OH Adduct Research Articles

Theoretical studies on the kinetics and mechanism of hydroxyl radical reaction with quinclorac and quinmerac herbicides in aqueous media

AbstractThe kinetic and mechanistic studies for the reaction of hydroxyl radical with two quinoline based herbicides, namely, quinclorac and quinmerac has been performed using various computational methods in aqueous media. Geometry optimizations were performed using Density Functional Theory (DFT) methods including water as the solvent. Local reactivity parameters of these herbicides towards the •OH radical are predicted using condensed Fukui function. Single point energies of various species were calculated using double hybrid method, namely, B2PLYP–D for better accuracy. The pKa values for these acid based herbicides allow them to exist in deprotonated form in aqueous condition. Hence, the calculations are also performed for the deprotonated or the anionic form apart from the neutral species. Individual rate coefficients for •OH radical addition reaction with each carbon atoms were evaluated using conventional transition state theory using one–dimensional tunneling corrections. The solvent effect on reaction is implemented through Collins–Kimball formulations. Both the approaches, namely, the Fukui index and individual rate constant determination confirms that the most reactive site for the •OH radical addition in these two herbicide is the carbon atom attached to the COOH group. The total rate constant for the •OH radical reaction with both neutral and anionic forms of these two herbicides are relatively high and equal to its diffusion‐limit value. Evaluation of the ecotoxicities of the parent herbicides and their OH adducts is estimated using the structure–activity relationship concept.

Revealing the crucial role of Carbon-Centered radicals in UV/PAA process for trace amounts of organic contaminants removal

Peracetic acid (PAA) is a promising peroxyacid to combine with ultraviolet (UV) to eliminate trace amounts of organic contaminants (TrOCs), with growing importance in water treatment. In spite of a large number of research on UV/PAA process, the underlying mechanism still require further investigation, due to the difficulties in analyzing the carbon-centered radicals. Here we demonstrated the crucial roles of the carbon-centered radicals (namely CH3C(O)OO), via electron paramagnetic resonance (EPR) and kinetic models, with concentration of 4 orders magnitude higher than that of OH. Then, the theoretical calculation using density functional theory (DFT) showed that CH3C(O)OO and OH tended to attack phenol mostly through the route of radical adduct formation. The generated CH3C(O)OO and OH adducts may undergo further reactions to produce catechol and hydroquinone. Further, the environmental applicability of UV/PAA process for TrOC abatement was evaluated, including the effects of pH, PAA dosage, water constituents, as well as the variation of acute toxicity. Different from the OH-dominated process, kinetic analysis revealed that CH3C(O)OO was almost immune to the presence of water constituents, leading to the outperformance of UV/PAA process in actual water samples. Our present study could inform the application of UV/PAA process in water and wastewater treatment.

Radiolytic Inactivation of Ponceau 4R Mutagenicity in Aqueous Solution

The electron-beam treatment of an aqueous solution of the azo dye Ponceau 4R (0.02 g/dm3) at a dose of 1.5 kGy in air led to the complete elimination of color and genotoxicity. The addition of OH radicals to the system of conjugated bonds of the dye (the formation of OH adducts) was a key radiolytic process, which led to the elimination of side groups with lower binding energies

The Evaluation of SHAPE-MaP RNA Structure Probing Protocols Reveals a Novel Role of Mn2+ in the Detection of 2'-OH Adducts.

Chemical probing, for decades, has been one of the most popular tools for studying the secondary structure of RNA molecules. Recently, protocols for simultaneous analysis of multiple RNAs have been developed, enabling in vivo transcriptome-wide interrogation of the RNA structure dynamics. One of the most popular methods is the selective 2'-hydroxyl acylation analyzed by primer extension and mutational profiling (SHAPE-MaP). In this study, we describe the evaluation of this protocol by addressing the influence of the reverse transcription enzymes, buffer conditions, and chemical probes on the properties of the cDNA library and the quality of mutational profiling-derived structural signals. Our results reveal a SuperScript IV (SSIV) reverse transcriptase as a more efficient enzyme for mutational profiling of SHAPE adducts and shed new light on the role of Mn2+ cations in the modulation of SSIV readthrough efficiency.

