Presence Of Nitroxyl Radicals Research Articles

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

A kinetic and spectroscopic characterization of the ferryl intermediate (APO-II) from APO, the heme-thiolate peroxygenase from Agrocybe aegerita, is described. APO-II was generated by reaction of the ferric enzyme with metachloroperoxybenzoic acid in the presence of nitroxyl radicals and detected with the use of rapid-mixing stopped-flow UV-visible (UV-vis) spectroscopy. The nitroxyl radicals served as selective reductants of APO-I, reacting only slowly with APO-II. APO-II displayed a split Soret UV-vis spectrum (370 nm and 428 nm) characteristic of thiolate ligation. Rapid-mixing, pH-jump spectrophotometry revealed a basic pKa of 10.0 for the Fe(IV)-O-H of APO-II, indicating that APO-II is protonated under typical turnover conditions. Kinetic characterization showed that APO-II is unusually reactive toward a panel of benzylic C-H and phenolic substrates, with second-order rate constants for C-H and O-H bond scission in the range of 10-10(7) M(-1)⋅s(-1). Our results demonstrate the important role of the axial cysteine ligand in increasing the proton affinity of the ferryl oxygen of APO intermediates, thus providing additional driving force for C-H and O-H bond scission.

Diverse copper(II) complexes with simple nitrogen ligands: Structural characterization and applications in aerobic alcohol oxidations in water

Four copper(II) complexes based on simple mono- or bi-dentate nitrogen ligands 2–5 were synthesized and structurally characterized by X-ray crystallography. Structural analysis revealed that diverse solid state structures of these Cu(II) complexes were formed, depending on the ligands used. The new copper(II) assembles include dinuclear tetra-ligand complex (6), mononuclear bis-ligand complex (7), one-dimensional (1-D) polymeric chain (8) and two-dimensional (2-D) hydrogen-bonded network composed of mononuclear bis-ligand complexes (9). In continuation with our previous findings of an efficient Cu/DMAP catalysis system for aerobic alcohol oxidation in water (DMAP=4-dimethylaminopyridine), we tested the catalytic activity of newly synthesized copper(II) complexes 6–9 under similar conditions. Both the isolated complexes and in-situ combinations of ligands with copper(I) or copper(II) salts were examined as catalysts toward aerobic oxidations of both primary and secondary alcohols in water in the presence of nitroxyl radicals (TEMPO or ABNO) and the results confirmed that complex 6 showed high catalytic activity, similar to the previously reported Cu/DMAP catalyst, while other complexes were less efficient catalysts. The remarkable differences on catalytic activity were tentatively attributed to their diverse molecular structures, in comparison with previous results in relevant systems.

Correction: One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate

Correction for ‘One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate’ by Sergiu Coseri et al., RSC Adv., 2015, 5, 85889–85897.

One-shot carboxylation of microcrystalline cellulose in the presence of nitroxyl radicals and sodium periodate

2,3,6-Tricarboxy cellulose has been synthesized in a one-shot reaction, combining two of the most common selective oxidation protocols for cellulose, i.e. nitroxyl mediated reaction and periodate oxidation.

Synthesis of block copolymers from polyvinyl chloride prepared in the presence of nitroxyl radicals of the imidazoline series

Specific features of radical polymerization of vinyl chloride in the presence of nitroxyl radicals of the imidazoline series (2-methyl-2,3-diphenyl-1,4-diazaspiro[4,5]dec-3-ene-1-oxyl, 2,2,5,5-tetramethyl-4-phenyl-2,5-dihydroimidazole-1-oxyl, 2,2,5-trimethyl-4,5-diphenyl-2,5-dihydroimidazole-1-oxyl) were studied. The possibility of preparing vinyl chloride-styrene block copolymers in the presence of these nitroxyl radicals was demonstrated, suggesting the occurrence of the polymerization by the reversible inhibition mechanism. The molecular-mass characteristics and the glass transition points of the synthesized samples were determined.

The Cu(II)-nitroxyl radicals system: the high activity and reactivation of the catalyst in the decomposition of hydroperoxides

Chelate copper(II) compounds were shown to exhibit high catalytic activity in the decomposition of hydroperoxides in the presence of nitroxyl radicals. The participation of nitroxyl radicals as cocatalysts increased the rate of the reaction and the number of catalytic cycles. Nitroxyl radicals provided catalyst reactivation after complete activity loss characteristic of the process in the absence of promoting admixtures. The promoting action of nitroxyl radicals was observed for hydroperoxides and was not for diacylperoxides. The special behavior of the catalytic system with respect to hydroperoxides presupposed the participation of nitroxyl radicals in one of the key stages of proton transfer from the copper(II)…hydroperoxide complex in the overall reaction mechanism. We, however, did not observe direct interaction between nitroxyl radicals and copper(II) ions by the electron transfer mechanism.

