates photoheterolysis of the Ar-OP/Ar-OS bond took place as the exclusive pathway.

Dichlorine radicals (Cl2•—) promote the photodegradation of propranolol in estuarine and coastal waters

Propranolol (PRO) is frequently detected in estuarine and coastal waters, which has adverse effects on estuarine and coastal ecosystems. In this study, the effects of halide ions and DOM from estuarine and coastal waters on the photochemical transformation of PRO were investigated. The results demonstrated that the presence of Br- alone exhibited slight effect on photochemical transformation of PRO, while photodegradation rates of PRO increased with the addition of 0.1–0.54 M Cl-. The quenching experiments and the laser flash photolysis experiments together demonstrated the generation of Cl2•— in the photolytic systems. Cl2•— is possibly produced through the charge separation of exciplex of 3PRO* and Cl- rather than via direct oxidation of Cl-. Additional experiments indicated that addition of seawater DOM inhibited the halide ions-sensitized photodegradation rates of PRO, which may be due to the quenching of Cl2•— by phenolic substances in DOM molecules. Compared with pure water, three new photochemical intermediates were identified in the presence of DOM or Cl-. The direct photolysis of PRO mainly reacted by hydroxyl additions, hydroxyl elimination and de-propylation, whereas electron transfer coupled with H-abstraction by Cl2•— and 3DOM* was proposed as the primary role for PRO degradation in the presence of Cl- or DOM.

P-Nitroterphenyl units for near-infrared two-photon uncaging of calcium ions

Near-infrared two-photon (NIR-2P) uncaging reactions are useful for precise investigations of the roles of biologically active compounds in various phenomena. In this study, we synthesized new NIR-2P-responsive chromophores with terphenyl backbones featuring an ethylene glycol tetraacetic acid (EGTA) unit as a Ca2+-selective chelator. The photolabile Ca2+ chelators were synthesized in high yields (83 % for 1a, 45 % for 1b) from commercially available starting materials. One- and two-photon photochemical excitations of the synthesized terphenyl derivatives featuring two different electron-donating groups (dimethylamino (NMe2) and methoxyl (OMe)), which were thermally stable at least 36 h at 37 C in HEPES buffer (pH 7.4), were conducted to clarify the substituent effect on the photophysical and photochemical properties. The NMe2 group, which is a stronger electron-donating group, has led to a higher two-photon cross-section (σ2 = 75 GM at 780 nm) compared to the OMe group (σ2 = 9 GM at 720 nm) in a pH = 7.4 HEPES buffer solution, although the molar extinction coefficient (ε) of the NMe2-substituted one (ε = 10,371 at λmax =363 nm) was lower than that of the OMe-derivative (ε = 14,351 at λmax = 344 nm). The quantum yield (Φ = 0.04) for the decomposition of the NMe2-substituted caged compound was lower than that for the OMe-substituted caged compound (Φ = 0.40). One and two-photon uncaging efficiency values of the OMe substituted compound, ε × Φ = 5740 and σ2 × Φ = 3.6, were higher than those of the NMe2-substituted compound, 415 and 3.0, respectively. Laser flash photolysis experiments clarified the decay rate constants (2.5 × 106 and 1.1 × 102 s−1) of aci-nitro intermediates formed during the uncaging process. A concentration jump of Ca2+ was confirmed in the photolysis of caged calcium compounds newly synthesized in this study, indicating the terphenyl derivatives can be also utilized in physiological experiments. The corresponding acetoxymethyl esters were also prepared for physiological studies.

Wavelength dependent photochemistry of BODIPY-phenols and their applications in the fluorescent labeling of proteins.

A series of BODIPY dyes were synthesized, that were at the 3, or 3 and 5 positions, substituted by photochemically reactive quinone methide (QM) precursor moieties. Fluorescence properties of the molecules were investigated and we demonstrated that the molecules undergo wavelength dependent photochemistry. Photodeamination to deliver QMs takes place only upon excitation to higher excited singlet states, showing unusual anti-Kasha photochemical reactivity. The findings were corroborated by TD-DFT computations. Laser flash photolysis experiments could not reveal QMs due to the low efficiency of their formation, but enabled the detection of phenoxyl radicals. The applicability of the molecules for the fluorescent labeling of bovine serum albumin as a model protein upon photoexcitation at 350 nm was demonstrated.

Reactivity of Chlorine Radicals (Cl• and Cl2•-) with Dissolved Organic Matter and the Formation of Chlorinated Byproducts.