Role of the Hydroxyl Radical-Generating System in the Estimation of the Antioxidant Activity of Plant Extracts by Electron Paramagnetic Resonance (EPR).

The scavenging activity of hydroxyl radicals, produced by the Fenton reaction, is commonly used to quantify the antioxidant capacity of plant extracts. In this study, three Fenton systems (Fe/phosphate buffer, Fe/quinolinic acid and Fe/phosphate buffer/quinolinic acid) and the thermal degradation of peroxydisulfate were used to produce hydroxyl radicals; the hydroxyl radical scavenging activity of plant extracts (ginger, blueberry juices and green tea infusion) and chemical compounds (EGCG and GA) was estimated by spin trapping with DMPO (5,5-dimethyl-1-pyrroline N-oxide) and EPR (Electron Paramagnetic Resonance) spectroscopy. Phosphate buffer was used to mimic the physiological pH of cellular systems, while quinolinic acid (pyridine-2,3-dicarboxylic acid) facilitates the experimental procedure by hindering the spontaneous oxidation of Fe(II). The EC50 (the concentration of chemical compounds or plant extracts which halves the intensity of the DMPO–OH adduct) values were determined in all the systems. The results show that, for both the chemical compounds and the plant extracts, there is not a well-defined order for the EC50 values determined in the four hydroxyl radical generating systems. The interactions of phosphate buffer and quinolinic acid with the antioxidants and with potential iron-coordinating ligands present in the plant extracts can justify the observed differences.

Rates and Yields of Unimolecular Reactions Producing Highly Oxidized Peroxy Radicals in the OH-Induced Autoxidation of α-Pinene, β-Pinene, and Limonene.

Recent ambient atmospheric measurements have detected highly oxygenated organic molecules (HOMs) at many sites and are a consequence of autoxidation processes occurring at ambient temperatures. Monoterpenes in particular have a propensity to autoxidize although they exhibit a wide range of HOM yields, which may be due to a variety of reasons including reactions with different oxidants like OH and O3, differing hydrogen (H) atom transfer or peroxy radical cyclization rates, numbers of available reaction pathways, and/or energy loss processes for activated HO-monoterpene or O3-monoterpene adducts. In this work, the autoxidation mechanisms of (+)-α-pinene, (+)-β-pinene, and (+)-limonene following initial OH oxidation and three successive O2 additions are examined using density functional theory (DFT) to understand what accounts for the disparity. Rates of different potential autoxidation pathways initiated by OH addition or abstraction reactions are quantified using transition-state theory (TST) and master equation approaches using the lowest-energy conformers. OH abstraction reactions do not appreciably influence HOM production in the pinenes and limit autoxidation for limonene because the subsequent autoxidation reactions are slow while OH addition reactions are found to be the main route to HOMs for all three monoterpenes. Generally, faster autoxidation rates are computed in later unimolecular reactions that produce RO7 radicals after OH addition (∼10 s-1 or greater) than rates for RO5 peroxy radical production (0.2-7 s-1). Mechanistic pathways that form RO7 peroxy radicals are similar for all three monoterpenes with a particular bicyclo RO7 radical involving a five-membered peroxide ring being favored for all three monoterpenes. The molar yields of RO7 radicals are 4.6% (+10.0/-2.4), 3.8% (+9.1/-2.6), and 7.6% (+13.1/-4.9) for α-pinene, β-pinene, and limonene, respectively, at 298 K and 1 ppb of NO and only significantly decline at NO concentrations exceeding 10 ppb. The higher yield for limonene relative to the pinenes is predominantly a consequence of the initial oxidation step: OH adducts of the bicyclic pinenes have to use the excess energy after OH addition to break one of the rings and make the molecule more flexible for autoxidation although this process is inefficient, while one of the prominent OH adducts for monocyclic limonene does not have to do this and may add O2 immediately before autoxidizing further. These insights may be used to guide a better representation of these processes in atmospheric models because they affect particulate matter (PM), NOx, and ozone concentrations via enhanced production of low-volatility species, less early-generation NOx cycling, and altered organic nitrate production.

Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil.

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (●OH) and azide (●N3) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ●OH with 2-SeU are dominated by an absorption band with an λmax = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2●). The dimeric radical (2●) is formed in the reaction of a selenyl-type radical (6●) with 2-SeU, and both radicals are in equilibrium with Keq = 1.3 × 104 M−1 at pH 4 (below the pKa of 2-SeU). Similar equilibrium with Keq = 4.4 × 103 M−1 was determined for pH 10 (above the pKa of 2-SeU), which admittedly involves the same radical (6●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2●−). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λmax = 390 nm, which was assigned to the ●OH adduct to the double bond at C5 carbon atom (3●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ●N3-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.

Hydroxylamine enhanced treatment of highly salty wastewater in Fe0/H2O2 system: Efficiency and mechanism study

• Fe 0 -HA/H 2 O 2 enhanced 7.9 times for tartrazine removal in salty system than Fe 0 /H 2 O 2 . • Fe 0 -HA/H 2 O 2 could widen the application of pH to neutral pH. • Fe 0 -HA/H 2 O 2 could obtain remarkable performance at wide salt concentration levels. • Fe 0 -HA/H 2 O 2 have good suitability for treatment of various types of salty wastewater. This study demonstrated a fast and efficient treatment of highly salty wastewater in a hydroxylamine (HA) enhanced Fe 0 /H 2 O 2 system. The apparent rate constant ( k ) in Fe 0 -HA/H 2 O 2 system could achieve 7.9 times higher than that in Fe 0 /H 2 O 2 system using tartrazine as the target pollutant and Na 2 SO 4 as the model salt. The value of k for removing tartrazine in the salty system by Fe 0 -HA/H 2 O 2 process even 12.7 times higher than that in the conventional system which was attributed to the faster Fe 0 corrosion rate in the salty system. Meanwhile, Fe 0 -HA/H 2 O 2 process could obtain remarkable performance for highly salty wastewater treatment at wide salt concentration levels (0–0.2 M) and could also widen the application of pH to neutral pH. The characterizations by SEM and XPS, the intensity of DMPO–OH adduct signals and detection of Fe 2+ and Fe 3+ ions in different systems confirmed the good performance was owing to the faster Fe 0 corrosion rate in the salty system and acceleration of Fe 3+ /Fe 2+ recycle owing the addition of HA inducing fast catalysis of H 2 O 2 and generation of hydroxyl radicals. Moreover, the value of k for removing tartrazine in various types of salty system by Fe 0 -HA/H 2 O 2 process was 2.9–30.8 times larger than Fe 0 /H 2 O 2 process demonstrating the good suitability of Fe 0 -HA/H 2 O 2 process at various types of salty system. Therefore, this study will shed lights on the reactivity of the Fe 0 -HA/H 2 O 2 system and provide an alternative pathway for removal of salty wastewater.

Effects of biochar-dissolved organic matter on the photodegradation of sulfamethoxazole and chloramphenicol in biochar solutions as revealed by oxygen reduction performances and free radicals