Intermolecular Radical Carboaminohydroxylation of Olefins with Aryl Diazonium Salts and TEMPO

Highly reactive aryl radicals can selectively be reacted with a broad variety of activated and nonactivated olefinic substrates in the presence of nitroxyl radicals. Direct recombination of the aryl radical and the nitroxide as well as telomerization of the olefin is successfully suppressed by the reaction conditions.

Feedback activation of ferrous 5-lipoxygenase during leukotriene synthesis by coexisting linoleic acid

Ferrous lipoxygenases seem to be activated through a feedback control mechanism via FA hydroperoxides generated from PUFAs by partially existing ferric lipoxygenases. However, during leukotriene synthesis, feedback activation of ferrous 5-lipoxygenase in the presence of arachidonic acid (AA) was not observed. In the present study, we examined the feedback activation of ferrous 5-lipoxygenase in the 5-lipoxygenase/AA system in the presence of linoleic aicd (LA), which is a predominant component of membrane phospholipids. When potato 5-lipoxygenase was incubated with AA and LA in the presence of nitroxyl radical, 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-N-oxyl (CmDeltaP), one-electron redox cycle reaction between ferric and ferrous 5-lipoxygenase was detected. For each revolution of the cycle, one molecule of PUFA and one molecule of its hydroperoxide were converted into PUFA-allyl radical-CmDeltaP adduct ([PUFA-H].-CmDeltaP) and PUFA-epoxyallyl radical-CmDeltaP adduct ([PUFA-H+O].-CmDeltaP), respectively. The ratios, [AA-H].-CmDeltaP/[LA-H].-CmDeltaP and [AA-H+O].-CmDeltaP/[LA-H+O].-CmDeltaP, were estimated to be 1.7 and 0.13, respectively. These facts indicate that ferrous 5-lipoxygenase is activated through feedback control in the presence of LA, and that resulting ferric 5-lipoxygenase catalyzes the stoichiometric synthesis of leukotrienes from AA. In conclusion, the biosynthesis of leukotrienes is remarkably efficient.

Thermolysis of polyethylene hydroperoxides in the melt 8. Mechanisms and formal kinetics in the presence of stable nitroxyl radicals

The presence of nitroxyl radicals affects mainly trans-vinylene and alcohol group formation. The influence of the nitroxyl radicals on the yield of carbonyl and γ-lactone formation is negligible. The effect on hydroperoxide decomposition is best explained by elimination of the chain reaction of induced hydroperoxide decomposition. This is a consequence of the reactions of nitroxyl radicals with alkyl radicals. These reactions involve geminate alkyl radicals as well as single alkyl radicals resulting from prior destruction of a radical pair by reaction with a nitroxyl radical. This is a major aspect of the kinetics of the reactions. The nitroxyl radicals do not disrupt rapidly enough the geminate radicals formed on hydroperoxide thermolysis to yield typical homogeneous, solution-type kinetics. They terminate chains on reaction with geminate and with single radicals. The increase of the rate of formation as well as of the maximum amount of trans-vinylene groups in the presence of nitroxyl radicals can be easily understood. It is caused by the disproportionation of macroalkyl radicals with nitroxyl radicals. It is more difficult to explain the significant decrease of the rate of formation and maximum amount alcohol groups. The only explanation in agreement with mechanisms and kinetics involves unchanged formation of alcohol groups in the presence of nitroxyl radicals. However, the alcohol groups formed by intramolecular hydrogen abstraction by alkoxy radicals are destroyed rapidly after formation of hydroxylamine ethers in the 4-position.

Thermolysis of polyethylene hydroperoxides in the melt7. Effect of stable nitroxyl radicals

The presence of nitroxyl radicals affects hydroperoxide thermolysis and oxidation product formation. With increasing concentration of a low molecular mass nitroxyl radical the rate of hydroperoxide decomposition at 190 °C decreases at first. Then the rate passes a minimum value and increases again to become at least comparable to that of the control experiment. For any nitroxyl radical concentration the rate of hydroperoxide decomposition remains constant during the entire decomposition. The rate is closer to typical first-order than even in the presence of a phenolic antioxidant. The oxidation products formed in the presence of nitroxyl radicals are mainly alcohols and carbonyl groups, trans-vinylene groups and γ-lactones are formed in smaller amounts. The effect of the nitroxyl radical concentration depends strongly on the oxidation product and the type of rate constant taken into account. The first-order rate constants of alcohol and carbonyl group formation decrease with increasing nitroxyl radical concentration in a way analogous to the rate of hydroperoxide thermolysis. These rates are increasing again for the largest nitroxyl radical concentration in the experiments. The first-order rate of trans-vinylene group formation does not follow the same pattern, it is increasing slightly with the nitroxyl radical concentration. The initial linear rates of product formation show significant differences with the corresponding first-order rates. The initial linear rate of alcohol formation decreases significantly with increasing nitroxyl radical concentration. The corresponding rate of carbonyl formation does not show much variation under the same conditions. However, the initial linear rate of trans-vinylene group formation increases steadily with the nitroxyl radical concentration. The maximum values of the oxidation product concentrations on completion of thermolysis parallel the variation of the initial linear rates with the nitroxyl radical concentration.

