Spin-trapping studies on the reaction of NO<Subscript>2</Subscript> with <Emphasis Type="Italic">β</Emphasis>-carotene

Absorption spectrophotometric studies indicated that in chloroform -carotene reacted with NO2 with a stoichiometry of 1:2. To understand the reaction mechanism, free radical intermediates produced during their reaction were trapped by spin traps 2-methyl-2-nitroso propane (MNP) and 4-pyridyl-1-oxide-N-1-butyl nitrone (POBN). The resulting stable spin adducts were detected by electron spin resonance (ESR) spectroscopy. NO₂ alone did not interact with spin traps, but introduction of -carotene-generated ESR signals with different nitrogen and hydrogen hyperfine coupling constants (A _N and A _H) characteristsic of the trapped radical. Two possible pathways exist for the reaction of NO₂ with -carotene, viz., electron transfer and addition to the polyene chain. Our ESR results supported an addition mechanism, where formation of carbon-centered [NO₂ · · · -carotene] free radical adduct was observed.     

ESR determination of the rate constant for the addition of CpW(CO)3 to spin traps

Conclusions The reaction of [CpW(CO)₃]₂ with PhCH₂Cl and the decomposition of CpW(CO)₃Me were studied by ESR spectroscopy using 2,4,6-tri-tert-butyinitrobenzene (BNB) and -phenyl-N-tert-butylnitrone (PNB) as spin traps. The rate constants for the addition of CpW(CO)₃ radicals to the spin traps at 20°C were determined by identification of the intermediate radicals in these reactions: k _ad ^BNB =6.7·10⁵-7.0·10⁵ and k _ad ^PNB =5.8·10⁴-6.1·10⁴ liters/mole · sec.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2631–2633, November, 1984.     

Direct ESR detection and spin trapping of radicals generated by reaction of oxygen radicals with sulfonated poly(ether ether ketone) (SPEEK) membranes

The stability of membranes under the strong oxidizing conditions in fuel cells is one of the major challenges in the development of fuel cells based on proton exchange membranes (PEMs). This study is centered on the determination of the susceptibility to degradation of SPEEK membranes exposed to OH radicals, using both direct ESR and spin trapping with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). In order to achieve a complete picture on SPEEK degradation, two types of experiments were performed: 1. UV irradiation at 77 K of SPEEK membranes swollen by aqueous solutions of H₂O₂; 2. UV irradiation of SPEEK membranes swollen by aqueous solutions of H₂O₂ in the presence of DMPO as a spin trap. UV irradiation without oxygen of SPEEK at 77 K in acid or basic form in the presence of H₂O₂/H₂O produced phenoxyl radicals as the predominant radicals detected by direct ESR or spin trapping methods. At pH 4, the oxygen radicals produced phenyl radicals as the predominant species detected by spin trapping methods. The hydroperoxyl radical, as DMPO/OOH adduct, was detected only when the DMPO/OH adduct was absent. The appearance of phenyl and phenoxyl radicals provides the evidence that OH radicals react with the aromatic ring of SPEEK or leading to the scission of its ether bridge.     

EPR/spin trapping study of the spontaneous addition of dioxygen on a dienol

The spontaneous addition of triplet oxygen on dienol 1, yielding endoperoxide 2, was followed by EPR/spin trapping. The use of nitroso and nitrone spin traps allowed the detection of two radical centers, showing that this reaction could likely follow a radical pathway.     

The fidelity of spin trapping with DMPO in biological systems

Unlike direct ESR, spin trap methodology depends on the absolute fidelity of the spin trap reaction. Two alternative reactions of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) leading to radical adduct artifacts have been discovered and investigated: inverted spin trapping and the Forrester-Hepburn nucleophilic mechanism. These two alternate pathways to radical adducts are a combination of one-electron oxidation and nucleophilic addition, in either order. In biological systems, serious artifacts have been reported due to the Forrester-Hepburn mechanism, which is initiated by the addition of a nucleophile to DMPO. It has recently been demonstrated that (bi)sulfite (hydrated sulfur dioxide) can react with DMPO via a nonradical, nucleophilic reaction, and it has been further proposed that DMPO/(?)SO(3)(-) formation in biological systems is an artifact and not the result of spin trapping of sulfur trioxide anion radical ((?)SO(3)(-)). The one-electron oxidation of (bi)sulfite catalyzed by horseradish peroxidase (HRP)/hydrogen peroxide (H(2)O(2)) has been reinvestigated by ESR spin trapping with DMPO and oxygen uptake studies to obtain further evidence for the radical reaction mechanism. In the absence of DMPO, the initial rate of (bi)sulfite-dependent oxygen and H(2)O(2) consumption was determined to be half of the initial rate of DMPO/(?)SO(3)(-) radical adduct formation as determined by ESR, demonstrating that, under our experimental conditions, DMPO exclusively forms the radical adduct by trapping the (?)SO(3)(-).     

