Apparent Second-order Rate Constants Research Articles

Reactivity of Cyanobacteria Metabolites with Ozone: Multicompound Competition Kinetics.

Cyanobacterial blooms occur at increasing frequency and intensity, notably in freshwater. This leads to the introduction of complex mixtures of their products, i.e., cyano-metabolites, to drinking water treatment plants. To assess the fate of cyano-metabolite mixtures during ozonation, a novel multicompound ozone (O3) competition kinetics method was developed. Sixteen competitors with known second-order rate constants for their reaction with O3 ranging between 1 and 108 M-1 s-1 were applied to cover a wide range of the O3 reactivity. The apparent second-order rate constants (kapp,O3) at pH 7 were simultaneously determined for 31 cyano-metabolites. kapp,O3 for olefin- and phenol-containing cyano-metabolites were consistent with their expected reactivity (0.4-1.7 × 106 M-1 s-1) while kapp,O3 for tryptophan- and thioether-containing cyano-metabolites were significantly higher than expected (3.4-7.3 × 107 M-1 s-1). Cyano-metabolites containing these moieties are predicted to be well abated during ozonation. For cyano-metabolites containing heterocycles, kapp,O3 varied from <102 to 5.0 × 103 M-1 s-1, giving first insights into the O3 reactivity of this class of compounds. Due to lower O3 reactivities, heterocycle- and aliphatic amine-containing cyano-metabolites may be only partially degraded by a direct O3 reaction near circumneutral pH. Hydroxyl radicals, which are formed during ozonation, may be more important for their abatement. This novel multicompound kinetic method allows a high-throughput screening of ozonation kinetics.

Reactions of hypobromous acid with dimethyl selenide, dimethyl diselenide and other organic selenium compounds: kinetics and product formation.

Selenium (Se) is an essential micronutrient for many living organisms particularly due to its unique redox properties. We recently found that the sulfur (S) analog for dimethyl selenide (DMSe), i.e. dimethyl sulfide (DMS), reacts fast with the marine oxidant hypobromous acid (HOBr) which likely serves as a sink of marine DMS. Here we investigated the reactivity of HOBr with dimethyl selenide and dimethyl diselenide (DMDSe), which are the main volatile Se compounds biogenically produced in marine waters. In addition, the reactivity of HOBr with further organic Se compounds was tested, i.e., SeMet (as N-acetylated-SeMet), and selenocystine (SeCys2 as N-acetylated-SeCys2), as well as the phenyl-analogs of DMSe and DMDSe, respectively, diphenyl selenide (DPSe) and diphenyl diselenide (DPDSe). Apparent second-order rate constants at pH 8 for the reactions of HOBr with the studied Se compounds were (7.1 ± 0.7) × 107 M-1 s-1 for DMSe, (4.3 ± 0.4) × 107 M-1 s-1 for DMDSe, (2.8 ± 0.3) × 108 M-1 s-1 for SeMet, (3.8 ± 0.2) × 107 M-1 s-1 for SeCys2, (3.5 ± 0.1) × 107 M-1 s-1 for DPSe, and (8.0 ± 0.4) × 106 M-1 s-1 for DPDSe, indicating a very high reactivity of all selected Se compounds with HOBr. The reactivity between HOBr and DMSe is lower than for DMS and therefore this reaction is likely not relevant for marine DMSe abatement. However, the high reactivity of SeMet with HOBr suggests that SeMet may act as a relevant quencher of HOBr.

Kinetic study and DFT calculation on the tetracycline abatement by peracetic acid

