Mechanism Of Hydroxylation Research Articles

Diverse Mechanisms for the Aromatic Hydroxylation: Insights into the Mechanisms of the Coumarin Hydroxylation by CYP2A6

Diverse Mechanisms for the Aromatic Hydroxylation: Insights into the Mechanisms of the Coumarin Hydroxylation by CYP2A6

Imidazolate-Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites.

Imidazole ligation of metals through histidine is extensive among metalloproteins and enzymes, yet the role of the imidazolate conjugate base is often neglected, despite its potential accessibility when bonded to a highly oxidized metal center. Using synthetic models of oxygenated tyrosinase enzymes with exclusive monodentate imidazole ligation, we find that deprotonation of the μ2-η2:η2-peroxidodicopper(II) species triggers redox isomerization to an imidazolate-ligated bis(μ2-oxido)dicopper(III) species. Formal two-electron oxidation to Cu(III) remains unprecedented in biological systems, yet is effected readily by addition of base in these model systems. Spectrophotometric titrations by UV/visible/near-IR and copper K-edge X-ray absorption spectroscopies are interpreted most simply as two cooperative, 2H+ transformations in which the peroxide O-O is cleaved in the first step. Elaboration from simple imidazoles to a protected histidine extends this isomerization into an amino acid environment. The role of phenolate as a base suggests this four-electron reduction of O2 is energetically viable in a biological context and requires only two copper centers, which act as two-electron shuttles when imidazole deprotonation assists. This existential precedent of viable imidazolate intermediates invites speculation into an alternative mechanism for phenol hydroxylation not previously considered at Type 3 copper sites such a tyrosinases.

Imidazolate‐Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites

Imidazole ligation of metals through histidine is extensive among metalloproteins and enzymes, yet the role of the imidazolate conjugate base is often neglected, despite its potential accessibility when bonded to a highly oxidized metal center. Using synthetic models of oxygenated tyrosinase enzymes with exclusive monodentate imidazole ligation, we find that deprotonation of the μ2‐η2:η2‐peroxidodicopper(II) species triggers redox isomerization to an imidazolate‐ligated bis(μ2‐oxido)dicopper(III) species. Formal two‐electron oxidation to Cu(III) remains unprecedented in biological systems, yet is effected readily by addition of base in these model systems. Spectrophotometric titrations by UV/visible/near‐IR and copper K‐edge X‐ray absorption spectroscopies are interpreted most simply as two cooperative, 2H+ transformations in which the peroxide O‐O is cleaved in the first step. Elaboration from simple imidazoles to a protected histidine extends this isomerization into an amino acid environment. The role of phenolate as a base suggests this four‐electron reduction of O2 is energetically viable in a biological context and requires only two copper centers, which act as two‐electron shuttles when imidazole deprotonation assists. This existential precedent of viable imidazolate intermediates invites speculation into an alternative mechanism for phenol hydroxylation not previously considered at Type 3 copper sites such a tyrosinases.

What is the Origin of the Regioselective C3-Hydroxylation of L-Arg by the Nonheme Iron Enzyme Capreomycin C?

The nonheme iron dioxygenase capreomycin C (CmnC) hydroxylates a free L-arginine amino acid regio- and stereospecifically at the C3-position as part of the capreomycin antibiotics biosynthesis. Little is known on its structure, catalytic cycle and substrate specificity and, therefore, a comprehensive computational study was performed. A large QM cluster model of CmnC was created of 297 atoms and the mechanisms for C3-H, C4-H and C5-H hydroxylation and C3-C4 desaturation were investigated. All low-energy pathways correspond to radical reaction mechanisms with an initial hydrogen atom abstraction followed by OH rebound to form alcohol product complexes. The work is compared to alternative L-Arg hydroxylating nonheme iron dioxygenases and the differences in active site polarity are compared. We show that a tight hydrogen bonding network in the substrate binding pocket positions the substrate in an ideal orientation for C3-H activation, whereby the polar groups in the substrate binding pocket induce an electric field effect that guides the selectivity.

