Bimolecular Rate Constants Research Articles

Characterization of acetonitrile ice irradiated by X-rays employing the procoda code – I. Effective rate constants and abundances at chemical equilibrium

ABSTRACT In this work, the chemical evolution of pure acetonitrile ice at 13 K irradiated with broad-band soft X-rays (from 6 eV to 2 keV) is determined by using a computational methodology (procoda code) to best fit the experimental data. To simulate the chemical evolution of the acetonitrile ice under an astrophysical analogous situation, the code employs 273 reaction rates involving 33 molecular species (5 species observed in the experiment and 28 non-observed or unknown). The considered reaction network describes 240 chemical reactions (including dissociation, bimolecular, and termolecular rates) and 33 individual desorption rates. The summed desorption yield was determined to be 0.23 molecules per photon, in agreement with previous estimates. Average values for dissociation, bimolecular, and termolecular effective rate constants were determined as 2.3 × 10−3 s−1, 9.7 × 10−26 cm3 molecule−1 s−1, and 3.2 × 10−47 cm6 molecule−2 s−1, respectively. Some branching ratios within reaction groups were also determined. Molecular abundances at chemical equilibrium were obtained, such as CH3CN (67.5 per cent), H (10.6 per cent), CN (6.7 per cent), CH2 (6.4 per cent), CH (2.5 per cent), CH3 (1.2 per cent), CH4 (1.1 per cent), C2N2 (0.8 per cent), HCN (0.8 per cent), and CH3NC (0.6 per cent). The results of this work can be employed in future astrochemical models to map chemical evolution embedded species in astrophysical regions in the presence of an ionizing radiation field.

Kinetics and stoichiometry of gallic acid and methyl gallate in scavenging DPPH radical as affected by the reaction solvent

The activity and capacity of gallic acid (GA) and methyl gallate (MG) in scavenging DPPH· were determined in different solvents. Based on the bimolecular rate constants k2, both antioxidants showed highest activities in EtOH, followed by in MeOH, t-BuOH, MeCN, 2-PrOH, acetone, THF, ethyl acetate, and 1,4-dioxane. GA indicated better activities (k2 value, M−1 s−1) than MG in the alcoholic solvents (51–1939 vs. 25–1530) and in MeCN (203 vs. 187) whereas MG was of higher activities in the polar aprotic solvents (1.7–41 vs. 1.6–13). The highest stoichiometries for GA vs. MG were in 2-PrOH (6.67 vs. 5.37), followed by EtOH (5.84 vs. 4.57), MeOH (5.34 vs. 3.8) ~ acetone (5.02 vs. 4.44), MeCN (3.68 vs. 3.05) ~ t-BuOH (3.14 vs. 2.99), THF (2.34 vs. 2.2), ethyl acetate (1.2 vs. 0.93), and 1,4-dioxane (0.34 vs. 0.35).

Effect of disinfection on the photoreactivity of effluent organic matter and photodegradation of organic contaminants

Chlorine, UV254, and ozone are three typical processes commonly used for wastewater disinfection, which could change the photoreactivity of dissolved organic matter (DOM) in effluents of wastewater treatment plants (WWTPs). The photoinduced reactive species (RS) from DOM, primarily including the excited triplet state of DOM (3DOM*), singlet oxygen (1O2), and hydroxyl radical (•OH), play important roles in the attenuation of contaminants. However, the effect of disinfection processes on the photosensitized degradation of contaminants is poorly understood. This paper presents the first evidence that 3DOM*, 1O2, and •OH interaction with three typical contaminants (diphenhydramine, cimetidine, and N,N-diethyl-m-toluamide (DEET)) was largely impacted by DOM after disinfection. The results of electron spin resonance (ESR) spectrometry and laser flash photolysis (LFP) experiments demonstrated that the chlorination increased the formation rate of 3DOM* and 1O2, while UV254 irradiation and ozonation decreased the formation rate of these RS. All these three disinfection processes promoted the photoproduction of •OH and increased the photodegradation rate constants (kobs) of DEET by 26−361%. The kobs of diphenhydramine, cimetidine, and DEET correlated positively with the formation rate of 3DOM*, 1O2, and •OH, respectively. The bimolecular reaction rate constant of 3DOM* with diphenhydramine increased by ∼41% after chlorination. These findings suggest that disinfection processes altered the photogeneration of RS from DOM, which significantly impacts the fate of trace pollutants in aquatic environments.

