Short-lived Radical Species Research Articles

Functionalized Polysaccharides Improve Sensitivity of Tyramide/Peroxidase Proximity Labeling Assays through Electrostatic Interactions.

High-throughput assays that efficiently link genotype and phenotype with high fidelity are key to successful enzyme engineering campaigns. Among these assays, the tyramide/peroxidase proximity labeling method converts the product of an enzymatic reaction of a surface expressed enzyme to a highly reactive fluorescent radical, which labels the cell surface. In this context, maintaining the proximity of the readout reagents to the cell surface is crucial to prevent crosstalk and ensure that short-lived radical species react before diffusing away. Here, we investigated improvements in tyramide/peroxidase proximity labeling for enzyme screening. We modified chitosan (Cs) chains with horseradish peroxidase (HRP) and evaluated the effects of these conjugates on the efficiency of proximity labeling reactions on yeast cells displaying d-amino acid oxidase. By tethering HRP to chitosan through different chemical approaches, we localized the auxiliary enzyme close to the cell surface and enhanced the sensitivity of tyramide-peroxidase labeling reactions. We found that immobilizing HRP onto chitosan through a 5 kDa PEG linker improved labeling sensitivity by over 3.5-fold for substrates processed with a low turnover rate (e.g., d-lysine), while the sensitivity of the labeling for high activity substrates (e.g., d-alanine) was enhanced by over 0.6-fold. Such improvements in labeling efficiency broaden the range of enzymes and conditions that can be studied and screened by tyramide/peroxidase proximity labeling.

Nanoporous anodic aluminum oxide-confined ZIF-67 for efficiently activating peroxymonosulfate to degrade organic pollutants

The limited mass transfer of short-lived radical species is an inherent drawback of heterogeneous oxidation processes. Herein, we fabricated zeolitic imidazolate framework (ZIF)-67 loaded nanoreactors with precise pore sizes based on anodic alumina oxide (AAO) templates to achieve nanoscale spatial confinement for enhanced radicals-mediated oxidation reactions. The prepared ZIF-67@AAO membrane/peroxymonosulfate (PMS) system exhibits perfect catalytic performance with a rhodamine B (RhB) removal of 100 % within 60 s under an AAO pore size of < 20 nm. The reaction rate constant of RhB degradation by the ZIF-67@AAO<20 nm/PMS system was 2.52 and 893 times that by the ZIF-67@AAO200–300 nm/PMS and ZIF-67/PMS system (no confinement), respectively. The ordered mesoporous structure of the ZIF-67@AAO nanoreactors provided many channels for PMS and RhB molecules to enter the confined inner space of the catalyst, leading to a significant improvement in catalytic performance under nanoconfinement. Radical quenching and electron paramagnetic resonance (EPR) results indicated that SO4−, ·OH, 1O2, and O2− were generated in the ZIF-67@AAO/PMS system, and their amounts were affected by spatial confinement and achieved the maximum at a pore size of < 20 nm. Furthermore, the ZIF-67@AAO catalytic membrane exhibited attractive long-term operation stability with perfect activity and neglectable Co leaching. The results of this study provide an important reference for designing novel catalytic membranes for water decontamination.

Bullseye Analysis: A Fluorescence Microscopy Technique to Detect Local Changes in Intracellular Reactive Oxygen Species (ROS) Production.

Reactive oxygen species (ROS) are naturally produced compounds that play important roles in cell signaling, gene regulation, and biological defense, including involvement in the oxidative burst that is central to the anti-microbial actions of macrophages. However, these highly reactive, short-lived radical species also stimulate cells to undergo programmed cell death at high concentrations, as well as causing detrimental effects such as oxidation of macromolecules at more moderate levels. Imaging ROS is highly challenging, with many researchers working on the challenge over the past 10-15 years without producing a definitive method. We report a new fluorescence microscopy-based technique, Bullseye Analysis. This methodology is based on concepts provided by the FRAP (Fluorescence Recovery after Photobleaching) technique and refined to evidence the spatiotemporal production of ROS, and the subsequent consequences, on a subcellular scale. To exemplify the technique, we have used the ROS-reporter dye, CellROX, and the ROS-inducing photosensitizer, LightOx58, a potent source of ROS compared with UV irradiation alone. Further validation of the technique was carried out using differing co-stains, notably Mitotracker and JC-1.