Combination of neural networks and DFT calculations for the comprehensive analysis of FDMPO radical adducts from fast isotropic electron spin resonance spectra

The 4-hydroxy-5,5-dimethyl-2-trifluoromethylpyrroline-1-oxide (FDMPO) spin trap is very attractive for spin trapping studies due to its high stability and high reaction rates with various free radicals. However, the identification of FDMPO radical adducts is a challenging task since they have very comparable Electron Spin Resonance (ESR) spectra. Here we propose a new method for the analysis and interpretation of the ESR spectra of FDMPO radical adducts. Thus, overlapping ESR spectra were analyzed using computer simulations. As a result, the N- and F-hyperfine splitting constants were obtained. Furthermore, an artificial neural network (ANN) was adopted to identify radical adducts formed during various processes (e.g., Fenton reaction, cleavage of peracetic acid over MnO(2), etc.). The ANN was effective on both "known" FDMPO radical adducts measured in slightly different solvents and not a priori "known" FDMPO radical adducts. Finally, the N- and F-hyperfine splitting constants of ·OH, ·CH(3), ·CH(2)OH, and CH(3)(C═O)O(·) radical adducts of FDMPO were calculated using density functional theory (DFT) at the B3LYP/6-31G(d,p)//B3LYP/6-31G++//B3LYP/EPR-II level of theory to confirm the experimental data.     

Assessing the accuracy of a QM/MM//MD combined protocol to compute spectromagnetic properties of polyfunctional nitroxides in solution

In the field of spin trapping chemistry, we proposed a promising new theoretical QM/MM//MD combined protocol to assist the development of new nitrone-based spin traps (Houriez et?al. in J Phys Chem B 114:11793–11803, 2010). In the present study, we test its accuracy and its transferability by investigating the spectromagnetic properties in water of DMPO–OH, the nitroxide spin adduct of the 5,5-dimethyl-1-pyrroline-N-oxide nitrone (DMPO) with the OH^· radical. Thanks to our theoretical method, we obtain quantitative estimates of the DMPO–OH hyperfine coupling constants (hcc’s) in very good agreement with experiment. Moreover, our study reveals that the DMPO–OH hcc values are related to the main features of an equilibrium between two major conformations of the nitroxide five-membered ring. Together with our earlier results, the present study clearly establishes the reliability of our theoretical protocol to investigate in condensed phase the behavior of flexible and large nitroxides. Particularly, note that with our method, it is possible to point out clearly fundamental differences in spectromagnetic properties even for two molecules very similar in geometrical structure.     

Effect of copper(II) ions on the stability of spin adducts formed by the reaction of 5,5-dimethylpyrroline-N-oxide with hydroxyl and hydroxylalkyl radicals

The reactions of copper(II) species with spin adducts formed by reaction of OH·, CH₃CHOH or (CH₃)₂COH with DMPO (5,5-dimethylpyrroline-N-oxide) have been investigated. The spin adducts prepared by -irradiation of appropriate solutions were detected using esr. Copper(II) in the form of aqua, phen, or 1-alanine complexes reacted rapidly with the OH· adduct to give products that gave no esr signal, but with the hydroxylalkyl adducts, reacted to give a new 9-line esr signal, identified as due to the H· adduct, (DMPO-H·). Measurement of the yield of Cu(phen) ₂ ⁺ formed by reaction of the copper(II) complex with the spin adduct showed that approximately 25% of the radiation-produced radicals formed esr detectable spin adducts. The implications of these findings for the use of spin trapping to detect radical intermediates in biological systems is discussed.     

High static pressure alters spin trapping rates in solution. Dependence on the structure of nitrone spin traps

Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (DeltaDeltaV(double dagger)) for the two spin trapping reactions were calculated to be -17-(-9) cm(3) mol(-1) for the 4-POBN system.     