Uncertainty Quantification of Reactivity Scales.

According to Mayr, polar organic synthesis can be rationalized by a simple empirical relationship linking bimolecular rate constants to as few as three reactivity parameters. Here, we propose an extension to Mayr's reactivity method that is rooted in uncertainty quantification and transforms the reactivity parameters into probability distributions. Through uncertainty propagation, these distributions can be transformed into uncertainty estimates for bimolecular rate constants. Chemists can exploit these virtual error bars to enhance synthesis planning and to decrease the ambiguity of conclusions drawn from experimental data. We demonstrate the above at the example of the reference data set released by Mayr and co‐workers [J. Am. Chem. Soc. 2001, 123, 9500; J. Am. Chem. Soc. 2012, 134, 13902]. As by‐product of the new approach, we obtain revised reactivity parameters for 36 π‐nucleophiles and 32 benzhydrylium ions.

Quinone binding in respiratory complex I: Going through the eye of a needle. The squeeze-in mechanism of passing the narrow entrance of the quinone site

At the joint between the membrane and hydrophilic arms of the enzyme, the structure of the respiratory complex I reveals a tunnel-like Q-chamber for ubiquinone binding and reduction. The narrow entrance of the quinone chamber located in ND1 subunit forms a bottleneck (eye of a needle) which in all resolved structures was shown to be too small for a bulky quinone to pass through, and it was suggested that a conformational change is required to open the channel. The closed bottleneck appears to be a well-established feature of all structures reported so-far, both for the so-called open and closed states of the enzyme, with no indication of a stable open state of the bottleneck. We propose a squeeze-in mechanism of the bottleneck passage, where dynamic thermal conformational fluctuations allow quinone to get in and out. Here, using molecular dynamics simulations of the bacterial enzyme, we have identified collective conformational changes that open the quinone chamber bottleneck. The model predicts a significant reduction—due to a need for a rare opening of the bottleneck—of the effective bi-molecular rate constant, in line with the available kinetic data. We discuss possible reasons for such a tight control of the quinone passage into the binding chamber and mechanistic consequences for the quinone two-electron reduction.Graphic abstract

Preparation a novel 1-pyreneacetic acid functionalized graphene/self-assembled monolayer modified gold electrode to immobilize and study interfacial electron transfer of cytochrome c by electrochemical approaches

A new hybrid of favorable orientated cytochrome c (cyt c) and a 1-Pyreneacetic acid (PA) functionalized graphene (Gr) on Au electrode (AuE) was prepared. In the hybrid system, Gr was functionalized with (PA) molecule through π-π interaction (PA-Gr). Afterward the PA-Gr attached to the AuE via Thionalide self-assembled monolayer (SAM), then conjugated with cyt c. The AuE/SAM/PA-Gr/cyt c hybrid system was characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) techniques. SECM approach curves were applied to measure the standard tunneling electron transfer (ET) and bimolecular rate constants which were 56.6 s−1 and 1 × 109cm3mol−1 s−1, respectively.

From Reduction to Oxidation: pH Controlled Reaction of 1-hydroxyethyl Radical with Caffeic Acid Analogues