Catalyst for the Generation of OH Radicals in Advanced Electrochemical Oxidation Processes: Present and Future Perspectives

Heterogeneous Advanced Oxidation Processes (H-AOPs) are considered a new process for removing emerging pollutants. In this case, the high reactivity of hydroxyl radicals is used to degrade persistent organic pollutants. This review explores the state-of-the-art catalyst for hydroxyl radical generation in AOPs. As a parasite reaction, chloride ions appear in alkaline conditions and compete with the active sites. The theoretical foundation of catalyst performance is explored, focusing on the fundamental principles that govern the efficiency and mechanism of hydroxyl or chloride radical production. The synthesis and electronic modification sections explore the modifications of catalysts. It discusses key methodologies for catalyst preparation, with a particular emphasis on electronic modification that enhances both activity and stability. Finally, laboratory and pilot applications highlight the effectiveness of novel or modified catalysts in different scenarios. These last findings provide insights into the future directions for research and application, aiming to draw attention to the gap between laboratory studies and real-world implementations.

QM/MM Study of the Reaction Mechanism of L-Tyrosine Hydroxylation Catalyzed by the Enzyme CYP76AD1.

We have studied the hydroxylation mechanism of l-Tyr by the heme-dependent enzyme CYP76AD1 from the sugar beet (Beta vulgaris). This enzyme has a promising biotechnological application in modified yeast strains to produce medicinal alkaloids, an alternative to the traditional opium poppy harvest. A generative machine learning software based on AlphaFold was used to build the structure of CYP76AD1 since there are no structural data for this specific enzyme. After model validation, l-Tyr was docked in the active site of CYP76AD1 to assemble the reactive complex, whose catalytic distances remained stable throughout the 100 ns of MD simulation. Subsequent QM/MM calculations elucidated that l-Tyr hydroxylation occurs in two steps: hydrogen abstraction from l-Tyr by CpdI, forming an l-Tyr radical, and subsequent radical rebound, corresponding to a rate-limiting step of 16.0 kcal·mol-1. Our calculations suggest that the hydrogen abstraction step should occur in the doublet state, while the radical rebound should happen in the quartet state. The clarification of the reaction mechanism of CYP76AD1 provides insights into the rational optimization of the biosynthesis of alkaloids to eliminate the use of opium poppy.

Effect of Substituted Pyridine Co-Ligands and (Diacetoxyiodo)benzene Oxidants on the Fe(III)-OIPh-Mediated Triphenylmethane Hydroxylation Reaction.

Iodosilarene derivatives (PhIO, PhI(OAc)2) constitute an important class of oxygen atom transfer reagents in organic synthesis and are often used together with iron-based catalysts. Since the factors controlling the ability of iron centers to catalyze alkane hydroxylation are not yet fully understood, the aim of this report is to develop bioinspired non-heme iron catalysts in combination with PhI(OAc)2, which are suitable for performing C-H activation. Overall, this study provides insight into the iron-based ([FeII(PBI)3(CF3SO3)2] (1), where PBI = 2-(2-pyridyl)benzimidazole) catalytic and stoichiometric hydroxylation of triphenylmethane using PhI(OAc)2, highlighting the importance of reaction conditions including the effect of the co-ligands (para-substituted pyridines) and oxidants (para-substituted iodosylbenzene diacetates) on product yields and reaction kinetics. A number of mechanistic studies have been carried out on the mechanism of triphenylmethane hydroxylation, including C-H activation, supporting the reactive intermediate, and investigating the effects of equatorial co-ligands and coordinated oxidants. Strong evidence for the electrophilic nature of the reaction was observed based on competitive experiments, which included a Hammett correlation between the relative reaction rate (logkrel) and the σp (4R-Py and 4R'-PhI(OAc)2) parameters in both stoichiometric (ρ = +0.87 and +0.92) and catalytic (ρ = +0.97 and +0.77) reactions. The presence of [(PBI)2(4R-Py)FeIIIOIPh-4R']3+ intermediates, as well as the effect of co-ligands and coordinated oxidants, was supported by their spectral (UV-visible) and redox properties. It has been proven that the electrophilic nature of iron(III)-iodozilarene complexes is crucial in the oxidation reaction of triphenylmethane. The hydroxylation rates showed a linear correlation with the FeIII/FeII redox potentials (in the range of -350 mV and -524 mV), which suggests that the Lewis acidity and redox properties of the metal centers greatly influence the reactivity of the reactive intermediates.