Water contamination by tetracycline (TC) has emerged as an environmental concern owing to its widespread use and antibiotic resistance. Application of peracetic acid (PAA) in the water and wastewater treatment has recently been proposed and demonstrated to be effective for TC abatement, yet the underlying reaction kinetics between the PAA and TC are not yet clear. To explore the reaction kinetics, the effect of solution pH on TC abatement by PAA is studied and the species-specific rate constants are calculated. The ability to donate and accept electrons for different species of TC and PAA is evaluated via density functional theory (DFT) calculations. The pH-dependent apparent second-order rate constants of TC abatement by PAA exhibits the parabolic shape with the maximum at pH 8.5 (9.75 L mol−1 s−1). This phenomenon is closely related to the speciation of TC and PAA, in which the reaction between PAAH and TTC2− possesses the highest species-specific rate constants according to the kinetic simulation. Further DFT calculations suggest that the HOMO of TTCH+, TTC, TTC−, TTC2− and the LUMO of PAAH and PAA− are –6.40, –6.26, –5.10, –4.94 eV and –0.24, 0.60 eV, respectively. According to the DFT calculations, deprotonation of TC and PAA leads to an increase of the HOMO value of TC and the LUMO value of PAA. Furthermore, the HOMOTC–LUMOPAA values is in good agreement with the trend of species-specific rate constants, which can be used to evaluate the reactivity between PAA and TC with different species. This study provides the kinetic data and theoretical basis for the reaction of PAA and TC, which is critical for the application of PAA in the treatment of water and wastewater.

Selective oxidation of histamine H2-Receptor antagonists by peracetic Acid: Kinetics and mechanism

Peracetic acid (PAA) is an emerging substitutive oxidant to chlorine disinfectants owing to the low generation of harmful by-products. Herein, we report PAA-induced oxidation of histamine H2-receptor antagonists (HRAs) via non-radical participation for the first time. Results showed that PAA can selectively oxidize HRAs with high reactivity to ranitidine (RNTD), nizatidine (NZTD), cimetidine (CMTD), and famotidine (FMTD), but low reactivity to roxatidine (RXTD). PAA-induced HRAs oxidation followed the second-order kinetic reaction, the apparent second-order reaction rate constants (k2, app) of RNTD, FMTD, CMTD and NZTD were 18.78 ± (2.4), 37.22 ± (6.1), 36.53 ± (7.8), 30.04 ± (2.4) M−1•s−1, respectively. The value of k2, app decreased as pH increased. Scavenging experiments indicated that PAA-induced HRAs oxidation proceeded via a non-radical process. The theoretical calculation and product analysis further reveal that the reaction involves the double-electron transfer from thioether sulfur in HRAs to peroxide bond. HRAs oxidation by PAA was not affected by water matrices (Cl-, HCO3–, HA), which has good applicability in surface water (SW) and wastewater (WW). Toxicity assessment indicated ecotoxicity of oxidation products was much lower than that of HRAs. This work provided a facile and emerging technology to eliminate the HRAs and their toxicity in the wastewater.

Developing a hybrid model for predicting the reaction kinetics between chlorine and micropollutants in water

Understanding the reactivities of chlorine towards micropollutants is crucial for assessing the fate of micropollutants in water chlorination. In this study, we integrated machine learning with kinetic modeling to predict the reaction kinetics between micropollutants and chlorine in deionized water and real surface water. We first established a framework to predict the apparent second-order rate constants for micropollutants with chlorine by combining Morgan molecular fingerprints with machine learning algorithms. The framework was tuned using Bayesian optimization and showed high prediction accuracy. It was validated through experiments and used to predict the unreported apparent second-order rate constants for 103 emerging micropollutants with chlorine. The framework also improved the understanding of the structure-dependence of micropollutants’ reactivity with chlorine. We incorporated the predicted apparent second-order rate constants into the Kintecus software to establish a hybrid model to profile the time-dependent changes of micropollutant concentrations by chlorination. The hybrid model was validated by experiments conducted in real surface water in the presence of natural organic matter. The hybrid model could predict how much micropollutants were degraded by chlorination with varied chlorine contact times and/or initial chlorine dosages. This study advances fundamental understanding of the reaction kinetics between chlorine and emerging micropollutants, and also offers a valuable tool to assess the fate of micropollutants during chlorination of drinking water.

Reactivity and mechanism of the reactions of 4-methylbenzoquinone with amino acid residues in β-lactoglobulin: A kinetic and product investigation

Quinones, produced by the oxidation of phenolic compounds, covalently bind to nucleophilic groups on amino acids or proteins. In this study, the reactions of 4-methylbenzoquinone (4MBQ) with β-lactoglobulin (β-LG) and amino acids at neutral pH were investigated. LC-MS analysis revealed that Cys121 was likely the most modified residue in β-LG. Identification of reaction products by LC-MS/MS showed that Michael addition occurred in all reactions with amino acids tested. The formation of Schiff base and a di-adduct was found in His and Trp samples. Apparent second-order rate constants (k2) were determined at 25 °C and pH 7.0 by stopped-flow spectrophotometry. The rate of reactions decreased in the order: β-LG > His > Trp > Arg > Nα-acetyl His > Nα-acetyl Arg > Nα-acetyl Trp. The rate constants correlated with the pKa values of the amino acids, showing that the amount of unprotonated amine is the major factor determining the reactivity.