Charge Transfer Salts of BO with Paramagnetic Isothiocyanato Complex Anions: (BO)[M(isoq)2(NCS)4]; M=Cr or Fe, isoq=isoquinoline and BO=Bis(ethylenedioxo)tetrathiafulvalene

The preparation, X-ray structures and magnetic properties of two isostructural new charge transfer salts: (BO)[M(isoq)₂(NCS)₄]; M=Cr^III(1), Fe^III(2) and isoq=isoquinoline are reported. Their structure consists of alternate organic and inorganic layers, each layer being formed by mixed columns of BO radical cations and paramagnetic metal complex anions. There are short intermolecular contacts between donor and anion (S2_anion· · ·S4_BO<3.5 Å) and between adjacent BO molecules (O· · · O1<3.2 Å). The two compounds are insulators. ESR measurements show single signal without separating the donor and anion spins. The magnetic measurements obey the Curie-Weiss law and revealed dominant antiferromagnetic interactions between anion spin and donor spin, but long-range magnetic ordering did not occur down to 2 K. This is directly related to structural reasons which were deduced from a comparison of the title compounds with other 1:1 salts containing same anion complexes and different donors.     

ESR study of weak exchange interactions in binuclear copper(ii) complexes with acyldihydrazones of fluorinated β-diketones

A series of binuclear copper(ii) complexes with acyldihydrazones of aliphatic dicarboxylic acids (from malonic to adipic) and fluorinated -diketones (trifluoro- and hexafluoroacetylacetone) of composition Cu₂L·2Py (H₄L is acyldihydrazone) were studied by ESR spectroscopy. The ESR spectra of solutions of complexes with trifluoroacetylacetone acyldihydrazones have an isotropic signal with a seven-line hyperfine structure from two equivalent copper nuclei (g = 2.112, a _Cu = (39—40)·10^–4 cm^–1), which is indicative of weak exchange interactions between the paramagnetic centers due to spin density delocalization through a chain of the -bonds of the polymethylene bridge. On going to hexafluoroacetylacetone derivatives, the coupling is suppressed and the ESR spectra of solutions of such complexes show a signal with a four-line hyperfine structure (g = 2.121—2.131, a _Cu = (55—63)·10^–4 cm^–1) typical of mononuclear copper complexes.     

ESR spectra of copper(ii) complexes with 2-hydroxy-5-methyl- and 5-chloro-2-hydroxyacetophenone acyldihydrazones

ESR spectra of binuclear copper(ii) complexes with 2-hydroxy-5-methyl- and 5-chloro-2-hydroxyacetophenone acyldihydrazones (H₄L) [Cu₂L·2Py], in which the coordination polyhedra are linked by the polymethylene chain with different lengths (from one to five units), were studied. The spectra of the complexes based on acyldihydrazones of malonic, succinic, glutaric, and adipic acids exhibit weak exchange interactions between the paramagnetic sites. These interactions induce seven HFS lines from two equivalent copper nuclei with the constant 40·10^–4 cm^–1 in the ESR spectra of liquid solutions. An increase in the polymethylene chain length to five units prevents the exchange interactions, and the ESR spectrum of the complex based on heptadioic acyldihydrazone contains the signal of four HFS lines with the constant 72·10^–4 cm^–1, which is common for the copper(ii) monomeric compounds.     

Superoxide dismutase versus ferricytochrome C: determining rate constants for the spin trapping of superoxide by cyclic nitrones

Given that spin trapping/electron paramagnetic resonance (EPR) spectroscopy has become the primary technique to identify important biologically generated free radicals, such as superoxide (O(2)(*-)), in vitro and in vivo models, evaluation of the efficiency of specific spin traps to identify this free radical is paramount. Recently, a family of ester-containing nitrones has been prepared, which appears to have distinct advantages for spin trapping O(2)(*-) compared to the well-studied spin traps 5,5-dimethyl-1-pyrroline N-oxide 1 and 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide 2. An important determinant in the selection of a spin trap is the rate constant (k(app)) for its reaction with O(2)(*-), and several different methods have been employed in estimating this k(app). In this paper, the two most frequently used scavengers of O(2)(*-), ferricytochrome c and Cu/Zn-SOD, were evaluated as competitive inhibitors for spin trapping this free radical. Data presented herein demonstrate that SOD is the preferred compound when determining the k(app) for the reaction of O(2)(*-) with spin traps. Using this model, the k(app) for the reaction of nitrone 1, 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide 3, and 5-methoxycarbonyl-5-methyl-1-pyrroline N-oxide 4 with O(2)(*)(-) was estimated to be 24.6 +/- 3.1, 73.0 +/- 12, and 89.4 +/- 1.0 M(-1) s(-1) at pH 7.0, respectively. Several other comparative studies between known spin traps were also undertaken.     