Mechanistic study of OH radical-promoted decomposition of polycyclic aromatic hydrocarbon oxyradicals

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized for their detrimental impact on human health and the atmospheric environment. One of the primary challenges in mitigating these effects is the efficient and rapid decomposition of PAH oxyradicals, a byproduct of PAH oxidation. This study aims to elucidate the promoting mechanism of hydroxyl (OH) radicals on the decomposition of PAH oxyradicals. Employing density functional theory and transition state theory, we systematically investigate the decomposition mechanism of PAH oxyradicals under the influence of OH radicals. Our findings indicate that the interaction between OH radicals and PAH oxyradicals constitutes a pivotal oxidation process within flames. Notably, PAH oxyradicals are more inclined to undergo pyrolysis subsequent to their reaction with OH radicals within a flame, as opposed to direct pyrolysis. This observation underscores the role of OH radicals in facilitating the decomposition of PAH oxyradicals in flame conditions. Further analysis reveals that OH radicals can effectively react with various forms of PAH oxyradicals—zigzag, free, and armchair—leading to the formation of OH-OPAH compounds characterized by HO-C-O functional groups. Importantly, this addition reaction between OH radicals and PAH oxyradicals is found to be independent of the edge types of the PAH oxyradicals. The pyrolysis of OH-OPAH predominantly yields products such as COOH, CO2, and PAHs with five-membered rings, with COOH emerging as the primary oxidation product. This pyrolysis predominantly proceeds via a ring-opening reaction, rather than through cyclization. This study also reveals that the likelihood of PAH oxyradical pyrolysis increases with the number of free edges surrounding the oxygen-containing functional groups. Furthermore, the oxidation activity of the HO-C-O functional group is demonstrated to be more potent than that of the C-O functional group, significantly enhancing the reactivity of PAH oxyradical degradation in the presence of OH radicals. These insights not only contribute to refining the combustion model of PAH oxidation but also advance our understanding of PAH oxyradical decomposition. Such knowledge is crucial for the development and optimization of control technologies targeting PAH pollutants emanating from combustion devices.

Oxygen vacancy-rich cuprous oxide microspheres exhibit strong absorption and high oxidative energy level for deep photodegradation of tetracycline

In this study, a Cu2O microsphere (WY1) was hydrothermal synthesized by employing a CTAB-cocoylsarcosine sodium composite micellar. WY1 has an unifirm hollow spherical structure formed by agglomeration of nanoparticles, and the results of X-ray photoelectron spectroscopy and electron paramagnetic resonance indicate that WY1 contains significantly increased oxygen vacancies. The degradation efficiency of WY1 for 40 mg L−1 TC reached 93.26% after 60 min visible-light irradiation, which was 2.04 times higher than that of the product prepared without soft template (WNT). Especially, 81.67% residual rate of total organic carbon in the degradation of TC over WY1 was 17.83 times higher than that of WNT (4.58%). The spectral and electrochemical data reveal that the position of valence band of WY1 was exceeded the H2O/·OH potential, and the ·OH species rarely played a key role in the photocatalytic degradation of tetracycline by cuprous oxide. In addition, the ·O2− species were generated by forming the electron transfer pathway with aid of oxygen vacancy-mediated trap, acting as another major active species, which followed a deep photodegradation mechanism of hydroxyl and superoxide radicals dual activities. The spatial agglomeration of cuprous oxide nanoparticles provides a new process for enhancing interfacial energy levels and inhibiting photocorrosion, which cater the antibiotic remediation.

Kinetics and mechanism of (re)hydroxylation in fired clay minerals

AbstractThree types of smectite with various crystallochemical compositions and interlayer cations as well as kaolinite and illite were fired at 800°C and 650°C and tested for their rehydroxylation's (RHX) potential and kinetics in the presence of water vapor at 200°C–350°C. A dehydroxylated structure of 2:1 Al‐rich mineral with the interlayer pillared by a large cation (like K+ or Cs+) is wide enough to allow H2O diffusion which results in advanced RHX, that is, restoring up to several tens of percent of the original OH content. Such 2:1 layer structures (beidellite, illite) do not follow the time‐to‐the‐quarter (TTTQ) kinetics during RHX and show a non‐Arrhenius behavior for isothermal RHX. Because TTTQ kinetics is assumed in an RHX dating of ceramic artifacts in archaeometry, fired‐clay ceramics prepared from material dominated by Al‐rich 2:1 minerals was found unfeasible for RHX dating.Kaolinite and Mg2+‐ or Ca2+‐exchanged smectites show the Arrhenius behavior and follow the TTTQ kinetics of RHX, resulting in apparent Ea increase with the progress of reaction (up to α ∼0.2) of ∼20–50 kJ/mol in beidellite and 15–35 kJ/mol in montmorillonite. We suggest using ‘hydroxylation’ rather than ‘rehydroxylation’ for structurally‐disordered fired clays gaining OH groups in an unknown position.