Chlorine radicals, including Cl• and Cl2•-, can be produced in sunlight waters (rivers, oceans, and lakes) or water treatment processes (e.g., electrochemical and advanced oxidation processes). Dissolved organic matter (DOM) is a major reactant with, or a scavenger of, Cl• and Cl2•- in water, but limited quantitative information exists regarding the influence of DOM structure on its reactivity with Cl• and Cl2•-. This study aimed at quantifying the reaction rates and the formation of chlorinated organic byproducts produced from Cl• and Cl2•- reactions with DOM. Laser flash photolysis experiments were conducted to quantify the second-order reaction rate constants of 19 DOM isolates with Cl• (kDOM-Cl•) and Cl2•- (kDOM-Cl2•-), and compare those with the hydroxyl radical rate constants (kDOM-•OH). The values for kDOM-Cl• ((3.71 ± 0.34) × 108 to (1.52 ± 1.56) × 109 MC-1 s-1) were orders of magnitude greater than the kDOM-Cl2•- values ((4.60 ± 0.90) × 106 to (3.57 ± 0.53) × 107 MC-1 s-1). kDOM-Cl• negatively correlated with the weight-averaged molecular weight (MW) due to the diffusion-controlled reactions. DOM with high aromaticity and total antioxidant capacity tended to react faster with Cl2•-. During the same experiments, we also monitored the formation of chlorinated byproducts through the evolution of total organic chlorine (TOCl) as a function of chlorine radical oxidant exposure (CT value). Maximum TOCl occurred at a CT of 4-8 × 10-12 M·s for Cl• and 1.1-2.2 × 10-10 M·s for Cl2•-. These results signify the importance of DOM in scavenging chlorine radicals and the potential risks of producing chlorinated byproducts of unknown toxicity.

Remediation of soil contaminated with ibuprofen by persulfate activated with gallic acid and ferric iron

This study investigated the ibuprofen (IBP) degradation by persulfate (PS) activated with Fe(III) and gallic acid (GA) in the soil system. This study aimed to find an environmentally friendly method for the purification of soil organic pollutants. Results showed that the addition of GA could greatly improve the IBP oxidation efficiency due to the GA reduction effect of Fe(III) to Fe(II). The optimum dosage of each reagent was obtained with Fe(III):GA:PS = 1:1:10 to achieve 80.7% deterioration of IBP in 2 h when the initial IBP concentration was 48.4 μmol kg−1. Quenching experiments suggested that hydroxyl radical (HO), sulfate radical (SO4−), and superoxide anion radicals (O2−) played important roles in the oxidation of IBP, and the contribution ratios were 28.5%, 25.5% and 44.4% respectively. The reaction started from the complexing between Fe(III) and GA. The Fe(III)-GA complex was transferred to quinone substance and Fe(II). PS was efficiently activated by Fe(II), and radicals were produced. The second order rate constants of the reactions between HO/SO4−and GA/IBP were determined with laser flash photolysis experiments (kHO, GA = 2.80 × 1010 M−1 s−1, kSO4−, GA = 2.01 × 109 M−1 s−1, and kHO,IBP = 3.11 × 109 M−1 s−1, kSO4−,IBP = 1.97 × 109 M−1 s−1). The soil organic matter (SOM) slightly decreased by 4.46%, and pH was changed from 7.8 to 6.2 after 240 min of reaction. The introduction of GA significantly increased the dissolved organic carbon (DOC), while the oxidation of the Fe(III)/GA/PS system decreased by 5.85% of the DOC content. Four chief byproducts were identified with GC–MS, and a potential degradation mechanism of IBP was proposed.

Turning Waste into Useful Products by Photocatalysis with Nanocrystalline TiO2 Thin Films: Reductive Cleavage of Azo Bond in the Presence of Aqueous Formate.

UV-photoexcitation of TiO2 in contact with aqueous solutions of azo dyes does not imply only its photocatalytic degradation, but the reaction fate of the dye depends on the experimental conditions. In fact, we demonstrate that the presence of sodium formate is the switch from a degradative pathway of the dye to its transformation into useful products. Laser flash photolysis experiments show that charge separation is extremely long lived in nanostructured TiO2 thin films, making them suitable to drive both oxidation and reduction reactions. ESR spin trapping and photoluminescence experiments demonstrate that formate anions are very efficient in intercepting holes, thereby inhibiting OH radicals formation. Under these conditions, electrons promoted in the conduction band of TiO2 and protons deriving from the oxidation of formate on photogenerated holes lead to the reductive cleavage of N=N bonds with formation and accumulation of reduced intermediates. Negative ion ESI–MS findings provide clear support to point out this new mechanism. This study provides a facile solution for realizing together wastewater purification and photocatalytic conversion of a waste (discharged dye) into useful products (such as sulfanilic acid used again for synthesis of new azo dyes). Moreover, the use of TiO2 deposited on an FTO (Fluorine Tin Oxide) glass circumvents all the difficulties related to the use of slurries. The obtained photocatalyst is easy to handle and to recover and shows an excellent stability allowing complete recyclability.

Generation of N,N-Di(4-bromophenyl)nitrenium Ion under Acidic Conditions: Search for a Nitrenium Dication.

The behavior of the N,N-di(4-bromophenyl)nitrenium ion under acidic aqueous conditions was examined via laser flash photolysis experiments. A long-lived species forms and can be assigned as the cation radical or the dication. This species is unreactive toward nucleophiles and reactive toward strong electron donors, consistent with a cation radical. Mechanistic analysis indicates that its formation is through a separate pathway compared to that of the nitrenium ion, suggestive of a triplet mechanism.

Application of a microbial siderophore desferrioxamine B in sunlight/Fe3+/persulfate system: from the radical formation to the degradation of atenolol at neutral pH.