Disulfides form persulfides at alkaline pH leading to potential overestimations in the cold cyanolysis method

It is well established that proteins and peptides can release sulfur under alkaline treatment, mainly through the β-elimination of disulfides with the concomitant formation of persulfides and dehydroalanine derivatives. In this study, we evaluated the formation of glutathione persulfide (GSSH/GSS-) by exposure of glutathione disulfide (GSSG) to alkaline conditions. The kinetics of the reaction between GSSG and HO- was investigated by UV–Vis absorbance, reaction with 5,5′-dithio-bis-(2-nitrobenzoic acid) (DTNB), and cold cyanolysis, obtaining an apparent second-order rate constant of ∼10-3 M-1 s-1 at 25 °C. The formation of GSSH and the dehydroalanine derivative was confirmed by HPLC and/or mass spectrometry. However, the mixtures did not equilibrate in a timescale of hours, and additional species, including thiol and diverse sulfane sulfur compounds were also formed, probably through further reactions of the persulfide. Cold cyanolysis is frequently used to quantify persulfides, since it measures sulfane sulfur. This method involves a step in which the sample to be analyzed is incubated with cyanide at alkaline pH. When cold cyanolysis was applied to samples containing GSSG, sulfane sulfur products that were not present in the original sample were measured. Thus, our results reveal the risk of overestimating the amount of sulfane sulfur compounds in samples that contain disulfides due to their decay to persulfides and other sulfane sulfur compounds at alkaline pH. Overall, our study highlights that the β-elimination of disulfides is a potential source of persulfides, although we do not recommend the preparation of GSSH from incubation of GSSG in alkali. Our study also highlights the importance of being cautious when doing and interpreting cold cyanolysis experiments.

Chlorination of amides: Kinetics and mechanisms of formation of N-chloramides and their reactions with phenolic compounds

Amides are common constituents in natural organic matter and synthetic chemicals. In this study, we investigated kinetics and mechanisms of the reactions of chlorine with seven amides, including acetamide, N-methylformamide, N-methylacetamide, benzamide, N-methylbenzamide, N-propylbenzamide, and N-(benzoylglycyl)glycine amide. Apparent second-order rate constants for the reactions of the amides with chlorine at pH 8 are in the range of 5.8 × 10−3 – 1.8 M−1s−1 and activation energies in the range of 62-88 kJ/mol. The second-order rate constants for the reactions of chlorine with different amides decrease with increasing electron donor character of the substituents on the amide-N and N-carbonyl-C in the amide structures. Hypochlorite (‒OCl) dominates the reactions of chlorine with amides yielding N-chloramides with species-specific second-order rate constants in the range of 7.3 × 10−3 – 2.3 M−1s−1. Kinetic model simulations suggest that N-chlorinated primary amides further react with HOCl with second-order rate constants in the order of 10 M−1s−1. The chlorination products of amides, N-chloramides are reactive towards phenolic compounds, forming chlorinated phenols via electrophilic aromatic substitution (phenol and resorcinol) and quinone via electron transfer (hydroquinone). Meanwhile, N-chloramides were recycled to the parent amides. At neutral pH, apparent second-order rate constants for the reactions between phenols and N-chloramides are in the order of 10−4-0.1 M−1s−1, comparable to those with chloramine. The findings of this study improve the understanding of the fate of amides and chlorine during chlorination processes.