Polymerization of styrene in the presence of nitroxyl radicals generated directly in the course of the polymer synthesis (in situ)

The features of radical polymerization of styrene in the presence of nitroxyl radicals generated directly in the course of the polymer synthesis (in situ) by irreversible reaction of stable organic compounds 2-methyl-2-nitrosopropane, C-phenyl-N-tert-butylnitrone, and nitrosodurene with propagating polymer radicals were studied.

Features of Radical Polymerization of Vinyl Chloride in the Presence of Nitroxyl Radicals

Radical polymerization of vinyl chloride in the presence of C-phenyl-N-tert-butylnitrone, 2-meth- yl-2-nitrosopropane, and 1-tert-butyl-3-phenyl-1-oxytriazene as potential sources of free radicals was studied.

Preparation of highly conducting and spin-probed polypyrroles using coupled electrochemical-chemical synthesis in the presence of nitroxyl radicals

A new method for the synthesis of conducting polypyrroles (PPy), based on chemical and combined electrochemical-chemical oxidation of pyrrole monomers in the presence of nitroxyl radical (TEMPOL-2,2,6,6-tetramethyl-4-hydroxy-1-oxy-piperidyl) as oxidation agent (in its oxidation state) and redox mediator respectively is described. The PPy films obtained are extremely porous and the electrical conductivity (σ) of the resulting PPy samples ranges from 1 to 100 S cm−1. PPy prepared in aqueous solutions has a lower conductivity σ∽1 S cm−1 as compared to the PPy prepared in acetonitrile solutions with conductivity of about 100 S cm−1. The PPy films are quite compact and thick films (1 mm) can be pealed off from the electrode surface and pressed as a disc for further studies. Only after partial reduction of polypyrrole film the spin-probed PPy was obtained. The concentration of nitroxyl radicals incorporated in the polymer matrix can be changed using various degrees of polymer reduction. The anisotropic broadening of the observed ESR lines indicates a low mobility of incorporated nitroxyl radicals in the PPy matrix. The electronic interaction between the nitroxyl groups and the paramagnetic centers of the polymer chains, polarons, causes an ESR line broadening of polaron signal both on air (1.1 mT) and in vacuum (0.7 mT) as compared to unmodified PPy (0.2 mT on air, 0.05 mT in vacuum).

Mechanism of Controlled/“Living” Radical Polymerization of Styrene in the Presence of Nitroxyl Radicals. Kinetics and Simulations

The polymerization of styrene in the presence of a stable radical, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), has been simulated using a Predici simulations package. On the basis of the experimental data, a kinetic model for the TEMPO-mediated polymerization of styrene is proposed. In order to simulate the experimental data properly, it was necessary to include thermal self-initiation, transfer, and irreversible decomposition of intermediate alkoxyamines in addition to the reversible cleavage of the TEMPO-polymeric radical adduct. This model, combined with the values of the rate constants of propagation (kp), termination (kt), transfer (ktrm), and alkoxyamines decomposition (kdecomp), was then employed to estimate the kinetic and thermodynamic parameters for the exchange between dormant and active species. The equilibrium constant, K, was estimated to be approximately 10-11 mol·L-1, the deactivation rate constant, kd = 8 × 107 mol-1·L·s-1, and the activation rate constant, ka = 8 × 10-4 s-1, for bulk styrene polymerization at 120 °C.

Synthesis and spectroscopic study of trans complexes of Pd(II) with the 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl radical

The authors synthesize the trans complexes L/sub 2/PdX/sub 2/ and use trans-(phCN)/sub 2/PdX/sub 2/ (X = C1, Br) as the initial compounds. The L/sub 2/-PdC1/sub 2/ (I) and L/sub 2/PdBr/sub 2/ (II) complexes obtained were studied by IR spectroscopy in the 4000/sup 2/-100 cm/sup -1/ region by Raman spectroscopy (4000-50 cm/sup -1/). The results of vibrational spectroscopy and ESR permit the unambiguous conclusion that the complexes I and II are biradicals and have the trans structure of the central coordination not PdN/sub 2/X/sub 2/. It is shown that the presence of nitroxyl radicals in ligands in the trans-arrangement does not lead to an increase in anticarcinogenic activity, which is in accord with information on the low cancerostatic activity of complexes with trans-amino ligands.

Kinetic relationships for the decomposition of cumyl hydroperoxide catalyzed by cobalt acetylacetonate in the presence of nitroxyl radicals

1. Using the scavenger 4-methyl-2-spirocyclohexyl-3,4,2′,3′-tetrahydrofurano-1,2,3,4-tetrahydroquinolin-1-oxyl the formation of peroxy radicals during the decomposition of cumyl hydroperoxide catalyzed by Co(acac)2 has been confirmed and their rate of formation has been measured. 2. The kinetic parametera, which characterizes the activity of radicals in chain propagation and termination reactions during the chain decomposition stage of cumyl hydroperoxide in the presence of Co(acac)2 has been determined.