Effect of long-term straw return on organic matter transformation by hydroxyl radical during paddy soil oxygenation

The generation of hydroxyl radicals (OH) mediated by Fe(II) has drawn increasing amounts of attention due to its significant role in the transformation of soil organic carbon (SOM) during redox fluctuations in paddy soil. This study further examined the effect of long-term (since 1984) straw return on Fe(II) on driving OH formation and SOM transformation upon paddy soils oxygenation. We found that long-term straw return could inhibit the OH and CO2 production in abiotic processes. The OH production decreased from 190.4 to 133.4 µM as the straw return amount increased from 0 to 19.16 t·hm−2·year−1. Moreover, 14.7–38.6 % of the CO2 production was associated with OH during oxygenation. By combining radical quenching and OM extraction, we determined that straw return could increase O2 reduction to H2O2 through a two-electron transfer pathway. Based on chemical extraction, Mössbauer spectra and multilinear regression model, we demonstrated that straw return resulted in more active Fe(II) species that controls the OH production. The results of 3DEEMs and FT-ICR MS showed that oxidative transformation can be accomplished by the introduction of OH to aromatic structures or olefins through ring opening and hydroxylation mechanisms. And OM at high straw return concentrations was likely oxidized by OH into bioavailable aliphatic compounds. Therefore, this study highlights the underappreciating effects of long-term straw return on Fe(II) oxidation in driving OH generation and SOM transformation in paddy soil redox fluctuation events.

Synergistic hydroxyl mechanism on halloysite-confined PtFe alloy boosting low-temperature CO-PROX performance

Both structural and adsorbed surface hydroxyl groups in solid materials are important carriers for oxygen species transport. The role of identifiable structural hydroxyl groups in catalytic reactions, however, is often underestimated. Here, we propose a synergistic hydroxyl reaction mechanism on Pt1Fe0.33/HNTs catalyst in CO-PROX reaction, which is critical to reveal the new way of oxygen transport in the oxidation reaction. Particularly, the Pt1Fe0.33 nanoparticles (NPs) have been confined in the cavity of halloysite nanotubes (HNTs) to activate structural hydroxyl groups linked to the AlVI via a two-step reduction treatment. The activated structural hydroxyl groups exhibit sensitivity to CO, as revealed by in situ DRIFTS and MS data. The experimental and DFT data demonstrate that the cooperation of Pt, Fe, and hydroxyl sites in the active units on Pt1Fe0.33/HNTs can induce a synergistic participation of structural and adsorbed hydroxyl groups in CO oxidation through the formate pathway. In such catalytic process, the oxygen mobility is dominated by oxygen-containing hydroxyl species rather than reactive oxygen species. This work contributes to a deeper understanding of the role of hydroxyl groups in catalysis and lays the foundation for establishing a comprehensive oxidation reaction mechanism in the future.

Unusual catalytic strategy by non-heme Fe(ii)/2-oxoglutarate-dependent aspartyl hydroxylase AspH.

Biocatalytic C-H oxidation reactions are of important synthetic utility, provide a sustainable route for selective synthesis of important organic molecules, and are an integral part of fundamental cell processes. The multidomain non-heme Fe(ii)/2-oxoglutarate (2OG) dependent oxygenase AspH catalyzes stereoselective (3R)-hydroxylation of aspartyl- and asparaginyl-residues. Unusually, compared to other 2OG hydroxylases, crystallography has shown that AspH lacks the carboxylate residue of the characteristic two-His-one-Asp/Glu Fe-binding triad. Instead, AspH has a water molecule that coordinates Fe(ii) in the coordination position usually occupied by the Asp/Glu carboxylate. Molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) studies reveal that the iron coordinating water is stabilized by hydrogen bonding with a second coordination sphere (SCS) carboxylate residue Asp721, an arrangement that helps maintain the six coordinated Fe(ii) distorted octahedral coordination geometry and enable catalysis. AspH catalysis follows a dioxygen activation-hydrogen atom transfer (HAT)-rebound hydroxylation mechanism, unusually exhibiting higher activation energy for rebound hydroxylation than for HAT, indicating that the rebound step may be rate-limiting. The HAT step, along with substrate positioning modulated by the non-covalent interactions with SCS residues (Arg688, Arg686, Lys666, Asp721, and Gln664), are essential in determining stereoselectivity, which likely proceeds with retention of configuration. The tetratricopeptide repeat (TPR) domain of AspH influences substrate binding and manifests dynamic motions during catalysis, an observation of interest with respect to other 2OG oxygenases with TPR domains. The results provide unique insights into how non-heme Fe(ii) oxygenases can effectively catalyze stereoselective hydroxylation using only two enzyme-derived Fe-ligating residues, potentially guiding enzyme engineering for stereoselective biocatalysis, thus advancing the development of non-heme Fe(ii) based biomimetic C-H oxidation catalysts, and supporting the proposal that the 2OG oxygenase superfamily may be larger than once perceived.