Chlorination of microcystin-LR in natural water: Kinetics, transformation products, and genotoxicity

Microcystin-LR (MC-LR), a type of cyanotoxin commonly found in natural water bodies (sources of drinking water), poses a threat to human health due to its high toxicity. It is essential to successfully remove this cyanotoxin from drinking water sources. In this study, chlorine was used to oxidize MC-LR in Milli-Q water (MQ) (control test) and natural water collected from Lake Longhu (LLW) as a drinking water source. The removal efficiency, proposed transformation pathways, and genotoxicity were investigated. In the chlorine dose range investigated (4.0 mg L−1 - 8.0 mg L−1), the apparent second-order rate constants for MC-LR chlorination varied from 21.3 M−1s−1 to 31.9 M−1s−1 in MQ, higher than that in LLW (9.06 M−1s−1 to 17.7 M−1s−1) due to a faster chlorine decay attributed to the water matrix (e.g., natural organic matter) of LLW. Eleven transformation products (TPs) of MC-LR were identified in the two waters. The conjugated diene moieties and benzene ring of Adda moiety (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid), and the double bond of Mdha moiety (N-methyldehydroalanine) were the major susceptible reaction sites. Attacking unsaturated bonds by hydroxyl and chlorine radicals to generate monochloro-hydroxy-MC-LR was the primary initial transformation pathway, followed by nucleophilic substitution, dehydration, and cleavage in MC-LR. Chlorine substitution on the benzene ring was also observed. Based on the bacterial reverse-mutation assay (Ames assay), TPs in treated natural water did not induce genotoxicity/mutagenicity. These findings shed light on the role of chlorination in controlling the risk of cyanotoxins in drinking water treatment plants.

Quantitative structure–activity relationship models for predicting apparent rate constants of organic compounds with ferrate (VI)

Ferrate (VI) (Fe (VI)) is a promising, environmentally friendly multifunctional oxidant widely applied in organic compound degradation. Oxidative kinetics of the apparent second-order rate constants (kapp) of Fe (VI) with organic compounds are critical for modeling oxidation processes. Herein, a quantitative structure–activity relationship (QSAR) model was developed using particle swarm optimization and an extreme learning machine to better understand the laws of the kapp values of organic compounds, including 33 aliphatic and aromatic hydrocarbon derivatives, during degradation by Fe (VI). Seven components—electronic hardness (H), electronic softness (S), ratio of oxygen to carbon atoms (On/Cn), energy of the highest occupied molecular orbital (EHOMO), vertical ionization potential (VIP), maximum nucleophilic reaction index (f(+)x), and minimum relative electrophilicity index (REn) constitute the critical molecular parameters. The developed QSAR model was verified on the basis of the coefficient of determination (R2) and the root mean square error (RMSE): for the training set, R2 = 0.924 and RMSE = 1.186, whereas for the test set, R2 = 0.996, and RMSE = 0.352. The applicability, reliability, and predictability of the model were verified by estimating the applicability domain (AD) of the model. Furthermore, QSAR models constructed using different methods were compared, and the main impact descriptors and conclusions obtained from previous studies were theoretically analyzed. Results indicate that constructing the QSAR model facilitates kapp prediction for Fe (VI) in the degradation of various organic compounds, improves the understanding of the degradation mechanism, and reduces the pressure on human and material resources caused by experiments.

Roles of MnO2 Colloids and Mn(III) during the Oxidation of Organic Contaminants by Permanganate.

Although intermediate manganese species can be generated during the reactions of permanganate (Mn(VII)) with organic pollutants in water, the role of the in situ generated MnO2 colloids in the Mn(VII) oxidation process remained controversial and the contribution of Mn(III) was largely neglected. This study showed that the apparent second-order rate constants (kapp) of Mn(VII) oxidation of methyl phenyl sulfoxide and carbamazepine remained constant with time. However, the degradation of four selected phenolic contaminants by Mn(VII) exhibited an autoaccelerating trend and a linear trend at pH 3.0-6.0 and pH 7.0-9.0, respectively. Multiple lines of evidence revealed that the occurrence of the autoaccelerating trend in the Mn(VII) oxidation process was ascribed to the oxidation of the phenolic organics by MnO2 colloids. The influence of pyrophosphate on the oxidation of different organic contaminants by MnO2 colloids suggests that Mn(III) was also responsible for the autoaccelerating oxidation of organic contaminants by Mn(VII) under specific reaction conditions. The kinetic models revealed that the overall contributions of MnO2 colloids and Mn(III) ranged within 6.6-67.9% during the autoaccelerating oxidation of phenolic contaminants by Mn(VII). These findings advance the understanding of the roles of MnO2 colloids and Mn(III) in the Mn(VII) oxidation process.