Reactivity of esters of 2-cyanoacrylic acid toward some free radicals

The rate constants for the addition of ^·CH(Ph)CH₂CCl₃, ^·CH₂Ph, ^·CH₂Prⁿ, and ^·CCl₃ radicals to the ethyl 2-cyanoacrylate molecule were determined by ESR spectroscopy using the spin trapping technique.     

Investigation of charge carriers in poly(3,4-butylenedioxythiophene) (PBuDOT) by means of ESR spectroelectrochemistry

In situ ESR spectroelectrochemical studies of poly(3,4-butylenedioxythiophene) (PBuDOT) have been performed, in order to investigate more closely the species responsible for the conductivity of this polymer in the doped state. In the process of electrochemical doping of the polymer, ESR spectra at progressively changed potentials were recorded. Then the subsequent dedoping process was studied accordingly. The results reveal that PBuDOTs ESR spectroscopic properties are markedly different form its close relative, poly(3,4-etylenedioxythiophene) (PEDOT). Firstly, the potential dependence of the spin concentration displays a clear peak-shaped transient with a gradual decrease at higher oxidation potentials. Similar behaviour is seen for the B_pp widths of the ESR signal, indicating that the type of interactions between paramagnetic centres is potential sensitive. The course of the reduction process of the polymer is more or less the reverse of the oxidation one, with only a slight hysteresis of the spin concentration and a barely discernible one of B_pp widths being observed. In PBuDOT, as in PEDOT, distinct narrow ESR lines revealing a noteworthy spin concentration in the reduced state of the polymer have also been observed. The presence of these residual spins in the dedoped polymer may indicate that the dedoping process of this polymer is indeed a slow one. The presence of trace amounts of impurities or oxygen may hinder the dedoping process, especially for thicker films.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Addition of Me2·CCN,Me2·CPh,and CCl3CH2·CHPh radicals to fullerene C60

The rate constants for addition of the Me₂ ^·CCN, Me₂ ^·CPh, and CCl₃CH₂ ^·CHPh radicals to fullerene C₆₀ at 22 °C were determined by ESR spectroscopy using spin trapping technique.     

Pulse radiolysis investigations on the reactions of primary radiolytic species of water with atropine

On pulse radiolysis of N₂O saturated aqueous solutions of atropine, an optical absorption band (_max at 320 nm,e=2.81·10³ dm³·mol^–1·cm^–1) was observed, which is assigned to the product of reaction of OH radicals with the solute. This absorption decayed following second order kinetics with a rate constant of 4.5·10⁸ dm³·mol^–1·s^–1. The rate constant for the reaction of OH radicals with atropine as estimated by following the build-up kinetics is 2.7·10⁹ dm³·mol^–1·s^–1. The H atoms also reacted with this compound to produce a transient absorption band behaving similarly to the one observed in the case of reaction with OH radicals. The transient species formed in both cases is assigned to a radical derived by H atom abstraction by H/OH radicals from the parent compound. This radical was unreactive towards 2-mercaptoethanol. e _aq ^– was found to react with atropine forming a transient band with _max at 310 nm (=3.55·10³ dm³·mol^–1). Its decay was also second order with a rate constant of 1.64·10⁹ dm³·mol^–1·s^–1. The bimolecular rate constant for the reaction of e _aq ^– with atropine as estimated from the decay of e _aq ^– absorption at 720 nm is 3.9·10⁹ dm³·mol^–1·s^–1. Specific one-electron oxidizing and reducing agents (such as Cl ₂ ^– , Tl²⁺, SO ₄ ^– and (CH₃)₂COH, CO ₂ ^– , respectively) failed to oxidize or reduce this compound in aqoues solutions. The radical anion of atropine formed by its reaction with e _aq ^– was found to reduce thionine and methyl viologen with bimolecular rate constant of 3.8·10⁹ and 3.2·10⁹ dm³·mol^–1·s^–1, respectively.     