Aims: The aim is to search for newer and better antioxidants through kinetic spectroscopic studies in combination with product analysis and computation. Background: Antioxidant effect of caffeic acid, its derivative, and analogues have been well reported. The anti-oxidative efficiencies are related to their molecular structure, and two reaction pathways are well accepted, H-atom transfer (HAT) or single electron transfer. 1-hydroxy ethyl radical (1-HER) being ethanol-derived free radical might be causing the onset of liver injury detected after alcohol administration. 1-HER has also been reported to react with fatty acids and endogenous antioxidants such as glutathione, ascorbic acid and alpha-tocopherol. Objective: The present study is an attempt to understand the reaction mechanism of 1- HER with caffeic acid, its derivative and analogues in detail. Methods: Pulse radiolysis with kinetic absorption spectroscopy has been employed to follow the reaction pathway and identify the intermediates produced in the reaction. The reaction products have been detected using LCMS/MS. Based on these studies, a consolidated mechanism has been proposed. Cyclic voltammetry measurements and computational calculations have been used in support of the proposed mechanism. Result: In the reaction of 1-hydroxy ethyl radical (1-HER) with caffeic acid and its oligomers, reduction takes place below the pKa1, while oxidation occurs with the deprotonated phenolic moiety. The reduction of caffeic acid generates a carbon-centered radical at the double bond of the side chain with a bimolecular rate constant of 1.4x1010 dm3 mol-1 s-1. Notably, a low concentration of oxygen is able to regenerate a part of the caffeic acid molecules in the reduction process. At pH 10 a phenoxyl radical is formed due to oxidation with a much lower bimolecular rate constant of (4.2x108 dm3 mol-1 s-1). In the case of di-hydrocaffeic acid, only phenoxyl radical is formed at pH 10 and, no reaction could be observed below pH 8. Conclusion: Change in reactive pattern from reduction to oxidation with change in pH within same set of reactants has been evidently established in the present study. The results point towards the importance of α−β unsaturation in the side chain of caffeic acid oligomers for their reaction with 1-HER at neutral pH. The effect of oxygen concentration on the antioxidative protection offered by this class of molecules might be intriguing for the quest of the effectiveness of antioxidants at low concentration. Other: It may be inferred that the effect of pH on the reactivity pattern as observed is not 1-HER, but substrate specific, in the present case phenolic acids. This study generates further scope for in-depth studies on other polyphenols where unsaturation exists in the side chain.

Temperature-dependent and nonlinear optical response of double perovskite Cs2AgBiBr6 nanocrystals

Cs2AgBiBr6 double perovskite nanocrystals (NCs) are materials within the halide perovskite family with promising application potential, owing to their nontoxicity and excellent stability. Herein, Cs2AgBiBr6 NCs were synthesized and their photo-physical properties were investigated in detail. The temperature-dependent photoluminescence (PL) test confirms that their PL mechanism is dependent on the competition between the PL originating from carrier recombination in the conduction band and that in the self-trapping states. Their carrier recombination constants (involving the monomolecular, bimolecular, and Auger recombination rate constants) were obtained from transient absorption data and indicated that the monomolecular recombination decreases with the lowering of temperature, while the bimolecular and Auger recombination show opposite trends. The nonlinear optical properties were studied based on the Z-scan technique, with a two-photon absorption cross section of approximately 1906 GM. Our results suggest that Cs2AgBiBr6 NCs can be extensively applied in the field of optoelectronics, and conventional electronic switches will benefit from the excellent carrier dynamics and nonlinear optical properties.

Comparative Study of Recombination Dynamics in Optimized Composition of Sn- Versus Pb-Based Perovskite Solar Cells.

The energy band gaps of Pb halide perovskites are higher than the optimal band gap required for single-junction solar cells, governed by the Shockley-Queisser radiative limit. The pure Sn and Pb-Sn mixed-based perovskites have drawn significant attention due to their ability to lead to lower band gaps and open a new door for all perovskite tandem applications. There has been continuous progress toward the rapid improvement in the power conversion efficiency of Sn and Pb-Sn mixed-based perovskite solar cells (PSCs). Along with efforts for efficiency, it is worth analyzing the in-depth recombination dynamics for further development of Sn-based PSCs. The lower bimolecular recombination rate constant (k) is often attributed to the high performance of PSCs. Herein, we study the role of "B" cations in charge carrier recombination dynamics (CCRD) of ABX3 (A = MA+, FA+, and Cs+; B = Pb2+, Sn2+, and X = I-)-based PSCs. We fabricated p-i-n configuration-based FA0.95Cs0.05PbI3 (pure Pb), MA0.20FA0.75Cs0.05SnI3 (pure Sn), and (MAPbI3)0.4(FASnI3)0.6 (Pb-Sn mixed) PSCs and compared the CCRD of all the three PSCs. We optimized the Sn-based perovskite thin film (pure Sn) in terms of moisture and thermal stability in order to minimize the error due to perovskite degradation. We note that despite having lower open-circuit voltage (VOC), a pure Sn-based PSC shows lower k than that of Pb-Sn mixed and pure Pb-based PSCs, which is a contradictory result. This slow relaxation lifetime of the charge carrier in Sn-based PSCs can be correlated with recombination through the defect states without introducing the quasi-Fermi-level splitting. Furthermore, our results suggest that the rate law of charge carrier decay has nonlinear dependence of k on n in Sn-based PSCs, whereas it is linear in the other two cases.