The present work reported a modified persulfate activation process with a microbial siderophore named desferrioxamine B (DFOB). DFOB was a natural complexing agent and could complex with Fe3+ strongly. The photochemical reactivity of Fe(III)-DFOB was studied. Fe2+ and HO• were produced from Fe(III)-DFOB photolysis. Furthermore, the degradation of atenolol (ATL) was followed in light/persulfate (PS)/Fe(III)-DFOB system. The main oxidative radicals were SO4•- in this system. The results of pH effect showed that there was no obviously fluctuation on ATL degradation efficiency with the pH increased from 2.5 to 8.4. Moreover, kSO4•-,DFOB was determined by laser flash photolysis (LFP) experiments. DFOB had positive effect on Fe2+ formation but negative effect on ATL degradation due to the high react rate constant between DFOB and SO4•-. The effects of chloride and carbonate ion were also investigated. The results in this study proposed the reaction mechanism of the modified persulfate activation process, and it could be applied in neutral and weak-alkaline pH range.

Degradation of Acetaminophen via UVA-induced advanced oxidation processes (AOPs). Involvement of different radical species: HO[rad], SO4[rad]− and HO2[rad]/O2[rad

In this work, UVA radiation that is part of solar light is taken as the irradiation source and radicals (HO, SO4− and HO2/O2−) are generated through activation of hydrogen peroxide (H2O2), sodium persulfate (Na2S2O8) and Bismuth catalyst (BiOCl), respectively. The distinguished performance in removing acetaminophen (ACTP), a model pharmaceutical pollutant, by these three radicals was compared for the first time. Effect of pH, halide ions concentration and interfacial mechanism have been investigated in detail. Interestingly, results show that heterogeneous UVA/BiOCl process has higher degradation efficiency than homogeneous UVA/H2O2 and UVA/Na2S2O8 systems whatever the solution’s pH. To explain these results, second order reaction rate constant (kradical, ACTP) have been determined with laser flash photolysis (LFP) or radical scavenging experiments. The strongly interfacial-depended HO2/O2− radicals have the lowest second order rate constant with ACTP but highest steady state concentration. BiOCl is much easier activated by UVA, and outstanding ACTP mineralization can be achieved. Combination of BiOCl and Na2S2O8 exhibits synergistic effects rather than antagonism effects with H2O2. This study highlights the relative effective utilization of solar light through interfacial directed BiOCl photocatalysis and its synergistic effects with traditional oxidants.

Independent Generation and Time-Resolved Detection of 2'-Deoxyguanosin-N2-yl Radicals.

Guanine radicals are important reactive intermediates in DNA damage. Hydroxyl radical (HO. ) has long been believed to react with 2'-deoxyguanosine (dG) generating 2'-deoxyguanosin-N1-yl radical (dG(N1-H). ) via addition to the nucleobase π-system and subsequent dehydration. This basic tenet was challenged by an alternative mechanism, in which the major reaction of HO. with dG was proposed to involve hydrogen atom abstraction from the N2-amine. The 2'-deoxyguanosin-N2-yl radical (dG(N2-H). ) formed was proposed to rapidly tautomerize to dG(N1-H). . We report the first independent generation of dG(N2-H). in high yield via photolysis of 1. dG(N2-H). is directly observed upon nanosecond laser flash photolysis (LFP) of 1. The absorption spectrum of dG(N2-H). is corroborated by DFT studies, and anti- and syn-dG(N2-H). are resolved for the first time. The LFP experiments showed no evidence for tautomerization of dG(N2-H). to dG(N1-H). within hundreds of microseconds. This observation suggests that the generation of dG(N1-H). via dG(N2-H). following hydrogen atom abstraction from dG is unlikely to be a major pathway when HO. reacts with dG.

Independent Generation and Time‐Resolved Detection of 2′‐Deoxyguanosin‐N2‐yl Radicals

AbstractGuanine radicals are important reactive intermediates in DNA damage. Hydroxyl radical (HO.) has long been believed to react with 2′‐deoxyguanosine (dG) generating 2′‐deoxyguanosin‐N1‐yl radical (dG(N1‐H).) via addition to the nucleobase π‐system and subsequent dehydration. This basic tenet was challenged by an alternative mechanism, in which the major reaction of HO. with dG was proposed to involve hydrogen atom abstraction from the N2‐amine. The 2′‐deoxyguanosin‐N2‐yl radical (dG(N2‐H).) formed was proposed to rapidly tautomerize to dG(N1‐H).. We report the first independent generation of dG(N2‐H). in high yield via photolysis of 1. dG(N2‐H). is directly observed upon nanosecond laser flash photolysis (LFP) of 1. The absorption spectrum of dG(N2‐H). is corroborated by DFT studies, and anti‐ and syn‐dG(N2‐H). are resolved for the first time. The LFP experiments showed no evidence for tautomerization of dG(N2‐H). to dG(N1‐H). within hundreds of microseconds. This observation suggests that the generation of dG(N1‐H). via dG(N2‐H). following hydrogen atom abstraction from dG is unlikely to be a major pathway when HO. reacts with dG.