Photodegradation plays an important role to directly and indirectly degrade residual antibiotics in environments. Biochar as an emerging material can be sustainably produced and used in various environmental remediation fields. However, very few efforts and mechanisms have been made to elucidate the photodegradation of sulfamethoxazole (SMX) and chloramphenicol (CAP) in the biochar (BCs), modified-biochar (MBCs), and biochar-derived dissolved organic matters (BDOMs) solutions under Xenon-lamp irradiation. In this study, different concentrations of BCs and MBCs (100, 250, 500, 1000 and 2000 mg L−1), and BDOMs (2, 4, 6 and 10 mg L−1) have been chosen to research the effects on the photodegradation of SMX and CAP. By studying the oxygen reduction performances and environmental persistent free radicals (EPFRs) of BCs or MBCs and DMPO-·OH adducts from the photodegradation process, the different photodegradation pathways were proposed. SMX and CAP can be directly photodegraded in the BCs or MBCs solutions by Xenon-lamp. In addition, in the low-concentration of BCs (100, 250, 500 mg L−1) and MBCs solutions, the oxygen reduction performances and EFPRs of BCs and MBCs can indirectly photodegrade SMX and CAP by promoting the oxidation process and electron transfer on BCs surface under Xenon-lamp irradiation. While the high-concentrations of BCs (1000 and 2000 mg L−1) and MBCs hindered the penetration of Xenon-lamp, resulting in decreasing the photodegradation efficiency. In particular, BDOMs released from BCs solutions significantly improved the photodegradation rates and efficiencies of SMX and CAP by generating ·OH radicals to photodegrade SMX and CAP indirectly. Therefore, this study provides a theoretical basis for the photodegradation of SMX and CAP with biochar and modified-biochar, and also gives a comprehensive evaluation of the effects and mechanisms of BDOMs on the photodegradation.

A Mononuclear Non-Heme Manganese(III)–Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer

Metal-oxygen complexes, such as metal-oxo [M(O2-)], -hydroxo [M(OH-)], -peroxo [M(O22-)], -hydroperoxo [M(OOH-)], and -superoxo [M(O2•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph3P). However, OAT of metal-aqua complexes, [M(OH2)]n+, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph3P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)MnII(OH2)]+, followed by nucleophilic attack of H2O in [(dpaq)MnII(OH2)]+ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)2C6H3S•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph3P occurs via electron transfer from Ph3P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph3P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.

Hydroxyl Radical Generation by the H2O2/CuII/Phenanthroline System under Both Neutral and Alkaline Conditions: An EPR/Spin-Trapping Investigation

The copper–phenanthroline complex CuI(Phen)2 was the first artificial nuclease studied in biology. The mechanism responsible for this activity involves CuII(Phen)2 and H2O2. Even if H2O2/Cu systems have been extensively studied in biology and oxidative chemistry, most of these studies were carried out at physiological pH only, and little information is available on the generation of radicals by the H2O2/CuII-Phen system. In the context of paper pulp bleaching to improve the bleaching ability of H2O2, this system has been investigated, mostly at alkaline pH, and more recently at near-neutral pH in the case of dyed cellulosic fibers. Hence, this paper aims at studying the production of radicals with the H2O2/CuII-Phen system at near-neutral and alkaline pHs. Using the EPR/spin-trapping method, HO• formation was monitored to understand the mechanisms involved. DMPO was used as a spin-trap to form DMPO–OH in the presence of HO•, and two HO• scavengers were compared to identify the origin of the observed DMPO–OH adduct, as nucleophilic addition of water onto DMPO leads to the same adduct. H2O2 decomposition was enhanced by the addition of CuII–Phen (and only slightly by addition of CuSO4), reaching a level similar to the Fenton reagent at near-neutral pH. This evidences the role of Phen, which improves the effect of CuII by tuning the electronic structure and structural properties of the corresponding CuII complexes.

Electrospun polymer nanofibers coated with TiO2 hollow spheres catalyze for high synergistic photo-conversion of Cr(VI) and As(III) using visible light