Mechanism of Selective Aromatic Hydroxylation in the Metabolic Transformation of Paclitaxel Catalyzed by Human CYP3A4.

Paclitaxel (PTX) is heralded as one of the most successful natural-product drugs for the treatment of refractory cancers. In humans, the hepatic metabolic transformation of PTX is primarily mediated by two cytochrome P450 enzymes (P450s): CYP3A4 and CYP2C8. The impact of P450 metabolism on the anticancer effectiveness of PTX is significant. However, the precise mechanism underlying selective P450-catalyzed reactions in PTX metabolism remains elusive. To address this knowledge gap, we conducted molecular docking and molecular dynamics simulations using multiple crystal structures of CYP3A4, which originally contained other ligands. These methods enabled us to determine the most plausible binding structure of PTX within the enzyme. By further employing hybrid quantum mechanics and molecular mechanics calculations, we successfully identified two primary pathways for the reaction between compound I (Cpd I) of CYP3A4 and PTX. One of these pathways involves the formation of an epoxide, while the other proceeds through a ketone intermediate.

Mechanism of Benzene Hydroxylation on Tri-Iron Oxo-Centered Cluster-Based Metal–Organic Frameworks

Mechanism of Benzene Hydroxylation on Tri-Iron Oxo-Centered Cluster-Based Metal–Organic Frameworks

Oxidation of 4-tert-butylphenol by potassium permanganate and reduced formation of disinfection byproducts from chlorination

4-tert-butylphenol (4-tBP), a typical endocrine disruptor, is widely used as pharmaceutical and agricultural additive and has been detected in the aqueous environment. In this work, we systematically investigated the oxidation process of 4-tBP by potassium permanganate (KMnO4). The degradation condition was optimized by adjusting pH and oxidant dose. The removal efficiency could reach > 93% in 45 min under pH = 6.0–9.0 and [4-tBP]0:[KMnO4]0 = 1:2, where the maximum second-order reaction rate constant (kapp) was (2.48 ± 0.23) × 10−2 M−1 min−1. Common anions (Cl-, SO42-, NO3- and HCO3-) and cations (K+, Ca2+, Mg2+, Fe3+ and Cu2+) did not show apparent influence, while humic acid played a positive role in the degradation. A total of 20 products were identified and two main pathways of the oxidation process were proposed, which included hydroxylation and coupling. Through the theoretical calculations, the mechanism of the 4-tBP hydroxylation was explained as mono-oxygen transfer at the ortho site of the hydroxyl group and coupling was from the extraction of hydrogen at the hydroxyl group. Meanwhile, the toxicity assessment showed lowered biotoxicity of products as compared to the substrate. In addition, chlorination of degradation products would yield lower concentrations of disinfection byproducts (dichloroacetic acid, 4.5 μg/L; trichloroacetic acid, 4.7 μg/L, in ultrapure water) than the initial 4-tBP solution. These findings confirmed that KMnO4 could be used as a safe and promising technology for 4-tBP degradation.

Surface hydroxylation mechanism of Zr[formula omitted]X(OH)[formula omitted] (X = C, N, or B) MXenes: A promising catalyst for CO[formula omitted] conversion into formic acid

Electrocatalytic transformation of CO2 to formic acid is typically realized via bi-hydrogenation process, which is remarkably challenging because of high thermodynamic stability and chemical inertness of the intermediate. Here, we explored the surface hydroxylation mechanism for CO2 conversion into formic acid over OH-terminated MXenes that can effectively circumvent the thermodynamic sink of the conventional bi-hydrogenation process by capturing additional hydrogen from the surface. The observed unique CO2RR activity can be attributed to the reactive H atom on the OH-terminated MXenes. The facile capture of (H) from OH-terminated MXenes confirm the viability of surface-hydroxylation mechanism. In addition, the ultra low work functions (1.72–2.27 eV) of such materials demonstrate the significance of functional groups in deciding the surface electrostatic potential of MXenes during the etching process. Overall, the present study bypass the thermodynamic constraints hampering the formation of formic acid by eliminating the intermediate step in the CO2 hydrogenation reaction and is likely to have considerable implications in the fields of catalytic chemistry and CO2 conversion.

