Peroxyl, alkoxyl, and carbon-centered radical formation from organic hydroperoxides by chloroperoxidase

A radical polymer membrane for simultaneous degradation of organic pollutants and water filtration

Integrating reactive radicals into membranes that resemble biological membranes has always been a pursuit for simultaneous organics degradation and water filtration. In this research, we discovered that a radical polymer (RP) that can directly trigger the oxidative degradation of sulfamethozaxole (SMX). Mechanistic studies by experiment and density functional theory simulations revealed that peroxyl radicals are the reactive species, and the radicals could be regenerated in the presence of O2. Furthermore, an interpenetrating RP network membrane consisting of polyvinyl alcohol and the RP was fabricated to demonstrate the simultaneous filtration of large molecules in the model wastewater stream and the degradation of ~ 85% of SMX with a steady permeation flux. This study offers valuable insights into the mechanism of RP-triggered advanced oxidation processes and provides an energy-efficient solution for the degradation of organic compounds and water filtration in wastewater treatment.

Preparation and characterization of carboxymethylated Aureobasidium pullulans β-(1 → 3, 1 → 6)-glucan and its in vitro antioxidant activity

Aureobasidium pullulans β-(1 → 3, 1 → 6)-glucan (APG) has a high degree of β-(1 → 6)-glucosyl branching and a regular triple helical structure similar to that of schizophyllan. In this study, APG was carboxymethylated to different degrees of substitution (DS = 0.51, 1.0, and 2.0, denoted CMAPG 1-3, respectively) using a heterogeneous reaction. With increasing DS, the triple-helix structure drastically decreased and converted to a random coil structure in CMAPG 3. Further, aqueous solutions of CMAPG changed from pseudoplastic fluids to perfect Newtonian liquids with increasing DS, indicating that the intra- and intermolecular hydrogen bonds had been cleaved by the substituents to form a random coil structure. In addition, APG and CMAPG solutions exhibited scavenging ability against hydroxyl, organic, and sulfate radicals. It was also found that the carboxymethylation of APG drastically enhanced the organic radical scavenging ability. On the basis of the relationship between the DS and radical scavenging ability of the CMAPG samples, we believe hydroxyl and organic radicals were preferably scavenged by the donation of hydrogen atoms from the glucose rings and the methylene moieties of the carboxymethyl groups, respectively. Considering the obtained results, CMAPG and APG are expected to have applications in pharmaceuticals, functional foods, and cosmetics as antioxidant polysaccharides.

Scalability of advanced oxidation processes (AOPs) in industrial applications: A review

Disinfection and decontamination of water by application of oxidisers is an essential treatment step across numerous industrial sectors including potable supply and industry waste management, however, could be greatly enhanced if operated as advanced oxidation processes (AOPs). AOPs destroy contaminants including pathogens by uniquely harnessing radical chemistry. Despite AOPs offer great practical opportunities, no reviews to date have highlighted the critical AOP virtues that facilitate AOPs' scale up under growing industrial demand. Hence, this review analyses the critical AOP parameters such as oxidant conversion efficiency, batch mode vs continuous-flow systems, location of radical production, radical delivery by advanced micro-/mesoporous structures and AOP process costs to assist the translation of progressing developments of AOPs into their large-scale applications. Additionally, the state of the art is analysed for various AOP inducing radical/oxidiser measurement techniques and their half-lives with a view to identify radicals/oxidisers that are suitable for in-situ production. It is concluded that radicals with short half-lives such as hydroxyl (10-4 μsec) and sulfate (30-40 μsec) need to be produced in-situ via continuous-flow reactors for their effective transport and dosing. Meanwhile, radicals/oxidisers with longer half-lives such as ozone (7-10 min), hydrogen peroxide (stable for several hours), and hypochlorous acid (10 min -17 h) need to be applied through batch reactor systems due to their relatively longer stability during transportation and dosing. Complex and costly synthesis as well as cytotoxicity of many micro-/mesoporous structures limit their use in scaling up AOPs, particularly to immobilising and delivering the short-lived hydroxyl and sulfate radicals to their point of applications. Overall, radical delivery using safe and advanced biocompatible micro-/mesoporous structures, radical conversion efficiency using advanced reactor design and portability of AOPs are priority areas of development for scaling up to industry.

Fighting Antibiotic-Resistant Bacterial Infections by Surface Biofunctionalization of 3D-Printed Porous Titanium Implants with Reduced Graphene Oxide and Silver Nanoparticles

Nanoparticles (NPs) have high multifunctional potential to simultaneously enhance implant osseointegration and prevent infections caused by antibiotic-resistant bacteria. Here, we present the first report on using plasma electrolytic oxidation (PEO) to incorporate different combinations of reduced graphene oxide (rGO) and silver (Ag) NPs on additively manufactured geometrically ordered volume-porous titanium implants. The rGO nanosheets were mainly embedded parallel with the PEO surfaces. However, the formation of ‘nano-knife’ structures (particles embedded perpendicularly to the implant surfaces) was also found around the pores of the PEO layers. Enhanced in vitro antibacterial activity against methicillin-resistant Staphylococcus aureus was observed for the rGO+Ag-containing surfaces compared to the PEO surfaces prepared only with AgNPs. This was caused by a significant improvement in the generation of reactive oxygen species, higher levels of Ag+ release, and the presence of rGO ‘nano-knife’ structures. In addition, the implants developed in this study stimulated the metabolic activity and osteogenic differentiation of MC3T3-E1 preosteoblast cells compared to the PEO surfaces without nanoparticles. Therefore, the PEO titanium surfaces incorporating controlled levels of rGO+Ag nanoparticles have high clinical potential as multifunctional surfaces for 3D-printed orthopaedic implants.