TiO2@PAN nanofibers with inorganic (TiO2 hollow spheres, PAN: polyacrylonitrile) and organic (pre-carbonized polyacrylonitrile) ordered co-combinations were exposed to Cr(VI) and As(III) under visible light irradiation (λ > 420 nm). Relative photocatalytic activity was as follows: TiO2@PAN-3(30 mg) > TiO2@PAN-4(40 mg) > TiO2@PAN-2(20 mg) > TiO2@PAN-1(10 mg) > PAN. TiO2@PAN-3 with 30 mg TiO2 hollow spheres exhibited a high photo-conversion rate (99% Cr(VI) photo-reduction efficiency and 83% As(V) formation rate) and stability (five cycles) relative to PAN and the other samples. The new insights of synergistic effect between Cr(VI) and As(III) provided a reference way for handling complex water pollutants. A series of instrumental analysis techniques demonstrated the electron-hole separation efficiency in TiO2@PAN-3. Electron Paramagnetic Resonance (EPR) of TiO2@PAN-3 depicted a stronger DMPO/·OH adduct signal. Furthermore, the smallest arc radius diameter was found on the TiO2@PAN-3 composite electrode Electrochemical Impedance Spectroscopy (EIS) Nyquist plot, which suggests that the TiO2@PAN-3 interface has minimal charge transfer resistance. In addition, TiO2@PAN-3 exhibited the lowest Photoluminescence (PL) intensity, indicating it displays the highest electron-hole separation efficiency. Based on these results, we envisage the future construction of TiO2@PAN-3 photocatalysts with high Cr(VI) photo-reduction (99%) and As(III) photo-oxidation performance, and high stability (the photo-reduction rate was still reached 82% after five cycles) for the removal of heavy metals from actual water samples.

Gold Nanoparticle as a Lewis Catalyst for Water Elimination of Tyrosine-•OH Adducts: A Radiation and Quantum Chemical Study.

The role of gold nanoparticles (AuNPs) in the degradation of tyrosine intermediates formed during the radiation-induced •OH reaction with tyrosine at pH 6.5 is investigated by measuring the radiolytic yields, G, of tyrosine (-Tyr), dityrosine (DT), and 3,4 dihydroxyphenylalanine (DOPA). The G(DT) is doubled, whereas G(-Tyr) calculated is halved in the presence of 6.0 × 10-10 mol dm-3 AuNPs. Pulse radiolysis studies are carried out to elucidate the mechanism and nature of the transient formed in the reaction of •OH and •N3 with tyrosine. The formation of tyrosyl radical in the presence of AuNPs is found to be a major pathway through the decay of tyrosine-•OH adducts via the water elimination reaction, which is found to be 3× faster in the presence of AuNPs. Quantum chemical calculations on the system showed favorable formation of the tyrosine-AuNP complex. A new plausible mechanism of tyrosine-AuNP complex acting as a Lewis type catalyst in the decay of tyrosine-•OH adducts leading to reduced DOPA formation is proposed. The proposed mechanism is also complemented by the electronic spectra and energetics of the reaction of •OH with tyrosine using density functional theory calculations. Significantly, the H-shift reaction of ortho-tyrosine-•OH adducts is also found to be energetically viable. The investigation provides a new physical insight into the effect of AuNPs on the decay of free-radical transient species and demonstrates the potential of radiation chemical techniques and quantum chemical calculations as a tool for understanding the impact of metal nanoparticles in free-radical oxidation of amino acids, which is important in the use of metal nanoparticles for biomedical applications.

Comparison of hydrogen atom and hydroxyl radical reactions with simple aromatic molecules in aqueous solution

The reactions of hydrogen atoms and hydroxyl radicals with benzene and 10 monosubstituted benzenes were investigated by measuring transient absorption spectra, calculating Gibbs free energies, absorption spectra of intermediates formed and determining the rate constants of H reactions. The basic reaction is addition to the aromatic ring forming cyclohexadienyl type radicals. Most radical exhibit absorption spectra with maxima in the 300–340 nm range, these maxima for molecules with strong electron withdrawing groups are shifted to 340–420 nm. The spectra of H and OH adducts are similar suggesting similar intermediates. In substituted benzenes the absorption band is due to several isomers. Ortho and para additions are important in reaction with all molecules studied. Both, H and OH can react with the –Cl, –Br and –NO2 substituents in exchange reaction without cyclohexadienyl radical. OH reacts with the aromatic ring of benzyl chloride in radical addition, while H eliminates Cl from the substituent.

Fe2+/HClO Reaction Produces FeIVO2+: An Enhanced Advanced Oxidation Process.