Synergetic effect of potassium persulfate on photodegradation of para-arsanilic acid in Fe(III) oxalate system

The p-arsanilic acid (p-ASA) is widely used in agriculture as a food additive to control parasites. It leaves the body almost unchanged and is subsequently destroyed by environmental factors with the formation of toxic forms of inorganic arsenic. UVA irradiation of p-ASA with the addition of the Fe(III) oxalate complex leads to an effective photodegradation of the target compound. However, this method needs high concentrations of reagents and the keeping high [oxalate]:[Fe(III)] ratio to maintain proper efficiency of Fe(III) oxalate system. In this work, to overcome these problems, potassium persulfate (PS) was used as an additional oxidizer to improve Fe(III) oxalate system. It was found that the sulfate radical produced upon PS activation reacts readily with both neutral and monoanionic forms of p-ASA yielding corresponding organic cation radical, bimolecular rate constants are (7.3 ± 0.6) × 109 and (2.4 ± 0.4) × 109 M−1s−1, accordingly. Addition of 1 mM of PS reduces to one order (from 1.6 to 0.12 mM) the working concentration of oxalate and allows to achieve the complete degradation of both p-ASA and organic byproducts to inorganic As(V). Also the proposed approach demonstrates high efficiency in a wide range (from <0.5 to 3 ppm) of p-ASA concentrations.

Electrochemical Oxidation of 6:2 Polyfluoroalkyl Phosphate Diester-Simulation of Transformation Pathways and Reaction Kinetics with Hydroxyl Radicals.

Polyfluoroalkyl phosphate diesters (diPAPs) are widely used for paper and cardboard impregnation and discharged via waste streams from production processes and consumer products. To improve the knowledge about the environmental fate of diPAPs, electrochemical oxidation (EO) was used to characterize the transformation pathways and reaction kinetics. 6:2 diPAP was transformed electrochemically to perfluorocarboxylic acids (C5-C7 PFCAs) and two intermediates (6:2 fluorotelomer carboxylic acid, FTCA, and 6:2 fluorotelomer unsaturated carboxylic acid, FTUCA). EO of potential intermediates 6:2 monoPAP and 6:2 fluorotelomer alcohol (FTOH) showed similar transformation products but with different ratios. We show that 6:2 diPAP is initiated by OH radical (•OH) reactions, as evidenced by the measured steady-state concentrations of •OH with the probe molecule terephthalic acid, quenching experiments, and pH dependency of the reaction. PFHpA was the main product of 6:2 diPAP oxidation, and it was formed in a pseudo-first-order reaction for which a bimolecular rate constant was estimated to be = 9.4(±1.4) × 107 M-1 s-1 by an initial rate approach. This can be utilized to estimate the environmental half-life of 6:2 diPAP for the reaction with •OH and the formation kinetics of persistent PFCAs.