Radical Intermediates in the Degradation of Hop Acids

Radical formation in isohumulones was investigated under different types of stress, including temperature, transition metal ions, and hydrogen peroxide. Including dihydroisohumulones and tetrahydroisohumulones, as relevant analogues, allowed us to evaluate critical functionalities in radical formation. Using spin-trapping methodology with 5,5-dimethyl-1-pyrroline N-oxide and N-tert-butyl-α-phenylnitrone as relevant traps, followed by simulation of corresponding spin adducts, identification of incipient radicals was attempted. The isohexenoyl side chain in isohumulones, but not present in dihydro- and tetrahydroisohumulones, was most sensitive to radical formation. Kinetic profiles further demonstrated that radical formation in this moiety was accelerated in the presence of ferrous ions. Reactivity of parent six-membered-ring humulones in radical formation was different, as scavenging of free radical species was more important. Lupulones, despite similarity with humulones, showed a different behavior with an obvious radical decay pathway during ageing, mainly ascribed to radical formation on the ring structure. Quantification of final spin adducts allowed us to determine absolute importance of the different degradation pathways. Eventually, mechanisms are presented explaining why isohumulones are more prone to radical processes in (aut)oxidation and thermal decay than close relatives such as dihydroisohumulones.

Does Tert-Butyl Alcohol Really Terminate the Oxidative Activity of •OH in Inorganic Redox Chemistry?

The hydroxyl radical, ^•OH, is one of the most reactive free radicals and plays significant roles in the oxidative degradation of organic pollutants and the electron transfer of inorganic ions in natural and engineered environmental processes. To quantitatively determine the contribution of ^•OH to oxidative reactions, a specific scavenger, such as tert-butyl alcohol (TBA), is usually added to eliminate ^•OH effects. Although TBA is commonly assumed to transform ^•OH into oxidatively inert products, this study demonstrates that utilizing TBA as an ^•OH scavenger generates the secondary peroxyl radical (ROO^•), influencing the oxidation of transition metals, such as Mn. Although ROO^• is less reactive than ^•OH, it has an extended half-life and a longer diffusion distance that enables more redox reactions, such as the oxidation of Mn²⁺(aq) to Mn^IV oxide solids. In addition to promoting Mn²⁺(aq) oxidation kinetics, TBA can also affect the crystalline phases, oxidation states, and morphologies of Mn oxide solids. Thus, the oxidative roles of ^•OH in aqueous redox reactions cannot be examined simply by adding TBA: the effects of secondary ROO^• must also be considered. This study urges a closer look at the potential formation of secondary radicals during scavenged oxidative reactions in environmental systems.

FeSe2 nanosheets as a bifunctional platform for synergistic tumor therapy reinforced by NIR-II light

Multi-functionality has been a constant pursuit in the development of next-generation drug carriers, as it will bring the potential for combination therapy by integrating diverse therapeutic modes. In this work, FeSe₂ nanosheets (NSs) have been prepared as a bifunctional platform to investigate their use in synergistic cancer therapy. Bifunctional FeSe₂ NSs exhibit exceptional Fenton-like activity that generates cytotoxic hydroxyl radical (˙OH) and strong broad photothermal performance including the second-infrared (NIR-II) spectral range, wherein the ˙OH production can be enhanced by NIR-II light irradiation. Furthermore, doxorubicin (DOX) was conjugated onto NSs via a pH-responsive hydrazone bond to achieve preferential drug release in an acidic microenvironment. Upon intratumoral administration, these bifunctional drug-carrying FeSe₂ NSs showed an NIR-II irradiation-reinforced strong tumor suppression effect, and no obvious toxicity to normal tissues was observed. This study provides a new paradigm for the design of advanced drug carriers relying on their inherent physicochemical properties.

Highly efficient visible-light photocatalytic ethane oxidation into ethyl hydroperoxide as a radical reservoir

Photocatalytic ethane conversion into value-added chemicals is a great challenge especially under visible light irradiation. The production of ethyl hydroperoxide (CH₃CH₂OOH), which is a promising radical reservoir for regulating the oxidative stress in cells, is even more challenging due to its facile decomposition. Here, we demonstrated a design of a highly efficient visible-light-responsive photocatalyst, Au/WO₃, for ethane oxidation into CH₃CH₂OOH, achieving an impressive yield of 1887 μmol g_cat ^-1 in two hours under visible light irradiation at room temperature for the first time. Furthermore, thermal energy was introduced into the photocatalytic system to increase the driving force for ethane oxidation, enhancing CH₃CH₂OOH production by six times to 11 233 μmol g_cat ^-1 at 100 °C and achieving a significant apparent quantum efficiency of 17.9% at 450 nm. In addition, trapping active species and isotope-labeling reactants revealed the reaction pathway. These findings pave the way for scalable ethane conversion into CH₃CH₂OOH as a potential anticancer drug.

Nanozyme-Initiated In Situ Cascade Reactions for Self-Amplified Biocatalytic Immunotherapy

Biocatalytic nanomaterials have been verified to modulate the immunosuppressive state of an extensive range of solid tumors and directly induce antitumor immune response, which effectively combats the holdbacks in cancer immunotherapy. Herein, biomimetic cascade enzyme-initiated toxic-radical-generating devices (GHZD NCs) are fabricated by enveloping glucose oxidase (GOx), artificial nanozyme hemin, and sesquiterpene lactone endoperoxide derived dihydroartemisinin (DHA) into zeolitic imidazolate framework (ZIF-8) for enhanced biocatalytic immunotherapy. The GHZD NCs exhibit amplified multienzyme-mimic (glucose oxidase, peroxidase, and glutathione peroxidase) cascade reactions in artificial nanoscale proximity. Concurrently, a glutathione (GSH)-stimulated labile iron-current amplifier boosts C-centered free radicals, which endows the GHZD NCs with tumor-specific and self-circulating generation ability of vicious C-centered free radicals. Irreversible free radicals (·C and ·OH) and sustainable H2 O2 from sequential catalytic processes logically and selectively elevate the oxidative stress in the tumor, which further triggers an efficient immunogenic cell death (ICD) progress. In addition, the in situ nanozyme-based immunotherapy employed for tumor suppression successfully elicits the long-lasting immunological memory effect, which hinders the growth of distant tumors and lung metastasis.