The reaction between Fe2+ and HClO constitutes a promising advanced oxidation process (AOP) for removing pollutants from wastewater, and •OH has been considered the dominant reactive oxidant despite limited evidence for this. Herein, we demonstrate that the Fe2+/HClO reaction enables the production of FeIVO2+ rather than •OH in acid medium, a finding that is strongly supported by multiple lines of evidence. Both X-ray absorption near-edge structure spectroscopic tests and Mössbauer spectroscopic tests confirmed the appearance of FeIVO2+ as the reactive intermediate in the reaction between Fe2+ and HClO. The determination of FeIVO2+ generation was also derived from the methyl phenyl sulfoxide (PMSO)-based probe experiments with respect to the formation of PMSO2 without •OH adducts and the density functional theory studies according to the lower energy barrier for producing FeIVO2+ compared with •OH. A dual-anode electrolytic system was established for the in situ generation of Fe2+ and HClO that allows the production of FeIVO2+. The system exhibits an enhanced capacity for oxidizing a model pollutant (e.g., phosphite) from industrial wastewater, making it an attractive and promising AOP for the abatement of aqueous contaminants.

Radiation Induced One-Electron Oxidation of 2-Thiouracil in Aqueous Solutions.

Oxidative damage to 2-thiouracil (2-TU) by hydroxyl (•OH) and azide (●N3) radicals produces various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by time-dependent density functional theory (TD-DFT) method. The transient absorption spectra recorded in the reactions of •OH with 2-TU depend on the concentration of 2-TU, however, only slightly on pH. At low concentrations, they are characterized by a broad absorption band with a weakly pronounced maxima located at λ = 325, 340 and 385 nm, whereas for high concentrations, they are dominated by an absorption band with λmax ≈ 425 nm. Based on calculations using TD-DFT method, the transient absorption spectra at low concentration of 2-TU were assigned to the ●OH-adducts to the double bond at C5 and C6 carbon atoms (3●, 4●) and 2c-3e bonded ●OH adduct to sulfur atom (1…●OH) and at high concentration of 2-TU also to the dimeric 2c-3e S-S-bonded radical in neutral form (2●). The dimeric radical (2●) is formed in the reaction of thiyl-type radical (6●) with 2-TU and both radicals are in an equilibrium with Keq = 4.2 × 103 M−1. Similar equilibrium (with Keq = 4.3 × 103 M−1) was found for pH above the pKa of 2-TU which involves admittedly the same radical (6●) but with the dimeric 2c-3e S-S bonded radical in anionic form (2●−). In turn, ●N3-induced oxidation of 2-TU occurs via radical cation with maximum spin location on the sulfur atom which subsequently undergoes deprotonation at N1 atom leading again to thiyl-type radical (6●). This radical is a direct precursor of dimeric radical (2●).

•OH oxidation of methionine in the presence of discrete water molecules: DFT, QTAIM and valence bond analyses

The first steps of the oxidation process of amino acid methionine (Met, CAS 63-68-3) by •OH radicals, leading to Met-OH• adduct and then to Met radical cation, were investigated theoretically over the last few years considering the aqueous environment as a continuum. In this work, following the same procedure that we used for the oxidation of dimethyl sulfide as reported by Domin et al. (J Phys Chem B, 121:9321), discrete water molecules, as well as relative positions, of the •OH radical to Met were taken from molecular dynamics calculations. The presence of water molecules strongly modifies the relative energies of Met-OH adducts and cations when water is properly modeled. Depending on the terminal functional groups and on the position of the •OH radical, several stable structures were found; however, the most stable radical is the N-centered or the S∴N radical cation. QTAIM analysis and valence bond (VB) treatment allowed for the characterization of the 2c∴3e nature of S∴N and S∴OH bonds. VB analysis estimated the probability of the heterolytic rupture of the •OH adduct that is modified by the presence of water molecules.

Photocatalytic Oxygenation Reactions Using Water and Dioxygen.