An experimental and statistical modeling study of the reactivity of Co+(CH3Br)n (n = 0,1) with methyl bromide

The temperature- and pressure-dependent kinetics of Co+(CH3Br)n (n = 0,1) + CH3Br are measured from 300 to 600 K and from 0.2 to 0.5 Torr. Results are interpreted using density functional calculations and modeled with statistical theory. In the n = 0 case, the associative product Co+(CH3Br) dominates with a rate constant between 5 and 30% of the collisional value, while a minor channel yields CoBr+ + CH3 with a rate constant increasing with temperature. 0K bond dissociation energies (BDE) are determined for Co+-Br (2.8 ± 0.1 eV) and Co+-CH3Br (2.4 ± 0.25 eV). Ligation of Co+ (i.e. n = 1) significantly increases the bimolecular reaction rate constant. CoBr+(CH3Br) + CH3 is formed and the reaction has a positive temperature dependence. Interestingly, this rate constant decreases with increasing pressure due to competition of the association channel. Ligation increases the (CH3Br)Co+-Br BDE (3.0 ± 0.1 eV) relative to Co+-Br, but decreases the (CH3Br)Co+-CH3Br BDE (1.95 ± 0.25) relative to Co+-CH3Br. Both bimolecular reactions proceed by a metal-insertion mechanism with a significantly submerged transition state that does not affect the kinetics. Instead, the endothermicity of the reaction is rate-limiting. Discussion about how energy, impact parameter, and angular momentum affect specific rate constants for dissociation to reactants, reaction to products, and association are presented.

Computational Investigation on the Formation and Decomposition Reactions of the C4H3O Compound.

Gas-phase mechanism and kinetics of the formation and decomposition reactions of the C4H3O compound, a crucial intermediate of the atmospheric and combustion chemistry, were investigated using ab initio molecular orbital theory and the very expensive coupled-cluster CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) method together with transition state theory and Rice–Ramsperger–Kassel–Macus kinetic predictions. The potential energy surface established shows that the C3H3 + CO addition reaction has four main entrances in which C3H3 + CO → IS1-cis (CHCCH2CO) is the most energetically favorable channel. The calculated results revealed that the bimolecular rate constants are positively dependent on both temperatures (T = 300–2000 K) and pressures (P = 1–76,000 Torr). Of these values, the k1 rate constant of the C3H3 + CO → IS1-cis addition channel is dominant over the 300–2000 K temperature range, increasing from 1.53 × 10–20 to 1.04 × 10–13 cm3 molecule–1 s–1 with the branching ratio reducing from 62% to 44%. The predicted unimolecular rate coefficients in the ranges of T = 300–2000 K and P = 1–76,000 Torr revealed that the intermediate products IS1-cis, IS1-trans, and IS2 are rather unstable and would rapidly decompose back to the reactants (C3H3 + CO), especially at high temperatures (T > 1000 K). The high-pressure limit rate constants for the C4H3O decomposition leading to products (C3H3 + CO), (CHCCHCO + H), and (CHCO + C2H2) have been found to be in excellent agreement with the available literature values proposed by Tian et al. (Combust. Flame, 2011,158, 756–773) without any adjustment from the ab initio calculations. Therefore, the predicted temperature- and pressure-dependent rate constants can be confidently used for modeling CO-related systems under atmospheric and combustion conditions.

Phototransformation of an emerging cyanotoxin (Aerucyclamide A) in simulated natural waters

Aerucyclamide A (ACA) is an emerging cyanopeptide toxin produced by cyanobacteria, and its transformation pathway has rarely been reported. In the present study, ACA was purified from cyanobacterial extracts, and photodegradation processes were investigated in dissolved organic matter (DOM) solutions. Under simulated solar irradiation, the photodegradation of ACA was dominated by •OH oxidation, accounting for ~72% of the indirect photodegradation. The bimolecular reaction rate constant of ACA with •OH was (6.4 ± 0.2) × 109M − 1s − 1. Our results indicated that the major reactive sites of ACA toward •OH are thiazoline and thiazole moieties. Product analysis via high-resolution mass spectrometry suggested that hydrogen abstraction and gradual hydroxylation are the main photodegradation pathways. The acute toxicity assessment indicate that the products generated in photolysis process did not show any measurable toxicity to Thamnocephalus platyurus. Photodegradation experiments with various DOM-phycocyanin mixtures demonstrated that the half-life of ACA is much longer than that of microcystin-LR.

Simplified Protocol for the Calculation of Multiconformer Transition State Theory Rate Constants Applied to Tropospheric OH-Initiated Oxidation Reactions.