Liquid Metal Nanoparticles as Initiators for Radical Polymerization of Vinyl Monomers

Sonication of gallium or gallium-based liquid metals in an aqueous solution of vinyl monomers leads to rapid free radical polymerization (FRP), without the need for conventional molecular initiators. Under ambient conditions, a passivating native oxide separates these metals from solution and renders the metal effectively inert. However, sonication generates liquid metal nanoparticles (LMNPs) of ∼100 nm diameter and thereby increases the surface area of the metal. The exposed metal initiates polymerization, which proceeds via a FRP mechanism and yields high molecular weight polymers that can form physical gels. Spin trapping EPR reveals the generation of free radicals. Time-of-flight secondary ion mass spectrometry measurements confirm direct polymer bonding to gallium, verifying the formation of surface-anchored polymer grafts. The grafted polymers can modify the interfacial properties, that is, the preference of the metal particles to disperse in aqueous versus organic phases. The polymer can also be degrafted and isolated from the particles using strong acid or base. The concept of physically disrupting passivated metal surfaces offers new routes for surface-initiated polymerization and has implications for surface modification, reduction reactions, and fabrication of mechanically responsive materials.

Contribution of 2-Propenesulfenic Acid to the Antioxidant Activity of Allicin

We re-examined the antioxidative mechanism of allicin as a radical scavenger on the basis of the reactivity toward 2, 2-diphenyl-1-picrylhydrazyl (DPPH) and peroxyl radicals. Initially, it was found that allicin decomposed more rapidly in n-hexane and chlorobenzene than in acetonitrile, ethanol, and ethanol/water solutions and decomposed into ajoene and vinyldithiins in these solvents. Furthermore, the decomposition of allicin and the following formations of ajoene and vinyldithiins from allicin were accelerated by the reaction of allicin with DPPH and peroxyl radicals. These results show that 2-propenesulfenic acid, which arises by Cope elimination from allicin, is proposed to contribute to scavenge these radicals because ajoene and vinyldithiins were produced from allicin through the use of 2-propenesulfenic acid. Next, allicin was more effective at inhibiting the linoleic acid oxidation at 50 °C than at 30 °C and in cyclohexane than in acetonitrile. These results indicate that allicin decomposed rapidly at high temperatures in a hydrogen-bond-acceptor solution to 2-propenesulfenic acid. In addition, 2-propene-1-sulfinothionic acid S-methyl ester, which does not produce sulfenic acid through Cope elimination, has no activity against the radicals. On the other hand, methanesulfinothionic acid S-2-propenyl ester, which produces methanesulfenic acid through Cope elimination, has the same or increased activity as its allicin against the radicals. Based on these results, the Cope elimination product, sulfenic acid, from thiosulfinates with an α-sulfenyl proton was found to make a larger contribution to the radical scavenger than that of allicin itself.

PRACTICAL APPLICATION

We examined the antioxidant activity of allicin on the oxidation of cumene and linoleic acid in homogeneous solutions. It is obvious from these results that 2-propenesulfenic acid was found to make a larger contribution to the radical scavenger than that of allicin itself.

Intermediate Tyrosyl Radical and Amyloid Structure in Peroxide-Activated Cytoglobin

We characterized the peroxidase mechanism of recombinant rat brain cytoglobin (Cygb) challenged by hydrogen peroxide, tert-butylhydroperoxide and by cumene hydroperoxide. The peroxidase mechanism of Cygb is similar to that of myoglobin. Cygb challenged by hydrogen peroxide is converted to a Fe⁴⁺ oxoferryl π cation, which is converted to Fe⁴⁺ oxoferryl and tyrosyl radical detected by direct continuous wave-electron paramagnetic resonance and by 3,5-dibromo-4-nitrosobenzene sulfonate spin trapping. When organic peroxides are used as substrates at initial reaction times, and given an excess of peroxide present, the EPR signals of the corresponding peroxyl radicals precede those of the direct tyrosyl radical. This result is consistent with the use of peroxide as a reducing agent for the recycling of Cygb high-valence species. Furthermore, we found that the Cygb oxidation by peroxides leads to the formation of amyloid fibrils. This result suggests that Cygb possibly participates in the development of degenerative diseases; our findings also support the possible biological role of Cygb related to peroxidase activity.

A protocol for quantifying lipid peroxidation in cellular systems by F2-isoprostane analysis

Cellular systems are essential model systems to study reactive oxygen species and oxidative damage but there are widely accepted technical difficulties with available methods for quantifying endogenous oxidative damage in these systems. Here we present a stable isotope dilution UPLC-MS/MS protocol for measuring F2-isoprostanes as accurate markers for endogenous oxidative damage in cellular systems. F2-isoprostanes are chemically stable prostaglandin-like lipid peroxidation products of arachidonic acid, the predominant polyunsaturated fatty acid in mammalian cells. This approach is rapid and highly sensitive, allowing for the absolute quantification of endogenous lipid peroxidation in as little as ten thousand cells as well as damage originating from multiple ROS sources. Furthermore, differences in the endogenous cellular redox state induced by transcriptional regulation of ROS scavenging enzymes were detected by following this protocol. Finally we showed that the F2-isoprostane 5-iPF_2α-VI is a metabolically stable end product, which is excreted from cells. Overall, this protocol enables accurate, specific and sensitive quantification of endogenous lipid peroxidation in cellular systems.

Reaction of ferric Caldariomyces fumago chloroperoxidase with meta-chloroperoxybenzoic acid: sequential formation of compound I, compound II and regeneration of the ferric state using one reactant

The mechanism of the reaction between ferric Caldariomyces fumago chloroperoxidase (CCPO) and meta-chloroperoxybenzoic acid (mCPBA) has been examined. It has previously been established that an Fe(IV) -oxo porphyrin radical species known as Compound I (Cpd I) is formed by two-electron oxidation of the native ferric enzyme by a variety of oxidants including organic peracids and hydroperoxides. Cpd I can return to the ferric state either by direct oxygen insertion into an organic substrate (e.g. a P450 oxygenase-like reaction), or by two consecutive one-electron additions, the first resulting in an intermediate Fe(IV) -oxo species known as Compound II (Cpd II). There has been much debate over the role of Cpd II and the details of its structure. In the present study, both CCPO Fe(IV) -oxo intermediates are formed, but unlike most CCPO reactions, Cpd I and Cpd II are formed using the same reactant, mCPBA. Thus, the peracid is used as an oxo donor to produce Cpd I and then as a reductant to reduce Cpd I to Cpd II, and finally, Cpd II to the ferric state. The observation of saturation kinetics with respect to mCPBA concentration for each step is consistent with the formation of CCPO-mCPBA complexes in each phase of the reaction. The original reaction mechanism for ferric CCPO with mCPBA was hypothesized to involve a scrambling mechanism with a unique Fe -OOO-C(O)R intermediate formed with no observed Cpd II intermediate. The data reported herein clearly demonstrate the formation of Cpd II in returning the oxidized enzyme back to its native ferric state.