Oxidative removal of dissolved manganese(II) by peroxymonosulfate: Non-radical mechanism and influencing factors

In this study, the oxidative removal of Mn(II) by peroxymonosulfate (PMS) was investigated. PMS was found to be a more efficient oxidant than chlorine for Mn(II) oxidation. Weak autocatalysis kinetics were observed in PMS/Mn(II) system suggesting that the Mn(II) oxidation consisted of both homogeneous (direct oxidation) and heterogeneous (amorphous MnO2 catalytic oxidation) processes. Homogeneous oxidation of Mn(II) by PMS showed strong pH dependency with apparent second-order rate constants of 3.1–249.9 M−1s−1 at pH 7.0–9.0. Stoichiometry of PMS to Mn(II) was about 1:1 suggesting a two-electron transfer pathway (O transfer), which was further confirmed by the XPS analysis results. However, the change in the coordination structure of Mn(II) was found to greatly affect the oxidation rate and mechanism of Mn(II). Complexion with inorganic or organic ligands could inhibit the oxidation rate of Mn(II) and alter the oxidation mechanism from two-electron to one-electron pathway. Furthermore, coexisting metal ions such as Ca(II), Zn(II), and Fe(II)/Fe(III) ions could greatly accelerate the oxidation of Mn(II) by PMS and chlorine by promoting the heterogeneous oxidation process. Oxidative removal of Mn(II) by PMS was more efficient than chlorine in real waters (56 %–100 % versus 12 %–17 %). The combination of PMS oxidation with coagulation could further increase the removal efficiency from 56 % to 92 %. These findings highlight that PMS oxidation is an attractive alternative technology for Mn(II) removal and may also provide a fundamental understanding of the Mn materials-based advanced oxidation processes.

Degradation of sulfonamide antibiotics and a structurally related compound by chlorine dioxide: Efficiency, kinetics, potential products and pathways

Sulfonamides (SAs) detected in all kinds of environmental water matrix may cause potential risk for aquatic environment and organisms. In this research, the oxidation of three SAs of sulfamethoxazole (SMX), sulfadiazine (SD) and sulfaguanidine (SGD), and a structurally related compound of 4-methylsulfonylaniline (MSA) by chlorine dioxide (ClO2) was investigated to identify removal efficiency, kinetics, potential products and pathways. At pH 7.0, the highest removal rates of 99.36 %, 100 % and 97.29 % were acquired for SMX, SD and SGD at the initial ClO2 concentration of 50 μM, while that of MSA was only 48.25 %. The difference among three SAs and MSA can be explained by structural influences of substituent attached to the sulfonyl amido-nitrogen. The reactions of SMX, SD and SGD with ClO2 followed the second-order kinetic model (R2 greater than 0.98) with the apparent second-order rate constants range of 5.60 × 102 to 2.73 × 103 M−1 s−1 at pH 7.0 and 25 ℃. The reaction rates were highly dependent on the pH and the trend of activation energy was SGD > SMX > SD at pH 7.0. Products evaluation indicated that the main potential degradation pathways of three SAs were the cleavage of SN and SC bonds, deamination and hydroxylation of aniline group. The sulfonyl amido-nitrogen and aniline amino-nitrogen were prone to be attacked by ClO2. Aniline and sulfanilic acid were found to be the common products of three SAs degradation. The results suggest that ClO2 has the potential to serve as a chemical oxidant for removing sulfonamides in practical water treatment conditions.

Heterogeneous activation of persulfate by FeS – Surface influence on selectivity

Recently, several studies have been published on sulfate radicals as strong oxidants and methods of their generation. A shift from homogeneous to heterogeneous methods can be observed for persulfate activation as a means of generating sulfate radicals. However, to date, the influence of the surface on the radical chemistry has not been examined in detail.In the present study, homogeneous persulfate activation methods (elevated temperature and dissolved iron(II)) are compared with heterogeneous activation by FeS. The selectivity patterns for the oxidation of chlorinated ethenes, ethanes and benzenes differ notably depending on the persulfate activation method. The obtained kinetic data are in conformity with the hypothesis that sulfate radicals generated at the FeS surface have different selectivities than freely dissolved radicals. The assumption of different reactive species is further confirmed by the determination of hydrogen kinetic isotope effects (H-KIEs) for the oxidation of methanol isotopologues with sulfate radicals using the set of activation methods. The H-KIE obtained in the heterogeneous system (H-KIEFeS = 3.0 ± 0.3) is significantly higher than for the homogeneous system (H-KIEhom = 2.3 ± 0.1). This leads us to the conclusion that sulfate radicals generated on the FeS surface react as surface-associated radicals, which show a different selectivity pattern than freely dissolved radicals.Furthermore, the apparent second-order rate constants in homogenous solution at ambient temperature, ranging from 5 · 107 to 1.7 · 109 M−1 s−1, were determined for the reaction of sulfate radicals with various chlorinated hydrocarbons under study.