Chemical kinetics plays a fundamental role in the understanding and modeling of tropospheric chemical processes, one of the most important being the atmospheric degradation of volatile organic compounds. These potentially harmful molecules are emitted into the troposphere by natural and anthropogenic sources and are chemically removed by undergoing oxidation processes, most frequently initiated by reaction with OH radicals, the atmosphere's "detergent". Obtaining the respective rate constants is therefore of critical importance, with calculations based on transition state theory (TST) often being the preferred choice. However, for molecules with rich conformational variety, a single-conformer method such as lowest-conformer TST is unsuitable while state-of-the-art TST-based methodologies easily become unmanageable. In this Feature Article, the author reviews his own cost-effective protocol for the calculation of bimolecular rate constants of OH-initiated reactions in the high-pressure limit based on multiconformer transition state theory. The protocol, which is easily extendable to other oxidation reactions involving saturated organic molecules, is based on a variety of freeware and open-source software and tested against a series of oxidation reactions of hydrofluoropolyethers, computationally very challenging molecules with potential environmental relevance. The main features, advantages and disadvantages of the protocol are presented, along with an assessment of its predictive utility based on a comparison with experimental rate constants.

Fluoroquinolone antibiotics sensitized photodegradation of isoproturon

Fluoroquinolone (FQ) antibiotics are a group of contaminants of emerging environmental concern. In the present study, we demonstrated that norfloxacin (NORF) and ofloxacin (OFLO), two typical FQs, have photochemical reactivity analogous to chromophoric dissolved natural organic matter (DOM) in surface waters and can sensitize the photodegradation of isoproturon (IPU), a phenylurea herbicide. Such photochemical reactivity is ascribed to the quinolone chromophore that is excited to a triplet state (3FQ*) upon UV-A irradiation. 3FQ* further reacts with dissolved oxygen to give rise to singlet oxygen. 3FQ* steady-state concentrations of 6.72 × 10−15 and 1.27 × 10−15 M were measured in 10 μM NORF and OFLO solutions, respectively, under UV365nm irradiation. The degradation of IPU was due to the reaction with 3FQ*, with bimolecular rate constants of 6.07 × 109 and 1.51 × 1010 for 3NORF* and 3OFLO*, respectively. Intriguingly, NORF and OFLO per se were unstable and photolyzed during UV-A irradiation, but the photochemical reactivities of the solutions were not lost accordingly. High-resolution mass spectrometry analysis revealed that defluorination and piperazine moiety oxidation were the main photolysis pathways, while the core quinolone structure remained intact. Thus, the photolysis products largely inherited the photochemical reactivity of the parent compounds. Since all FQs share the same quinolone structure, similar photochemical reactivity is expected. The presence of FQs in surface water would affect the transformation and fate of coexisting compounds. To the best of our knowledge, this is the first study examining the environmental behavior of FQs as photosensitizers. The findings greatly advance the understandings of the influence of FQs in aquatic environment.

Inhibition Kinetics of 16 Organophosphorus Pesticides or Their Active Metabolites on Erythrocyte Acetylcholinesterase From Humans and Rats.

Inhibition kinetics assays were conducted with 16 commercial organophosphate (OP) pesticides or their metabolites on acetylcholinesterase (AChE) in erythrocyte "ghost" preparations from 18 individual humans (both sexes; adults, juveniles, and cord blood samples; mixed races/ethnicities) and pooled samples from adult rats (both sexes). A well-established spectrophotometric assay using acetylthiocholine as substrate and a chromogen was employed. The kinetic parameters bimolecular rate constant (ki), dissociation constant (KI), and phosphorylation constant (kp) were calculated for each compound. As expected, a wide range of potencies were displayed among the tested compounds. Statistical analysis of the resultant data indicated no differences in sex, age, or race/ethnicity among the human samples that are unexpected based on chance (4.2% statistically significant out of 48 parameters calculated) and no differences between the sexes in rats. The bimolecular rate constants for 10 of the compounds were not statistically different between rats and humans. The data indicate that, consistent with the high level of conservation of AChE among species and the fact that AChE at different locations within a species arises from the same gene, the inhibition kinetic parameters calculated from rat erythrocyte ghost preparations should be useful in estimating potencies of OP compounds on target AChE in humans. Additionally, the data indicate that differences in sensitivities among individual humans were not apparent.