Electronic and ESR spectroscopic study of thiocyanate-containing coordination compounds of titanium(III) in alcohol solutions

In an investigation of electronic and ESR spectra, it has been established that titanium(III) with the thiocyanate ion (in ethanol) forms distorted tetrahedral and octahedral complexes. The tetrahedral compounds, which are hydroxo complexes of titanium(III) that contain one hydroxyl group and three thiocyanate ions in the coordination sphere, give signals in the ESR spectra with g_o=1.960, A_o=15.0·10^–4 cm^–1 g=1.958, A ^Ti =26.4·10^–4cm^–1, g= 1.691, A^Ti=9.0·10^–4 cm^–1. In the ESR spectra of liquid solutions, additional splitting on the nitrogen atoms is manifested(A _o ^N =2.2·10^–4 cm^–1), which indicates bonding of thiocyanate ions through the nitrogen atom. The pseudo-octahedral complexes of titanium(III), containing from two to six thiocyanate ions in the coordination sphere, are characterized by d-d absorption bands in the 520–590 nm region; and at 77°K, they give anisotropic ESR signals. On the basis of the temperature dependences of the equilibrium constants for the reaction of conversion of the tetrahedral complexes to the octahedral complexes, and also the lifetimes of the tetrahedral complexes, values have been estimated for the heat of reaction (H=26.8 kJ/mole) and the entropy change (S=–1.66 kJ/mole·°K) of the equilibrium process, and also the activation energy for the reaction of titanium(III) thiocyanate formation (E=37 kJ/mole).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 560–567, September–October, 1985.     

Direct Access to Dithiobenzoate RAFT Agent Fragmentation Rate Coefficients by ESR Spin‐Trapping

The β‐scission rate coefficient of tert‐butyl radicals fragmenting off the intermediate resulting from their addition to tert‐butyl dithiobenzoate—a reversible addition–fragmentation chain transfer (RAFT) agent—is estimated via the recently introduced electron spin resonance (ESR)‐trapping methodology as a function of temperature. The newly introduced ESR‐trapping methodology is critically evaluated and found to be reliable. At 20 °C, a fragmentation rate coefficient of close to 0.042 s⁻¹ is observed, whereas the activation parameters for the fragmentation reaction—determined for the first time—read E_A = 82 ± 13.3 kJ mol⁻¹ and A = (1.4 ± 0.25) × 10¹³ s⁻¹. The ESR spin‐trapping methodology thus efficiently probes the stability of the RAFT adduct radical under conditions relevant for the pre‐equilibrium of the RAFT process. It particularly indicates that stable RAFT adduct radicals are indeed formed in early stages of the RAFT poly­merization, at least when dithiobenzoates are employed as controlling agents as stipulated by the so‐called slow fragmentation theory. By design of the methodology, the obtained fragmentation rate coefficients represent an upper limit. The ESR spin‐trapping methodology is thus seen as a suitable tool for evaluating the fragmentation rate coefficients of a wide range of RAFT adduct radicals.

     

Molecular structures and ESR spectra of copper(ii) complexes with 2-hydroxypropiophenone acyldihydrazones

The dinuclear copper(ii) complexes with 2-hydroxypropiophenone acyldihydrazones (H₄L) having the composition [Cu₂L·mPy], where the L ligand contains the polymethylene chain with different lengths (from two to five units), were synthesized and studied. The crystal and molecular structures of the 2-hydroxypropiophenone adipoylhydrazone complex [Cu₂L·4Py]·Py were established by X-ray diffraction analysis. Copper atoms are 8.212 distant from each other, and their nearest environment has the tetragonal pyramidal geometry. The ESR spectra of solutions of the complexes based on acyldihydrazones of succinic, glutaric, and adipic acids contain seven HFS lines with the constant 40·10^–4 cm^–1 from two equivalent copper atoms. The spectra were interpreted as a result of the spin-spin exchange interaction of two unpaired electrons. An increase in the polymethylene chain length to five units prevents exchange interactions. The ESR spectrum of the complex with acyldihydrazone of pimelic acid contains a signal of four HFS lines with a _Cu = 73.4·10^–4 cm^–1, which is typical of mononuclear copper(ii) complexes.