Oxidative degradation of bisphenol A in municipal wastewater reverse osmosis concentrate (ROC) using ferrate(VI)/hydrogen peroxide

The combination of ferrate(VI) with hydrogen peroxide (H2O2) was used for the degradation of bisphenol A (BPA) spiked in municipal wastewater reverse osmosis concentrate (ROC). The results showed that the combined Fe(VI) and H2O2 oxidant provided an additive effect to achieve greater degradation efficiencies of BPA in ROC compared to the use of Fe(VI) or H2O2 alone. The degradation efficiency of BPA was studied in detail by assessing the operating conditions, including Fe(VI) dosage, H2O2 dosage, the dosing strategy of Fe(VI) and H2O2, solution pH, and the initial concentration of BPA. It was found that over 99.0% of BPA was degraded (50 µg L−1) by using [Fe(VI)]/[H2O2] at a ratio of 1050:5000 and at pH 8.0. In addition, the degradation was also accompanied by 37.2% of dissolved organic carbon (DOC) removal and 94.0% of UV254 reduction. However, when the excess amount of H2O2 was added, it was found that H2O2 started to inhibit the degradation and the degradation efficiency of BPA in ROC lowered. It was also found that the simultaneous addition of Fe(VI) and H2O2 provided satisfactory BPA degradation. Similar degradation efficiency could be achieved by injecting the second oxidant into the system 10 min after adding the first oxidant at time zero. The apparent second-order reaction rate constant (kapp) obtained was 67.9 × 103 M−1 s−1 at a [Fe(VI)]/[H2O2] ratio of 1050:5000. The overall ecotoxicity of the treated water samples as measured by the Microtox bioassay technique decreased after the Fe(VI)/H2O2 treatment, which indicated that an insignificant amount, if not none, toxic degradation by-products was formed by the oxidative action of the Fe(VI)/H2O2 combined oxidant. The use of Fe(VI)/H2O2 as an alternative treatment option of cost-effectiveness than Fe(VI) alone in water and wastewater industries.

Linear Relationship between Temperature and the Apparent Reaction Rate Constant of Hydroxyl Radical with 4-chlorobenzoic Acid

ABSTRACT 4-Chlorobenzoic acid (p-CBA) is frequently used as a hydroxyl radical (HO·) probe substance in studies of ozonation and advanced oxidation processes. However, the temperature dependence of the reaction between HO· and p-CBA remains unclear. In this context, we identified the relationship between temperature ( T , K) and the apparent second-order reaction rate constant of HO· with p- CBA ( k H O ⋅ , p − C B A T , M−1 s−1): 10 10 . They were measured by a novel competitive method using 2-methylpropan-2-ol (tert-butyl alcohol) as a reference substance in the range of 1.0–40.0℃. The linear regression equation was more appropriate than the exponential regression equation to express this relationship. More generally, our simulation shows that the linear regression equation can be more accurate than the exponential regression equation to express the relationship between temperature and apparent reaction rate constants of HO.