Usage of Cell-Free Protein Synthesis in Post-Translational Modification of μ-Conopeptide PIIIA.

The post-translational modifications of conopeptides are the most complicated modifications to date and are well-known and closely related to the activity of conopeptides. The hydroxylation of proline in conopeptides affects folding, structure, and biological activity, and prolyl 4 hydroxylase has been characterized in Conus literatus. However, the hydroxylation machinery of proline in conopeptides is still unclear. In order to address the hydroxylation mechanism of proline in μ-PIIIA, three recombinant plasmids encoding different hybrid precursors of μ-PIIIA were constructed and crossly combined with protein disulfide isomerase, prolyl 4 hydroxylase, and glutaminyl cyclase in a continuous exchange cell-free protein system. The findings showed that prolyl 4 hydroxylase might recognize the propeptide of μ-PIIIA to achieve the hydroxylation of proline, while the cyclization of glutamate was also formed. Additionally, in Escherichia coli, the co-expression plasmid encoding prolyl 4 hydroxylase and the precursor of μ-PIIIA containing pro and mature regions were used to validate the continuous exchange cell-free protein system. Surprisingly, in addition to the two hydroxyproline residues and one pyroglutamyl residue, three disulfide bridges were formed using Trx as a fusion tag, and the yield of the fusion peptide was approximately 20 mg/L. The results of electrophysiology analysis indicated that the recombinant μ-PIIIA without C-terminal amidate inhibited the current of hNaV1.4 with a 939 nM IC50. Our work solved the issue that it was challenging to quickly generate post-translationally modified conopeptides in vitro. This is the first study to demonstrate that prolyl 4 hydroxylase catalyzes the proline hydroxylation through recognition in the propeptide of μ-PIIIA, and it will provide a new way for synthesizing multi-modified conopeptides with pharmacological activity.

Molecular Dynamics of Iron Porphyrin-Catalyzed C-H Hydroxylation of Ethylbenzene.

Quasi-classical molecular dynamics (MD) simulations were carried out to study the mechanism of iron porphyrin-catalyzed hydroxylation of ethylbenzene. The hydrogen atom abstraction from ethylbenzene by iron-oxo species is the rate-determining step, which generates the radical pair of iron-hydroxo species and the benzylic radical. In the subsequent radical rebound step, the iron-hydroxo species and benzylic radical recombine to form the hydroxylated product, which is barrierless on the doublet energy surface. In the gas-phase quasi-classical MD study on the doublet energy surface, 45% of the reactive trajectories lead directly to the hydroxylated product, and this increases to 56% in implicit solvent model simulations. The percentage of reactive trajectories leading to the separated radical pair is 98-100% on high-spin (quartet/sextet) energy surfaces. The low-spin state reactivity dominates in the hydroxylation of ethylbenzene, which is dynamically both concerted and stepwise, since the time gap between C-H bond cleavage and C-O bond formation ranges from 41 to 619 fs. By contrast, the high-spin state catalysis is an energetically stepwise process, which has a negligible contribution to the formation of hydroxylation products.

Hydroxylation of some emerging disinfection byproducts (DBPs) in water environment: Halogenation induced strong pH-dependency

In this work, the hydroxylation mechanisms and kinetics of some emerging disinfection byproducts (DBPs) have been systematically investigated through theoretical calculation methods. Five chlorophenols and eleven halogenated pyridinols were chosen as the model compounds to study their pH-dependent reaction laws in UV/H2O2 system. For the reactions of HO• with 37 different dissociation forms, radical adduct formation (RAF) was the main reaction pathway, and the reactivity decreased with the increase of halogenation degree. The kapp values (at 298 K) increased with the increase of pH from 0 to 10, and decreased with the increase of pH from 10 to 14. Compared with phenol, the larger the chlorination degree in chlorophenols was, the stronger the pH sensitivity of the kapp values; compared with chlorophenols, the pH sensitivity in halogenated pyridinols was further enhanced. As the pH increased from 2 to 10.5, the degradation efficiency increased at first and then decreased. With the increase of halogenation degree, the degradation efficiency range increased, the pH sensitivity increased, the optimal degradation efficiency slightly increased, and the optimal degradation pH value decreased. The ecotoxicity and bioaccumulation of most hydroxylated products were lower than their parental compounds. These findings provided meaningful insights into the strong pH-dependent hydroxylation of emerging DBPs on molecular level.