An advanced Electron Spin Resonance (ESR) spin-trapping and LC/(ESR)/MS technique for the study of lipid peroxidation

There are two types of nutritionally important polyunsaturated fatty acids (PUFAs), namely ω-6s and ω-3s. PUFAs and their metabolites generated from lipid peroxidation via cyclooxygenase (COX) and lipoxygenase (LOX) are believed to be involved in a variety of physiological and pathological processes in the human body. Both COX- and LOX-catalyzed PUFA peroxidation are complex events that generate a series of radicals, which may then bind proteins, target DNA/RNA, and lead to a number of biological changes. However, due to the lack of an appropriate method, it was not possible until recently to identify the short-lived PUFA-derived radicals in COX-/LOX-catalyzed peroxidation. Failure to characterize free radicals during peroxidation has greatly restricted our knowledge about COX/LOX biology in human health. Here we review the development and refinement of combined ESR spin trapping and LC/ESR/MS to characterize PUFA-derived radicals formed from in vitro (cell-free) peroxidation. We also present the most recent approach for studying peroxidation in cells which allows us to directly assess the potential bioactivity of PUFA-derived free radicals. This advanced technique has resulted in a major breakthrough in radical structural characterization, as well as assessment of free radical-associated cell growth response, thereby greatly improving our knowledge of PUFAs, COX-/LOX-catalyzed lipid peroxidation, and their related biological consequences.

Alkene cleavage catalysed by heme and nonheme enzymes: reaction mechanisms and biocatalytic applications

The oxidative cleavage of alkenes is classically performed by chemical methods, although they display several drawbacks. Ozonolysis requires harsh conditions (−78°C, for a safe process) and reducing reagents in a molar amount, whereas the use of poisonous heavy metals such as Cr, Os, or Ru as catalysts is additionally plagued by low yield and selectivity. Conversely, heme and nonheme enzymes can catalyse the oxidative alkene cleavage at ambient temperature and atmospheric pressure in an aqueous buffer, showing excellent chemo- and regioselectivities in certain cases. This paper focuses on the alkene cleavage catalysed by iron cofactor-dependent enzymes encompassing the reaction mechanisms (in case where it is known) and the application of these enzymes in biocatalysis.

Identification of mitochondrial electron transport chain-mediated NADH radical formation by EPR spin-trapping techniques

The mitochondrial electron transport chain (ETC) is a major source of free radical production. However, due to the highly reactive nature of radical species and their short lifetimes, accurate detection and identification of these molecules in biological systems is challenging. The aim of this investigation was to determine the free radical species produced from the mitochondrial ETC by utilizing EPR spin-trapping techniques and the recently commercialized spin-trap, 5-(2,2-dimethyl-1,3-propoxycyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO). We demonstrate that this spin-trap has the preferential quality of having minimal mitochondrial toxicity at concentrations required for radical detection. In rat heart mitochondria and submitochondrial particles supplied with NADH, the major species detected under physiological pH was a carbon-centered radical adduct, indicated by markedly large hyperfine coupling constant with hydrogen (a(H) > 2.0 mT). In the presence of the ETC inhibitors, the carbon-centered radical formation was increased and exhibited NADH concentration dependency. The same carbon-centered radical could also be produced with the NAD biosynthesis precursor, nicotinamide mononucleotide, in the presence of a catalytic amount of NADH. The results support the conclusion that the observed species is a complex I derived NADH radical. The formation of the NADH radical could be blocked by hydroxyl radical scavengers but not SOD. In vitro experiments confirmed that an NADH-radical is readily formed by hydroxyl radical but not superoxide anion, further implicating hydroxyl radical as an upstream mediator of NADH radical production. These findings demonstrate the identification of a novel mitochondrial radical species with potential physiological significance and highlight the diverse mechanisms and sites of production within the ETC.

Synthesis of allylic hydroperoxides and EPR spin-trapping studies on the formation of radicals in iron systems as potential initiators of the sensitizing pathway

Many terpenes used as fragrance compounds autoxidize when exposed to air, forming allylic hydroperoxides that have the potential to be skin contact allergens. To trigger the immunotoxicity process that characterizes contact allergy, these hydroperoxides are supposed to bind covalently to proteins in the skin via radical pathways. We investigated the formation of reactive radical intermediates from 7-hydroperoxy-3,7-dimethylocta-1,5-dien-3-ol and 2-hydroperoxylimonene, responsible for the sensitizing potential acquired by autoxidized linalool and limonene. Both compounds were synthesized through new short and reproducible synthetic pathways. The hydroperoxide decomposition catalyzed by Fe(II)/Fe(III) redox systems, playing a key role in degradating peroxides in vivo, was examined by spin-trapping-EPR spectroscopy. Alkoxyl and carbon-centered free radicals derived from the hydroperoxides were successfully trapped by the spin-trap 5,5-dimethyl-1-pyrroline N-oxide, whereas peroxyl radicals were characterized by spin-trapping studies with 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide. Using liquid chromatography combined with mass spectrometry, we demonstrated the formation of adducts, via radical mechanisms induced by Fe(II)/Fe(III), between the hydroperoxides and N-acetylhistidine methyl ester, a model amino acid that is prone to radical reactions. Free radicals derived from these hydroperoxides can thus induce amino acid chemical modifications via radical mechanisms. The study of these mechanisms will help to understand the sensitizing potential of hydroperoxides.