A Multidimensional Approach to Carbodiphosphorane-Bismuth Coordination Chemistry: Cationization, Redox-Flexibility, and Stabilization of a Crystalline Bismuth Hydridoborate.

Bismuth complexes stabilized by carbon-based donor ligands are underserved by their instability, often due to facile ligand dissociation and deleterious protonolysis. Herein, we show that the ortho-bismuthination of hexaphenylcarbodiphosphorane enables a robust framework with geometrically constrained carbone-bismuth bonding interactions, which are highly tunable by cationization. The carbodiphosphorane bismuth halides (1 and 2) are remarkably air-stable and feature unprecedented trans carboneC-Bi-X ligation, resulting in highly elongated Bi-X bonds. In contrast to known carbone-bismuth complexes, hydrolytic activation of the carbone yields well-defined organobismuth complexes, and subsequent dehydrohalogenation is feasible using potassium bis(trimethylsilyl)amide or N-heterocyclic carbenes. The redox-flexibility of this framework was evaluated in the high catalytic activity of 1 and 2 for silylation of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) under mild conditions (50 °C, 24-96 h) and low catalyst loadings (5-10 mol %), which suggests the accessibility of short-lived hydridic and radical bismuth species. The reaction of 1, PhSiH3, and tris(pentafluorophenyl)borane (BCF) yields the first crystallographically characterized bismuth hydridoborate complex as an ionic species (9), presumably by BCF-mediated hydride abstraction from an unobserved [Bi]-H intermediate. All isolated compounds have been characterized by heteronuclear NMR spectroscopy and X-ray crystallography, and the bonding situation in representative complexes (1, 2, 5, and 9) were further evaluated using density functional theory.

Mechanism of Heterogeneous Fenton Reaction Kinetics Enhancement under Nanoscale Spatial Confinement.

Nanoscale catalysts that can enable Fenton-like chemistry and produce reactive radicals from hydrogen peroxide activation have been extensively studied in order to overcome the limitations of homogeneous Fenton processes. Despite several advantageous features, limitation in mass transfer of short-lived radical species is an inherent drawback of the heterogeneous system. Here, we present a mechanistic foundation for the way spatial confinement of Fenton chemistry at the nanoscale can significantly enhance the kinetics of radical-mediated oxidation reactions-pollutant degradation in particular. We synthesized a series of Fe3O4-functionalized nanoreactors with precise pore dimensions, based on an anodized aluminum oxide template, to enable quantitative analysis of nanoconfinement effects. Combined with computational simulation of spatial distribution of radicals, we found that hydroxyl radical concentration was strongly dependent on the distance from the surface of Fenton catalysts. This distance dependency significantly influences the gross reaction kinetics and accounts for the observed nanoconfinement effects. We further found that a length scale below 25 nm is critical to avoid the limitation of short-lived species diffusion and achieve kinetics that are orders of magnitude faster than those obtained in a batch suspension of heterogeneous catalysts. These findings suggest a new strategy to develop an innovative heterogeneous catalytic system with the most effective use of hydroxyl radicals in oxidation treatment scenarios.

Methyl groups matter: Photo-CIDNP characterizations of the semiquinone radicals of FMN and demethylated FMN analogs.

In this contribution, the relative hyperfine couplings are determined for the 1H nuclei of the flavin mononucleotide (FMN) radical in an aqueous environment. In addition, three structural analogs with different methylation patterns are characterized and the influence of the substituents at the isoalloxazine moiety on the electronic structure of the radicals is explored. By exploiting nuclear hyperpolarization generated via the photo-CIDNP (chemically induced dynamic nuclear polarization) effect, it is possible to study the short-lived radical species generated by in situ light excitation. Experimental data are extracted by least-squares fitting and supported by quantum chemical calculations and published values from electron paramagnetic resonance and electron-nuclear double resonance. Furthermore, mechanistic details of the photoreaction of the investigated flavin analogs with l-tryptophan are derived from the photo-CIDNP spectra recorded at different pH values. Thereby, the neutral and anionic radicals of FMN and three structural analogs are, for the first time, characterized in terms of their electronic structure in an aqueous environment.

Nitrosylation vs. oxidation - How to modulate cold physical plasmas for biological applications.