Water (H2 O) is the most environmentally benign reductant and is oxidized to evolve dioxygen (O2 )-the greenest oxidant-in photosystem II. This Minireview focuses on photocatalytic oxygenation of substrates with H2 O as an oxygen source and O2 as an oxidant. Metal complexes can be oxidized by two molecules of one-electron oxidants with H2 O to produce high-valent metal-oxo complexes, which act as active oxidants for oxygenating organic substrates. When an appropriate oxidant is employed for the substrate oxidation, the reduced oxidant can be oxidized by dioxygen to regenerate the oxidant when water and dioxygen are used as an oxygen source and an oxidant, respectively. Photoinduced electron transfer from a substrate (S) to the excited state of complex [(L)MIII ]+ produces a substrate radical cation (S.+ ), accompanied by the regeneration of [(L)MII ]. S.+ then reacts with H2 O to produce an OH adduct radical that is oxidized by [(L)MIII ]+ to yield an oxygenated product (SO), in which the oxygen atom originates from H2 O, accompanied by regeneration of [(L)MII ]. Photocatalytic oxidation of H2 O by O2 to produce H2 O2 is combined with the catalytic oxygenation of substrates with H2 O2 to produce the oxygenated products, in which the oxygen atom originates from O2 at the beginning but later from water. This Minireview provides a promising strategy for oxygenation of substrates by using H2 O as an oxygen source and O2 as the greenest oxidant.

Sulfate and hydroxyl radicals-initiated degradation reaction on phenolic contaminants in the aqueous phase: Mechanisms, kinetics and toxicity assessment

Phenolic organic contaminants (POCs) in water environment have been usually degraded by advanced oxidation processes (AOPs) based on sulfate radicals (SO4·-) and hydroxyl radicals (OH). In this paper, the first step of SO4·-/·OH-initiated oxidation reactions toward 19 POCs were investigated using density functional theory (DFT) in order to explore and compare the reactivity of POCs with two free radicals in the aqueous phase. The oxidation reactions initiated by SO4·-/·OH were confirmed that POCs can follow three reaction mechanisms: radical adduct formation (RAF), hydrogen atom abstraction (HAA), and single electron transfer (SET). The rate constants of all primary oxidation reactions were calculated using transition state theory (TST). The results turn out that the stronger the electron donating effects of the substituent on POCs, the better the reactivity of POCs with two free radicals. SET mechanisms are main reaction pathways for SO4·--initiated oxidation reactions. Furthermore, the ecotoxicity assessment shows that most OH adducts have higher toxic on aquatic organisms than corresponding reactants. For all the POCs covering in this work, the order of acute toxicity is p-DP > o-AP > m-AP (p-AP) > MP > CP > NP > m-DP (o-DP) > phenol > YP, while the chronic toxicity is in the order p-DP > o-AP > m-AP (p-AP) > MP > CP > NP > YP > phenol > m-DP (o-DP). Thus, the application of AOPs for the removal of POCs should be taken seriously.

Melatonin and its metabolites as chemical agents capable of directly repairing oxidized DNA.

Oxidative stress mediates chemical damage to DNA yielding a wide variety of products. In this work, the potential capability of melatonin and several of its metabolites to repair directly (chemically) oxidative lesions in DNA was explored. It was found that all the investigated molecules are capable of repairing guanine-centered radical cations by electron transfer at very high rates, that is, diffusion-limited. They are also capable of repairing C-centered radicals in the sugar moiety of 2'-deoxyguanosine (2dG) by hydrogen atom transfer. Although this was identified as a rather slow process, with rate constants ranging from 1.75 to 5.32×102 M-1 s-1 , it is expected to be fast enough to prevent propagation of the DNA damage. Melatonin metabolites 6-hydroxymelatonin (6OHM) and 4-hydroxymelatonin (4OHM) are also predicted to repair OH adducts in the imidazole ring. In particular, the rate constants corresponding to the repair of 8-OH-G adducts were found to be in the order of 104 M-1 s-1 and are assisted by a water molecule. The results presented here strongly suggest that the role of melatonin in preventing DNA damage might be mediated by its capability, combined with that of its metabolites, to directly repair oxidized sites in DNA through different chemical routes.