Spin trapping investigation of peroxide- and isoniazid-induced radicals in Mycobacterium tuberculosis catalase-peroxidase

A new approach, the immuno-spin trapping assay, used a novel rabbit polyclonal anti-DMPO (5,5-dimethyl-1-pyrroline N-oxide) antiserum to detect protein radical-derived DMPO nitrone adducts in the hemoprotein Mycobacterium tuberculosis catalase-peroxidase (KatG). This work demonstrates that the formation of protein nitrone adducts is dependent on the concentrations of tert-BuOOH and DMPO as shown by Western blotting and an enzyme-linked immunosorbent assay (ELISA). We have also detected protein-protein cross-links formed during turnover of Mtb KatG driven by tert-butyl peroxide ( tert-BuOOH) or enzymatic generation of hydrogen peroxide. DMPO inhibits this dimerization due to its ability to trap the amino acid radicals responsible for the cross-linkage. Chemical modifications by tyrosine and tryptophan blockage suggest that tyrosine residues are one site of formation of the dimers. The presence of the tuberculosis drug isoniazid (INH) also prevented cross-linking as a result of its competition for the protein radical. Protein-DMPO nitrone adducts were also generated by a continuous flux of hydrogen peroxide. These findings demonstrated that the protein-based radicals were formed not only during Mtb KatG turnover with alkyl peroxides but also in the presence of hydrogen peroxide. Furthermore, the formation of protein-DMPO nitrone adducts was accelerated in the presence of isoniazid.

Free radical trapping properties of several ethyl-substituted derivatives of 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO)

The spin trapping behavior of several ethyl-substituted EMPO derivatives, cis- and trans-5-ethoxycarbonyl-3-ethyl-5-methyl-pyrroline N-oxide (3,5-EEMPO), 5-ethoxycarbonyl-4-ethyl-5-methyl-pyrroline N-oxide (4,5-EEMPO), cis- and trans-5-ethoxycarbonyl-5-ethyl-3-methyl-pyrroline N-oxide (5,3-EEMPO), and 5-ethoxycarbonyl-5-ethyl-4-methyl-pyrroline N-oxide (5,4-EEMPO), toward a series of different oxygen- and carbon-centered radicals, is described. Considerably different stabilities of the superoxide adducts (ranging from about 12 to 55 min) as well as the formation of other radical adducts were observed.

Antioxidant activity of thiosulfinates derived from garlic

Garlic extract significantly inhibited the oxidation of methyl linoleate in homogeneous acetonitrile solution, whereas the antioxidant effect of allicin-free garlic extract, prepared by removing allicin by prepared by removing allicin by preparative HPLC, was much lower than that of the garlic extract. These results suggest that the antioxidant properties are mostly attributed to the presence of allicin in the garlic extract. Allicin a major component of the thiosulfinates in garlic extract, was found to be effective for inhibiting methyl linoleate oxidation, but its efficiency was less than that of alpha-tocopherol. Next, the reactivity of allicin toward the peroxyl radical, which is a chain-propagating species, was investigated by direct ESR detection. The addition allicin to 2,2'-azobis(2,4-dimethylvaleronitrile)-peroxyl radical solution caused the signal intensity of the peroxyl radical to dose-dependently decrease, indicating that allicin is capable of scavenging the the peroxyl radical and acting as an antioxidant. Finally, we studied the structure-anioxidant activity relationship for thiosulfinates and suggested that the combination of the allyl group (-CH2CH=CH2) and the -S(O)S- group is necessary for the antioxidant action of thiosulfinates in the garlic extract. In addition, one of the two possible combinations, -S(O)S-CH2CH=CH2, was found to make a much larger contribution to the antioxidant activity of the thiosulfinates than the other, CH2=CH-CH2-S(O)S-.

Mild Manganese(III) Acetate Catalyzed Allylic Oxidation: Application to Simple and Complex Alkenes

Manganese(III) acetate catalyzed allylic oxidation of alkenes to the corresponding enones was investigated, showing excellent regioselectivity and chemoselectivity (functional group compatibility). Delta(5)-Steroids were transformed into bioactive Delta(5)-en-7-ones under a nitrogen atmosphere, whereas simple alkenes were converted into the corresponding enones under an oxygen atmosphere in good yields.

Spin trapping of C- and O-centered radicals with methyl-, ethyl-, pentyl-, and phenyl-substituted EMPO derivatives

In order to develop spin traps with an optimal ratio between hydrophilic and lipophilic properties, low toxicity, and high stability of spin adducts (especially with superoxide radicals), several EMPO-derived spin traps have recently been synthesized forming more stable superoxide adducts (t(1/2) > 20 min) than DMPO or DEPMPO. In this study, ESR-, 1H-, and 13C-NMR data of several phenyl- or n-pentyl-substituted EMPO derivatives are presented with full signal assignment. Methyl groups at position 3 or 4 stabilized the superoxide adducts considerably. Spin adducts from other oxygen- and carbon-centered radicals (e.g., derived from methanol or linoleic acid hydroperoxide) are also described.

Very stable superoxide radical adducts of 5-ethoxycarbonyl- 3,5-dimethyl-pyrroline N-oxide (3,5-EDPO) and its derivatives

Oxygen radicals are involved in the onset of many diseases. Adequate spin traps are required for identification and localisation of free radical formation in biological systems. Superoxide spin adducts with half-lives up to 20 min at physiological pH have recently been reported to be formed from derivatives of the spin trap 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO). This is a major improvement over DMPO (t(1/2) ca. 45 s), and even DEPMPO (t(1/2) ca. 14 min). In this study, an additional methyl group was introduced into position 3 or 4 of the pyrroline ring which greatly increases the stability of the respective superoxide spin adducts. In addition, the ethoxy group of EMPO was exchanged by either a propoxy- or an iso-propoxy group in order to test the influence of increasing lipophilic properties of the investigated spin traps. The structure of all compounds was confirmed by (1)H and (13)C-NMR with full signal assignment. In comparison with EMPO (t(1/2) ca. 8 min) or DEPMPO (t(1/2) ca. 14 min), the superoxide adducts of all novel spin traps were considerably higher (t(1/2) ca. 12-55 min). In addition, various other spin adducts obtained from oxygen-centered as well as carbon-centered radicals (e.g. derived from methanol or linoleic acid hydroperoxide) were also detected.