Thiol moieties are major targets for cold plasma-derived nitrogen and oxygen species, making CAPs convenient tools to modulate redox-signaling pathways in cells and tissues. The underlying biochemical pathways are currently under investigation but especially the role of CAP derived RNS is barely understood. Their potential role in protein thiol nitrosylation would be relevant in inflammatory processes such as wound healing and improving their specific production by CAP would allow for enhanced treatment options beyond the current application. The impact of a modified kINPen 09 argon plasma jet with nitrogen shielding on cysteine as a thiol-carrying model substance was investigated by FTIR spectroscopy and high-resolution mass spectrometry. The deposition of short-lived radical species was measured by electron paramagnetic resonance spectroscopy, long-lived species were quantified by ion chromatography (NO2-, NO3-) and xylenol orange assay (H2O2). Product profiles were compared to samples treated with the so-called COST jet, being introduced by a European COST initiative as a reference device, using both reference conditions as well as conditions adjusted to kINPen gas mixtures. While thiol oxidation was dominant under all tested conditions, an Ar + N2/O2 gas compositions combined with a nitrogen curtain fostered nitric oxide deposition and the desired generation of S-nitrosocysteine. Interestingly, the COST-jet revealed significant differences in its chemical properties in comparison to the kINPen by showing a more stable production of RNS with different gas admixtures, indicating a different •NO production pathway. Taken together, results indicate various chemical properties of kINPen and COST-jet as well as highlight the potential of plasma tuning not only by gas admixtures alone but by adjusting the surrounding atmosphere as well.

Ultra-sensitive spectroscopy of OH radical in high-temperature transient reactions.

The hydroxyl (OH) radical is arguably the most important transient radical in high-temperature gas-phase combustion reactions, yet it is very difficult to measure because of its high reactivity and, thus, short lifetime and low concentration. This work reports the development of a novel method for ultra-sensitive, quantitative, and microsecond-resolved detection of OH based on UV frequency-modulation spectroscopy (FMS). To the best of the authors' knowledge, this is the first FMS demonstration in the near-UV spectral region for detection of short-lived radical species. Shot-noise-limited detection was achieved at an optical power of 25mW. A proof-of-concept experiment in a tabletop H2O/He microwave discharge cell has reached a 1σ minimum detectable absorbance (MDA) of less than 2×10-4 over 1MHz measurement bandwidth. High-temperature OH measurement was demonstrated in a 15cm diameter shock tube, where a typical MDA of 3.0×10-4 was achieved at 1330K, 0.38atm, and 1MHz. These preliminary results have outperformed the previous best MDA by more than a factor of 3; further improvement by another order of magnitude is anticipated, following the strategies outlined at the end of this Letter. The current method paves the path to parts per billion (ppb) -level OH detection capability and offers prospects to significantly advance fundamental combustion research by enabling direct observation of OH formation and scavenging kinetics during key stages of fuel oxidation that were inaccessible with previous methods.

ChemInform Abstract: Silicon Based Radicals, Radical Ions, Diradicals and Diradicaloids

AbstractReview: [93 refs.

Ethionamide biomimetic activation and an unprecedented mechanism for its conversion into active and non-active metabolites.

Ethionamide (ETH), a second-line anti-tubercular drug that is regaining a lot of interest due to the increasing cases of drug-resistant tuberculosis, is a pro-drug that requires an enzymatic activation step to become active and to exert its therapeutic effect. The enzyme responsible for ETH bioactivation in Mycobacterium tuberculosis is a monooxygenase (EthA) that uses flavin adenine dinucleotide (FAD) as a cofactor and is NADPH- and O2-dependant to exert its catalytic activity. In this work, we investigated the activation of ETH by various oxygen-donor oxidants and the first biomimetic ETH activation methods were developed (KHSO5, H2O2, and m-CPBA). These simple oxidative systems, in the presence of ETH and NAD+, allowed the production of short-lived radical species and the first non-enzymatic formation of active and non-active ETH metabolites. The intermediates and the final compounds of the activation pathway were well characterized. Based on these results, we postulated a consistent mechanism for ETH activation, not involving sulfinic acid as a precursor of the iminoyl radical, as proposed so far, but putting forward a novel reactivity for the S-oxide ethionamide intermediate. We proposed that ETH is first oxidized into S-oxide ethionamide, which then behaves as a "ketene-like" compound via a formal [2 + 2] cycloaddition reaction with peroxide to give a dioxetane intermediate. This unstable 4-membered intermediate in equilibrium with its open tautomeric form decomposes through different pathways, which would explain the formation of the iminoyl radical and also that of different metabolites observed for ETH oxidation, including the ETH-NAD active adduct. The elucidation of this unprecedented ETH activation mechanism was supported by the application of isotopic labelling experiments.