Oxidation of pyrazolone pharmaceuticals by peracetic acid: Kinetics, mechanism and genetic toxicity variations

Peracetic acid (PAA) oxidation is an emerging technology in water disinfection and purification. This study evaluated the oxidation of three pyrazolone pharmaceuticals (i.e., Aminopyrine (AMP), Antipyrine (ANT), and Isopropylphenazone (PRP) by PAA. Experimental results showed that PAA exhibited structure selectivity to the above three pharmaceuticals and oxidized AMP with the highest reactivity. The degradation kinetics of AMP was investigated by calculating the apparent second-order rate constants (kapp) under different initial pH. Through kinetic simulation, the second-order rate constants of elementary reactions between AMP (i.e., neutral (AMP0) and protonated (AMP+) species) with PAA (i.e., neutral (PAA0) and anionic (PAA−) species) were obtained to be 0.34 ± 0.077 M−1 s−1(k”AMP+, PAA0), 0.89 ± 0.091 M−1 s−1(k”AMP0, PAA-) and 5.94 ± 0.142 M−1 s−1(k”AMP0, PAA0), respectively. The PAA could oxidize AMP via electrophilic attack, and the degradation site of AMP was confirmed to be the central nitrogen of –N(CH3)2 with the highest relative electrophilicity (sk-/sk+, 48.8614) by Density Functional Theory (DFT) calculation. The intermediates/products of AMP degradation were identified by high-performance liquid chromatography-mass spectrometry (LC-MS/MS), and the transformation pathways of AMP during PAA oxidation were inferred to be hydroxylation, demethylation, and CC cleavage. The genetic toxicity of AMP contaminated water could be reduced after PAA oxidation, which was evaluated by the micronucleus test of Vicia faba root tips.

Redox-Active Moieties in Dissolved Organic Matter Accelerate the Degradation of Nitroimidazoles in SO4•--Based Oxidation.

The presence of dissolved organic matter (DOM) is known to inhibit the degradation of trace organic contaminants (TrOCs) in SO4•--based advanced oxidation processes (AOPs) due to filtering of the photochemically active light and radical scavenging effects. This study revealed an unexpected contribution for DOM in the degradation of nitroimidazoles (NZs) in the UV/persulfate AOP. The apparent second-order rate constants of NZs with SO4•- increased by 2.05 to 4.77 times in the presence of different DOMs. The increments were linearly related to the total electron capacity of DOM. Quinone and polyphenol moieties were found to play a dominant role. The reactive species generated from SO4•-'s oxidation of DOM, including semiquinone radical (SQ•-) and superoxide (O2•-), were found to react with NZs via Michael addition and O2•- addition. The second-order rate constants of tinidazole with SQ•- is determined to be (5.69 ± 0.59) × 106 M-1 s-1 by laser flash photolysis. Reactive species potentially generated from DOM may be considered in designing processes for the abatement of different types of TrOCs.

HaloTag Engineering for Enhanced Fluorogenicity and Kinetics with a Styrylpyridium Dye.

HaloTag is a small self‐labeling protein that is frequently used for creating fluorescent reporters in living cells. The small‐molecule dyes used with HaloTag are almost exclusively based on rhodamine scaffolds, which are often expensive and challenging to synthesize. Herein, we report the engineering of HaloTag for use with a chemically accessible, inexpensive fluorophore based on the dimethylamino‐styrylpyridium dye. Through directed evolution, the maximum fluorogenicity and the apparent second‐order bioconjugation rate constants could be improved up to 4‐fold and 42‐fold, respectively. One of the top variants, HT‐SP5, enabled reliable imaging in mammalian cells, with a 113‐fold fluorescence enhancement over the parent protein. Additionally, crystallographic characterization of selected mutants suggests the chemical origin of the fluorescent enhancement. The improved dye system offers a valuable tool for imaging and illustrates the viability of engineering self‐labeling proteins for alternative fluorophores.

Giant nanotubes equipped with horseradish peroxidase active sites: a powerful nanozyme co-assembled from supramolecular amphiphiles for glucose detection

Construction of biomimetic nanozymes has attracted considerable attention because these artificial enzymes not only exhibited catalytic effects similar to those of natural enzymes, but were also more economical and stable than natural enzymes for a wide range of practical applications. Herein, a peroxidase-like nanozyme was constructed by supramolecular co-assembled strategy, in which horseradish peroxidase active sites (histidine-heme-histidine) were loaded on giant supramolecular tubular scaffolds. The giant nanotube carried a large number of histidine-heme-histidine active sites on the surface thus exhibiting admirable activity with the high apparent second-order rate constant (kcat[Heme]/Km) of 572.2 M−1s−1 per single heme molecule and also showing high stability. Furthermore, the nanozyme was further applied to detect glucose through test paper by loading with extremely low levels of nanozyme, revealing its potential application in the rapid diagnosis of diabetes.