Spin adduct formation from lipophilic EMPO-derived spin traps with various oxygen- and carbon-centered radicals

Free radicals are involved in the onset of many diseases, therefore the availability of adequate spin traps is crucial to the identification and localization of free radical formation in biological systems. In recent studies several hydrophilic compounds of 2-ethoxycarbonyl-2-methyl-pyrroline-N-oxide (EMPO) have been found to form rather stable superoxide spin adducts with half-lives up to twenty minutes at physiological pH. This is a major improvement over DMPO (t1/2=ca. 45 s), and even over DEPMPO (t1/2=ca. 14 min), the best commercially available spin trap for the unambiguous detection of superoxide radicals. In order to allow the detection of superoxide and also other radicals in lipid environment a series of more lipophilic derivatives of EMPO was synthesized and their structure unambiguously characterized by 1H and 13C NMR spectroscopy. In this way, six different compounds with a n-butyl group in position 5 and either an ethoxy- (EBPO), propoxy- (PBPO), iso-propoxy- (iPBPO), butoxy- (BBPO), sec-butoxy- (sBBPO) or tert-butoxycarbonyl group (tBBPO) in position 5 of the pyrroline ring were obtained and fully analytically characterized (NMR, IR). The stability of the superoxide adducts of all investigated spin traps were comparable with EMPO (t1/2=ca. 8 min), except for the two compounds bearing an additional methyl group in position 3 or 4 of the pyrroline ring, 5-butyl-5-ethoxycarbonyl-3-methyl-pyrroline-N-oxide (BEMPO-3) and 5-butyl-5-ethoxycarbonyl-4-methyl-pyrroline-N-oxide (BEMPO-4), of which the superoxide adducts were stable for more than 30 min. Spin adducts of other carbon- and oxygen-centered radicals were also investigated.

Spin adducts of several N-2-(2-alkoxycarbonyl-propyl)-alpha-pyridylnitrone derivatives with superoxide, alkyl and lipid-derived radicals

Several derivatives of N-t-butyl-alpha-phenylnitrone (PBN) such as N-2-(2-ethoxycarbonyl-propyl)-alpha-phenylnitrone (EPPN) have recently been reported to form superoxide spin adducts (t(1/2) ca. 2-7 min at pH 7.0), which are considerably more stable than their respective PBN or DMPO adducts (t(1/2) ca. 10 and 45 s, respectively). In continuation of our studies on structure optimization of EPPN derivatives, a series of 12 novel spin traps with 2-, 3- and 4-pyridinyl substituents was synthesized and fully characterized by 1H NMR, 13C NMR and IR spectroscopy. In addition to the replacement of the phenyl ring by a 2-, 3- or 4-pyridinyl substituent, the ethoxy group of the parent compound EPPN was replaced by either a propoxy, iso-propoxy, or cyclopropylmethoxy moiety. Superoxide adducts of all PPyN derivatives were considerably more stable than those of the respective EPPN derivatives with half-lives ranging from about 6 to 11 min. In addition, alkoxyl radical adducts were also considerably more stable than those of the EPPN series. Hydroxyl radical adducts were not detected, on the other hand, very stable spin adducts were formed from a series of carbon centered radicals, e.g. from the methyl or hydroxymethyl radical. The novel spin traps are offering an alternative to PBN or POBN, especially where the higher stability of oxygen-centered radical adducts is of major importance. All of them can easily be synthesized from commercially available compounds in two or three steps.

Tyrosine radical formation in the reaction of wild type and mutant cytochrome P450cam with peroxy acids: a multifrequency EPR study of intermediates on the millisecond time scale

We report a multifrequency (9.6-, 94-, 190-, and 285-GHz) EPR study of a freeze-quenched intermediate obtained from reaction of substrate-free cytochrome P450cam (CYP101) and its Y96F and Y96F/Y75F mutants with peroxy acids. It is generally assumed that in such a shunt reaction an intermediate [Fe(IV)=O, porphyrin-pi-cation radical] is formed, which should be identical to the species in the natural reaction cycle. However, for the wild type as well as for the mutant proteins, a porphyrin-pi-cation radical is not detectable within 8 ms. Instead, EPR signals corresponding to tyrosine radicals are obtained for the wild type and the Y96F mutant. Replacement of both Tyr-96 and Tyr-75 by phenylalanine leads to the disappearance of the tyrosine EPR signals. EPR studies at 285 GHz on freeze-quenched wild type and Y96F samples reveal g tensor components for the radical (stretched g(x) values from 2.0078 to 2.0064, g(y) = 2.0043, and g(z) = 2.0022), which are fingerprints for tyrosine radicals in a heterogeneous polar environment. The measurements at 94 GHz using a fundamental mode microwave resonator setup confirm the 285-GHz study. From the simulation of the hyperfine structure in the 94-GHz EPR spectra the signals have been assigned to Tyr-96 in the wild type and to Tyr-75 in the Y96F mutant. We suggest that a transiently formed Fe(IV)=O porphyrin-pi-cation radical intermediate in P450cam is reduced by intramolecular electron transfer from these tyrosines within 8 ms.

Spin trapping of superoxide, alkyl- and lipid-derived radicals with derivatives of the spin trap EPPN

The N-t-butyl-alpha-phenylnitrone derivative N-2-(2-ethoxycarbonyl-propyl)-alpha-phenylnitrone (EPPN) has recently been reported to form a superoxide spin adduct (t(1/2)=5.25 min at pH 7.0), which is considerably more stable than the respective N-t-butyl-alpha-phenylnitrone or 5,5-dimethylpyrroline N-oxide adducts (t(1/2) approximately 10 and 45s, respectively). In continuation of our previous studies on structure optimization of 5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide derivatives, a series of six different EPPN derivatives was synthesized and characterized by 1H NMR, 13C NMR and IR spectroscopy. The ethoxy group of EPPN was replaced by a propoxy, iso-propoxy, n-butoxy, sec-butoxy, and tert-butoxy moiety, as well as the phenyl by a pyridyl ring. Electron spin resonance spectra and stabilities of the superoxide adducts of the propoxy derivatives were found to be similar to those of the respective EPPN adduct, whereas the electron spin resonance spectra of the superoxide adducts of N-2-(2-ethoxycarbonyl-propyl)-alpha-(4-pyridyl) nitrone and the butoxy derivatives were accompanied by decomposition products. In contrast to the 5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide series, no significant improvement of the superoxide adduct stability could be obtained when the ethoxy group was replaced by other substituents. Carbon centered radical adducts derived from methanol, ethanol, formic acid and linoleic acid hydroperoxide were more stable than those of 5,5-dimethylpyrroline N-oxide, whereas among the alkoxyl radicals only the methoxyl radical adduct could be detected.