Reactivity of Cationic Triruthenium Carbonyl Clusters: From Pyrimidinium Ligands to N-Heterocyclic Carbenes

The reaction of [Ru3(CO)12] with pyrimidine (Hpym) leads to the isomeric cyclometalated derivatives [Ru3(μ-H)(μ-κ2N1,C6-pym)(CO)10] (1) and [Ru3(μ-H)(μ-κ2N1,C2-pym)(CO)10] (2), which have been treated with [Me3O][BF4] to give the corresponding N-methylpyrimidinium derivatives [Ru3(μ-H)(μ-κ2N1,C6-pymMe)(CO)10][BF4] (3[BF4]) and [Ru3(μ-H)(μ-κ2N1,C2-pymMe)(CO)10][BF4] (4[BF4]). The reaction of 4[BF4] with K-Selectride renders a separable mixture of two neutral trinuclear products, [Ru3(μ-H){μ-κ2N1,C2-(6-HpymMe)}(CO)9] (5) and [Ru3(μ-H){μ-κ2N1,C2-(4-HpymMe)}(CO)9] (6), which arise from the nucleophilic attack of a hydride at the ligand C6 and C4 carbon atoms of 4+. This cationic complex can be reduced to the short-lived radical species 4•, which spontaneously dimerizes by forming an intermolecular C−C bond between the ligands of two such radicals. Three hexanuclear derivatives, [Ru6(μ-H)2{μ6-κ4N1,N1′,C2,C2′-(4,6′-bipymMe2)}(CO)18] (isomers 7 and 8) and [Ru6(μ-H)2{μ6-κ4N1,N1′,C2,C2′-(4,4′-bipymMe2)}(CO)18] (is...

Vitamin E Forms Domains with Pufa in Membranes: An EPR Spin Trapping Study of Lipid Peroxidation in Model Membranes

Spin trapping in conjunction with EPR spectroscopy is a widely used technique for measurement of short-lived free radical species of biological interest due to its high sensitivity and specificity. In spin trapping a molecule (the spin trap) reacts with the free radical producing a spin adduct that is sufficiently stable to be detected by EPR. In this study, we employ lipid-soluble N-tert-butyl-α-phenylnitrone (PBN) as our spin trap. PBN spin adducts detected by EPR are believed to be lipid hydroperoxides that are secondary species resulting from free radical attack on cell membranes. Thus the presence of the spin-adducts is reflective of ROS levels. Lipid auto oxidation will be monitored in model membranes containing 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC) with various concentrations of α-tocopherol, the major lipid soluble antioxidant in membranes and a primary component of vitamin E. Preliminary data of PDPC without α-tocopherol show auto oxidation that leads to a free radical cascade. By measuring the reduction in lipid peroxidation due to the presence of α-tocopherol, we test the hypothesis that the vitamin co-localizes with polyunsaturated lipids in membrane domains to ensure its close proximity to the most vulnerable lipid species.

Efficiency of Clay−TiO2 Nanocomposites on the Photocatalytic Elimination of a Model Hydrophobic Air Pollutant

Clay-supported TiO2 photocatalysts can potentially improve the performance of air treatment technologies via enhanced adsorption and reactivity of target volatile organic compounds (VOCs). In this study, a benchtop photocatalytic flow reactor was used to evaluate the efficiency of hectorite-TiO2 and kaolinite-TiO2, two novel composite materials synthesized in our laboratory. Toluene, a model hydrophobic VOC and a common indoor air pollutant, was introduced in the air stream at realistic concentrations, and reacted under UVA (lamda(max) = 365 nm) or UVC (lamda(max) = 254 nm) irradiation. The UVC lamp generated secondary emission at 185 nm, leading to the formation of ozone and other short-lived reactive species. Performance of clay-Ti02 composites was compared with that of pure TiO2 (Degussa P25), and with UV irradiation in the absence of photocatalyst under identical conditions. Films of clay-TiO2 composites and of P25 were prepared by a dip-coating method on the surface of Raschig rings, which were placed inside the flow reactor. An upstream toluene concentration of approximately 170 ppbv was generated by diluting a constant flow of toluene vapor from a diffusion source with dry air, or with humid air at 10, 33, and 66% relative humidity (RH). Toluene concentrations were determined by collecting Tenax-TA sorbent tubes downstream of the reactor, with subsequent thermal desorption--GC/MS analysis. The fraction of toluene removed, %R, and the reaction rate, Tr, were calculated for each experimental condition from the concentrations measured with and without UV irradiation. Use of UVC light (UV/TiO2/O3) led to overall higher reactivity, which can be partially attributed to the contribution of gas phase reactions by short-lived radical species. When the reaction rate was normalized to the light irradiance, Tr/Ilamda,the UV/TiO2 reaction under UVA irradiation was more efficient for samples with a higher content of TiO2 (P25 and Hecto-TiO2), but notfor Kao-TiO2. In all cases, reaction rates peaked at 10% RH, with Tr values between 10 and 50% higherthan those measured under dry air. However, a net inhibition was observed as RH increased to 33% and 66%, indicating that water molecules competed effectively with toluene for reactive surface sites and limited the overall photocatalytic conversion. Compared to P25, inhibition by coadsorbed water was less significant for Kao-TiO2 samples, but was more dramatic for Hecto-TiO2 due to the high water uptake capacity of hectorite.