Flavoenzyme-catalyzed single-electron reduction of nitroaromatic antiandrogens: implications for their cytotoxicity

The therapeutic action of nitroaromatic antiandrogens nilutamide and flutamide may be complicated by their cytotoxicity, whose mechanisms are still incomprehensively understood. In particular this concerns the enzymatic redox cycling of flutamide and its metabolites, and its impact on their cytotoxicity. In this work, we examined the single-electron reduction of nilutamide, flutamide, its metabolites 2-hydroxyflutamide and 4-nitro-3-trifluorormethyl-phenylamine, and a topical antiandrogen (3-amino-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl)-phenyl) propanamide by NADPH:cytochrome P-450 reductase and adrenodoxin reductase/adrenodoxin. The obtained steady-state bimolecular rate constants of oxidant reduction (k cat/K m) enabled to establish single-electron reduction midpoint potentials (E 1 7) of compounds, −0.377 – −0.413 V, which were in line with enthalpies of formation of their free radicals, obtained by quantum mechanical calculations. Using murine hepatoma MH22a cells, the obtained cytotoxicity vs. E 1 7 correlation based on the data of model nitroaromatic compounds shows that redox cycling and oxidative stress could be the main factor of cytotoxicity of nitroaromatic antiandrogens. Other minor cytotoxicity factors could be their redox metabolism involving NAD(P)H:quinone oxidoreductase (NQO1) and cytochromes P-450.

Lignin-Derived Non-Heme Iron and Manganese Complexes: Catalysts for the On-Demand Production of Chlorine Dioxide in Water under Mild Conditions.

A lignin-derived ligand, bis(2-hydroxy-3-methoxy-5-propylbenzyl)glycine (DHEG), was synthesized from 2-methoxy-4-propylphenol (dihydroeugenol (DHE)) and the amino acid glycine. Two mononuclear iron and manganese complexes of DHEG were prepared, characterized, and employed for the oxidation of chlorite to chlorine dioxide in aqueous solution. Peroxyacetic acid (PAA) was used as a "green" oxidant in the redox reactions to initiate the formation of high-valent Fe and Mn (IV)-OH intermediates. EPR studies verified the formation of a high-valent MnIV species. Both Fe and Mn complexes catalyzed chlorite oxidation with bimolecular rate constants of 32 and 144 M-1 s-1, respectively, at pH 4.0 and 25 °C. The Mn complex was found to be more efficient for chlorite oxidation with a turnover frequency of 17 h-1 and remained active during subsequent additions of PAA. The rate of ClO2 decomposition with PAA/Mn-DHEG was first order in PAA and increased significantly as pH increased. A mechanism that accounts for all observations is presented.

Bioorthogonal Reactions Utilizing Nitrones as Versatile Dipoles in Cycloaddition Reactions.

Bioorthogonal chemical reactions have emerged as convenient and rapid methods for incorporating unnatural functionality into living systems. Different prototype reactions have been optimized for use in biological settings. Optimization of 3 + 2 dipolar cycloadditions involving nitrones has resulted in highly efficient reaction conditions for bioorthogonal chemistry. Through substitution at the nitrone carbon or nitrogen atom, stereoelectronic tuning of the reactivity of the dipole has assisted in optimizing reactivity. Nitrones have been shown to react rapidly with cyclooctynes with bimolecular rate constants approaching k2 = 102 M-1 s-1, which are among the fastest bioorthogonal reactions reported (McKay et al. Org. Biomol. Chem. 2012, 10, 3066-3070). Nitrones have also been shown to react with trans-cyclooctenes (TCO) in strain-promoted TCO-nitrone cycloadditions reactions. Copper catalyzed reactions involving alkynes and nitrones have also been optimized for applications in biology. This review provides a comprehensive accounting of the different bioorthogonal reactions that have been developed using nitrones as versatile reactants, and provides some recent examples of applications for probing biological systems.