Separation and identification of DMPO adducts of oxygen-centered radicals formed from organic hydroperoxides by HPLC-ESR, ESI-MS and MS/MS

Many electron spin resonance (ESR) spectra of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) radical adducts from the reaction of organic hydroperoxides with heme proteins or Fe(2+) were assigned to the adducts of DMPO with peroxyl, alkoxyl, and alkyl radicals. In particular, the controversial assignment of DMPO/peroxyl radical adducts was based on the close similarity of their ESR spectra to that of the DMPO/superoxide radical adduct in conjunction with their insensitivity to superoxide dismutase, which distinguishes the peroxyl adducts from the DMPO/superoxide adduct. Although recent reports assigned the spectra suggested to be DMPO/peroxyl radical adducts to the DMPO/methoxyl adduct based on independent synthesis of the adduct and/or (17)O-labeling, (17)O-labeling is extremely expensive, and both of these assignments were still based on hyperfine coupling constants, which have not been confirmed by independent techniques. In this study, we have used online high performance liquid chromatography (HPLC or LC)/ESR, electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) to separate and directly characterize DMPO oxygen-centered radical adducts formed from the reaction of Fe(2+) with t-butyl or cumene hydroperoxide. In each reaction system, two DMPO oxygen-centered radical adducts were separated and detected by online LC/ESR. The first DMPO radical adduct from both systems showed identical chromatographic retention times (t(R) = 9.6 min) and hyperfine coupling constants (a(N) = 14.51 G, a(H)(beta) = 10.71 G, and a(H)(gamma) = 1.32 G). The ESI-MS and MS/MS spectra demonstrated that this radical was the DMPO/methoxyl radical adduct, not the peroxyl radical adduct as was thought at one time, although its ESR spectrum is nearly identical to that of the DMPO/superoxide radical adduct. Similarly, based on their MS/MS spectra, we verified that the second adducts (a(N) = 14.86 G and a(H)(beta) = 16.06 G in the reaction system containing t-butyl hydroperoxide and a(N) = 14.60 G and a(H)(beta) = 15.61 G in the reaction mixture containing cumene hydroperoxide), previously assigned as DMPO adducts of t-butyloxyl and cumyloxyl radical, were indeed from trapping t-butyloxyl and cumyloxyl radicals, respectively.

Radical production from free and peptide-bound methionine sulfoxide oxidation by peroxynitrite and hydrogen peroxide/iron(II)

Methionine sulfoxide is a post-translational protein modification that has been receiving increasing attention in the literature. Here we used electron paramagnetic resonance spin trapping techniques to show that free and peptide-bound methionine sulfoxide is oxidized by hydrogen peroxide/iron(II)-EDTA and peroxynitrite through the intermediacy of the hydroxyl radical to produce both *CH3 and *CH2CH2CH radicals. The results indicate that methionine sulfoxide residues are important targets of reactive oxygen- and nitrogen-derived species in proteins. Since the produced protein-derived radicals can propagate oxidative damage, the results add a new antioxidant route for the action of the enzyme peptide methionine sulfoxide reductase.

Metallophthalocyanines photosensitize the breakdown of (hydro)peroxides in solution to yield hydroxyl or alkoxyl and peroxyl free radicals via different interaction pathways

Interactions of organic peroxides (R'OOR) and hydroperoxides (R'OOH), including H2O2, with excited triplet and singlet state metallophthalocyanines (MPc, M = Zn, Al) have been studied by T-T absorption decay and fluorescence quenching. The ensuing photochemical processes result in decomposition of (hydro)peroxides as assessed by photo-EPR (electron paramagnetic resonance) and spin trapping. In argon-saturated apolar solutions and low MPc concentrations, alkoxyl free radicals (*OR) were identified as the primary products of (hydro)peroxide breakdown. Similarly, photosensitized decomposition of symmetric disulfides results in the formation of sulfur-centered radicals. In air-free aqueous solutions, ROOH photosensitization always gave rise to a mixture of hydroxyl and peroxyl radical (*OOR) adducts in varying molar ratios. At high MPc concentrations, both in polar and in apolar solutions, the most abundant products of ROOH decomposition were identified as *OOR. This indicates a change in the predominant interaction pathway, most likely mediated by MPc exciplexes and involving H-atom abstraction from ROOH by MPc-cation radicals. The prevalence of MPc singlet vs. triplet state interactions was confirmed by the much higher singlet quenching rate constants (log kq up to 9.5; vs. log kT < or = 4.5). In contrast to the triplet quenching, singlet quenching rates were found to depend on the (hydro)peroxide structure, following closely the trend of varying *OR yields for different substrates. Thermodynamic calculations were performed to correlate experimental results with models for electronic energy and charge transfer processes in agreement with the Marcus theory (Rhem and Weller approximation) and Savéant's model for a concerted dissociative electron transfer mechanism.