Hybrid QM/MM Investigations into the Structure and Properties of Oxygen-Donating Species in TS-1

The active oxidizing species in the H2O2/TS-1 catalytic system is investigated using a hybrid quantum mechanical/molecular mechanical approach. In this computational technique, the site of interest is described with the density functional theory using the BB1K exchange and correlation functional, and the remainder of the system is treated with a valence force field. We have examined the formation of dioxygen adsorbate structures with η1 and η2 Ti-peroxo configurations, including radical species, which arise from the attack of hydrogen peroxide on a tetrahedral TiIV site along with effects of water co-adsorption. Our results show that hydrogen peroxide physisorbed on tetra- and tripodal Ti sites, as well as coadsorbed H2O2/H2O, is energetically favorable and involves minor structural modifications in the framework, as Ti atoms change their coordination from four to six. Furthermore, our calculations of various Ti-peroxo complexes formed from tripodal sites and H2O2 strongly suggest that Ti-η2(OOH), Ti-η1(OOH), and Ti-η1(O(H)OH) complexes will all form in the pores of TS-1 under reactions with H2O2 with a preferential coordination of six in the presence of water. In particular, both six-coordinate Ti-η1(OOH) and Ti-η2(OOH) compare favorably with EXAFS data obtained from the mesoporous materials with the isomorphous Ti active sites. These results indicate that water is not just a medium for transporting reactants and products at the catalytic sites, but has an active role in stabilizing the peroxo species present on the working catalyst. The Ti−O bond distances are also close to those in the five-coordinated complexes reported for TS-1 (also based on EXAFS data). Although, we find that both anhydrous and hydrated η2(O2·) type species are unlikely to be the predominant oxygen-donating species, we have demonstrated that our calculated g-tensors for the short-lived radical species are in line with EPR data, which supports our assertion of the tripodal Ti site as the main active site in TS-1 for partial oxidation catalysis.

Incoherent broad-band cavity-enhanced absorption spectroscopy of the marine boundary layer species I2, IO and OIO

The novel combination of incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) and a discharge-flow tube for the study of three key atmospheric trace species, I(2), IO and OIO, is reported. Absorption measurements of I(2) and OIO at lambda=525-555 nm and IO at lambda=420-460 nm were made using a compact cavity-enhanced spectrometer employing a 150 W short-arc Xenon lamp. The use of a flow system allowed the monitoring of the chemically short-lived radical species IO and OIO to be conducted over timescales of several seconds. We report detection limits of approximately 26 pmol mol(-1) for I(2) (L=81 cm, acquisition time 60 s), approximately 45 pmol mol(-1) for OIO (L=42.5 cm, acquisition time 5 s) and approximately 210 pmol mol(-1) for IO (L=70 cm, acquisition time 60 s), demonstrating the usefulness of this approach for monitoring these important species in both laboratory studies and field campaigns.

A Bystander Effect in Alpha-Particle Irradiations of Human Prostate Tumor Cells

Alpha-particle exposures were used to determine whether cells of the human prostate carcinoma cell line DU-145 can produce and respond to a bystander effect signal. An apparatus for alpha-particle irradiation of cells growing as a monolayer on a 1.4-microm-thick Mylar membrane directly above an 241Am alpha-particle source was constructed and calibrated. At the cell irradiation position, the alpha-particle fluence was 998 counts/mm2 s(-1), the average alpha-particle energy was 3.14 MeV, and the average linear energy transfer was 128 keV/microm. The average dose rate to the cells growing on the Mylar surface was 1.2 Gy/min. A co-culture system was used to examine bystander effects transmitted through the medium from the directly targeted cells to tumor cells growing on an insert well beyond the range of the alpha particles. Alpha-particle doses from 0.1 to 6.0 Gy to the targeted cells on the Mylar membrane, followed by a 2-h co-incubation of the cells on the insert in the irradiated medium above the irradiated cells, all caused an approximately 50% increase in micronucleus formation in the nontargeted co-cultured cells. Addition of the radical scavenger DMSO to the medium during the irradiation and the 2-h postirradiation incubation period completely blocked the bystander effect, whereas addition of a nitric oxide scavenger had no effect. Irradiation of medium containing serum, followed by a 2-h incubation, caused no bystander effect in the co-cultured cells. When the co-cultured cells on the insert were placed into the irradiated medium above the directly targeted cells immediately (approximately 1 min) after the irradiation and co-incubated for 2 h, there was no bystander effect. These data indicate that the observed bystander effect requires that the co-cultured cells be present in the medium during the irradiation of the directly targeted cells and suggest the involvement of a short-lived radical species.

Spin evolution of radical pair with radical containing two groups of equivalent magnetic nuclei

Analytical solution is obtained for time-resolved magnetic field effects (TR-MFE) on recombination fluorescence of radical-ion pair (RIP) containing radical ion with two groups of magnetically equivalent nuclei. The present theoretical approach is applied to three experimental systems: RIPs containing radical cations of 2,3-dimethylbutane, 2,2,6,6-tetramethylpiperidine, or diisopropylamine and radical anion of p-terphenyl-d14 in nonpolar alkane solutions. Good agreement between theory and experiment is found for all the three systems, hyperfine coupling constants of radical cations are obtained by fitting the experimental TR-MFE traces. The potential of the TR-MFE technique for studying radical ions with nonequivalent nuclei is discussed in detail. The wide applicability of the theoretical model and the experimental technique make them useful for studying short-lived radical species that are often beyond the reach of the conventional electron paramagnetic resonance spectroscopy.

Carotenoid–radical interactions

Carotenoids have been reported to react with virtually any radical species likely to be encountered in a biological system. The products of such reactions are frequently short-lived radical species that can decay to more stable products. In some cases, stable adducts can be observed, but in the majority of interactions with radicals, carotenoids break down to degradation products very similar to what is seen with oxidative degradation. It is only recently that the biological activity of these breakdown products has begun to be investigated.

Electron paramagnetic resonance induced by K-shell resonance excitation in DNA bases in solid state

DNA base radicals were examined around oxygen K-edge region using an EPR spectrometer installed in a synchrotron soft X-ray beamline (BL23SU) in SPring-8. In situ measurements of EPR spectrum of guanine and thymine revealed that short-lived transient radical species are induced by photoexcitation of O 1s electron to σ * orbital. They promptly disappear by “beam-off”. On the other hand, long-lived radicals whose EPR spectra are consistent with previous reports for guanine cation and thymine anion radical are accumulated during the irradiation. These results indicate that chemically stable DNA base lesions, such as 8-oxo-G, would result from transient species that are inferred to be one electron oxidized radicals after decay of Auger final state.

Transfer of mass‐independent fractionation in ozone to other oxygen‐containing radicals in the atmosphere

Mass‐independent fractionation (MIF) of ozone has been observed in both the troposphere and stratosphere. Because ozone is a photochemically active species, its MIF signature can be imparted to other atmospheric molecules. Using a photochemical equilibrium model for short‐lived radical species, I have computed the expected MIF for typical mid‐latitude conditions. The model accounts for about ∼70% of recent measurements of Δ177O for H2O2 in rainwater, and predicts large MIF for NOx and ClO species (∼40–70‰), and their products (ClONO2 and HNO3). Furthermore, in the stratosphere oxygen exchange reactions between OH and NOx yield OH with Δ17O from 2 to 45‰. Stratospheric water produced during H abstraction by OH would be similarly mass‐independently fractionated. In the troposphere rapid exchange between OH and H2O erases any MIF signature in OH. These model results depend on several O exchange reactions with unknown activation energies or with rate coefficients known only as upper limits.