Spin adducts of superoxide, alkoxyl, and lipid-derived radicals with EMPO and its derivatives

The compound 5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide (EMPO) is a hydrophilic cyclic nitrone spin trap, which, in contrast to DMPO, forms a relatively stable superoxide adduct (t(1/2)=8.6 min) with an EPR spectrum similar to the respective DMPO adduct. In order to find the optimal degree of lipophilicity of this novel type of spin trap with respect to the detection of radicals formed during lipid peroxidation, the ethoxy group of EMPO was replaced by alkoxy substituents of increasing chain length, leading to the methoxy- (MeMPO), 1-propoxy- (PrMPO), 1-butoxy- (BuMPO), and 1-octyloxy- (OcMPO) derivatives of EMPO. The stability of their superoxide adducts was found to be strongly dependent on the size of the alkoxycarbonyl group. Increasing chain length of the alkoxyl substituent decreased the stability of alkoxyl radical adducts of MeMPO, EMPO, and PrMPO, but increased the stability of OcMPO adducts. The stability of alkoxyl radical adducts of BuMPO, on the other hand, were practically independent of the size of the alkoxyl group. Detection of lipid alkoxyl radicals formed by peroxidizing linoleic acid in a stationary system was therefore only possible with the most lipophilic spin trap, OcMPO. However, with the more hydrophilic spin traps MeMPO, EMPO, PrMPO, and BuMPO optimal EPR signal intensity could be obtained when a slow-flow system was used. Thus, within this series EMPO is the best spin trap for the detection of superoxide; OcMPO, on the other hand, is most suitable for the detection of lipid alkoxyl radicals.

ESR analysis of spin adducts of alkoxyl and lipid-derived radicals with the spin trap Trazon

Detection of oxygen-centered radicals was performed using the spin trap 1,3,3-trimethyl-6-azabicyclo[3.2.1]oct-6-ene-N-oxide (Trazon), a bicyclic nitrone spin trap that is easily synthesized from the corresponding amine via hydrogen peroxide mediated oxidation in the presence of the catalyst, sodium tungstate. Compared to monocyclic spin traps such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO) or 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO), the ESR spectra of Trazon spin adducts provide additional structural information due to long-range hyperfine splitting constants and also due to the fact that different stereoisomers can be distinguished. This is especially helpful for the detection of lipid-derived alkoxyl radicals which can be identified according to their characteristic hyperfine splitting pattern. Due to the relatively high stability of the Trazon spin adducts with lipid alkoxyl radicals, which were formed from peroxidizing linoleic acid, ESR experiments could be performed using a stationary system, whereas a slow-flow system is recommended for DMPO. A series of structurally different alkoxyl radical adducts were synthesized by iron-catalyzed nucleophilic addition of the respective alcohol to the spin trap Trazon and the spectra were analyzed by computer simulation. Both the molecular weight of the alcohol and the position of the alcoholic hydroxyl group were of significant influence on the ESR spectra. Two stereochemically different spin adducts were formed in a ratio typical of the alcohol used, thus allowing structural classification of the alkoxyl radical trapped.

Antioxidant effect of capsaicin on lipid peroxidation in homogeneous solution, micelle dispersions and liposomal membranes

The antioxidant activity of capsaicin (CAP) was measured in the oxidation of methyl linoleate (ML) in homogeneous solution, of ML micelles in aqueous dispersions and also of soybean phosphatidylcholine liposomal membrane, and was compared to that of alpha-tocopherol (alpha-TOH) which is one of the most important antioxidants in vivo. The reactivity of CAP toward galvinoxyl (a model phenoxyl radical) in acetonitrile solution was found to be much smaller than that of alpha-TOH, suggesting that the radical scavenging activity of CAP is much weaker than that of alpha-TOH. In fact, in homogeneous acetonitrile solution where the antioxidant activity is determined primarily by the chemical activity of the antioxidant toward peroxyl radicals, CAP inhibited the oxidation of ML much less efficiently than alpha-TOH and a clear induction period was not observed. The antioxidant activity of CAP was found to be about 60 times smaller than that of alpha-TOH in homogeneous solution. However, in micelle oxidation, the difference in antioxidant activity of the two antioxidants was much smaller than in homogeneous solution. Furthermore, in the membrane, CAP inhibited the oxidation almost as effectively as alpha-TOH. These results suggest that CAP can act as an antioxidant in the biomembrane.

Spin trapping of lipid radicals with DEPMPO-derived spin traps: detection of superoxide, alkyl and alkoxyl radicals in aqueous and lipid phase

The spin trap 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) forms a superoxide adduct with a half-life of almost 15 min. DEPMPO is very hydrophilic and its use for the detection of radicals in the lipid phase (lipid-derived radicals and superoxide generated in the lipid phase) is therefore limited due to its very low concentration in the lipid phase. For the detection of lipid-derived radicals, three derivatives of DEPMPO with increasing degree of lipid solubility have been investigated: 5-(di-n-propoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DPPMPO), 5-(di-n-butoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DBPMPO), and 5-(bis-(2-ethylhexyloxy)phosphoryl)-5-methyl-1-pyrroline N-oxide (DEHPMPO). As compared with the spin trap DMPO, the half-lives of the respective superoxide adducts were clearly higher in aqueous solutions of the spin traps, which facilitates qualitative ESR measurements. The stability of the superoxide spin adducts formed with the various lipophilic spin traps in aqueous buffer were similar to those observed with DEPMPO (half-life: 7-11 min.). In model experiments using Fe(3+)-catalyzed nucleophilic addition of methanol or tert-butanol to the respective spin trap the respective alkoxyl radical adducts were formed in aqueous solution as transient species in the presence of high concentrations of the alcohol. Upon dilution with water the alkoxyl group was substituted by water, giving the respective hydroxyl adduct of the spin trap. Care must therefore be taken when Fenton-type reactions are used for the generation of radicals such as the use of Fe(2+) complexes with phosphate or DTPA or inactivation of iron by addition of "Desferal" (Novarti's Pharma GmbH, Vienna, Austria) after a short incubation time. Addition of Fe(2+) under anaerobic conditions to an aqueous suspension of linoleic acid hydroperoxide and the spin trap resulted in the detection of three different species: a carbon-centered radical adduct, an acyl radical adduct, and the hydroxyl adduct. In the presence of oxygen a different species was observed with DEPMPO, DPPMPO, and DBPMPO, which was only slightly suppressed upon the addition of SOD, possibly the respective spin adduct of either the alkylperoxyl radical or, in analogy to DMPO, a secondary alkoxyl radical.