Radical Spin Adduct Research Articles

Spin Trapping Analysis of Radical Intermediates on the Thermo-Oxidative Degradation of Polypropylene.

The purpose of this study is to investigate the thermo-oxidative degradation behavior of polypropylene (PP) by comparing three types of pristine PP granules (consisting of homopolymer, random copolymer, and block copolymer) with their corresponding oxidized analogues. These analogues were intensely oxidized under oxygen at 90 °C for 1000 h by using the electron spin resonance (ESR) spin trapping method that can detect short-lived radical intermediates during the degradation. The degrees of oxidation could be evaluated by chemiluminescence (CL) intensity, which was related to the concentration of hydroperoxide groups generated in the PP chain. In the pristine PP samples, a small amount of hydroperoxides were found to be formed unintentionally, and their homolysis produces alkoxy radicals, RO•, which then undergo β-scission to yield chain-end aldehydes or chain-end ketones. These oxidation products continue to take part in homolysis to produce their respective carbonyl and carbon radicals. On the other hand, in the oxidized PP granules, because of their much higher hydroperoxide concentration, the two-stage cage reaction and the bimolecular decomposition of hydroperoxides are energetically favorable. Carbonyl compounds are formed in both reactions, which are then homolyzed to form the carbonyl radical species, •C(O)-. PP homopolymer produced the largest amount of carbonyl radical spin adduct; thus, it was found that the homopolymer is most sensitive to oxygen attack, and the presence of ethylene units in copolymers enhances the oxidation resistance of PP copolymers.

EPR spectroscopic evidence of iron-catalysed free radical formation in chronic mountain sickness: Dietary causes and vascular consequences

Chronic mountain sickness (CMS) is a high-altitude (HA) maladaptation syndrome characterised by elevated systemic oxidative-nitrosative stress (OXNOS) due to a free radical-mediated reduction in vascular nitric oxide (NO) bioavailability. To better define underlying mechanisms and vascular consequences, this study compared healthy male lowlanders (80 m, n = 10) against age/sex-matched highlanders born and bred in La Paz, Bolivia (3600 m) with (CMS+, n = 10) and without (CMS-, n = 10) CMS. Cephalic venous blood was assayed using electron paramagnetic resonance spectroscopy and reductive ozone-based chemiluminescence. Nutritional intake was assessed via dietary recall. Systemic vascular function and structure were assessed via flow-mediated dilatation, aortic pulse wave velocity and carotid intima-media thickness using duplex ultrasound and applanation tonometry. Basal systemic OXNOS was permanently elevated in highlanders (P = <0.001 vs. lowlanders) and further exaggerated in CMS+, reflected by increased hydroxyl radical spin adduct formation (P = <0.001 vs. CMS-) subsequent to liberation of free ‘catalytic’ iron consistent with a Fenton and/or nucleophilic addition mechanism(s). This was accompanied by elevated global protein carbonylation (P = 0.046 vs. CMS-) and corresponding reduction in plasma nitrite (P = <0.001 vs. lowlanders). Dietary intake of vitamins C and E, carotene, magnesium and retinol were lower in highlanders and especially deficient in CMS + due to reduced consumption of fruit and vegetables (P = <0.001 to 0.028 vs. lowlanders/CMS-). Systemic vascular function and structure were also impaired in highlanders (P = <0.001 to 0.040 vs. lowlanders) with more marked dysfunction observed in CMS+ (P = 0.035 to 0.043 vs. CMS-) in direct proportion to systemic OXNOS (r = −0.692 to 0.595, P = <0.001 to 0.045). Collectively, these findings suggest that lifelong exposure to iron-catalysed systemic OXNOS, compounded by a dietary deficiency of antioxidant micronutrients, likely contributes to the systemic vascular complications and increased morbidity/mortality in CMS+. Trial registryClinicalTrials.gov; No: NCT01182792; URL: www.clinicaltrials.gov.

Free-Radical Generation from Bulk Nanobubbles in Aqueous Electrolyte Solutions: ESR Spin-Trap Observation of Microbubble-Treated Water.

Microbubbles are very fine bubbles that shrink and collapse underwater within several minutes, leading to the generation of free radicals. Electron spin resonance spectroscopy (ESR) confirmed the generation of hydroxyl radicals under strongly acidic conditions. The drastic environmental change caused by the collapse of the microbubbles may trigger radical generation via the dispersion of the elevated chemical potential that had accumulated around the gas-water interface. The present study also confirmed the generation of ESR signals from the microbubble-treated waters even after several months had elapsed following the dispersion of the microbubbles. Bulk nanobubbles were expected to be the source of the spin-adducts of hydroxyl radicals. Such microbubble stabilization and conversion might be caused by the formation of solid microbubble shells generated by iron ions in the condensed ionic cloud around the microbubble. Therefore, the addition of a strong acid might cause drastic changes in the environment and destroy the stabilized condition. This would restart the collapsing process, leading to hydroxyl radical generation.

Antioxidant Properties of Galanin and Its N-Terminal Fragments in in vitro and in vivo Oxidative Stress Modeling.

Antioxidant properties of rat galanin GWTLNSAGYLLGPHAIDNHRSFSDKHGLT-NH2 (Gal), N-terminal fragment of galanin (2-15 aa) WTLNSAGYLLGPHA (G1), and its modified analogue WTLNSAGYLLGPβAH (G2) were studied in vivo in the rat model of regional myocardial ischemia and reperfusion and in vitro in the process of Cu2+-induced free radical oxidation of human blood plasma low-density lipoproteins. Intravenous administration of G1, G2, and Gal to rats after ischemia induction reduced the infarction size and activities of the necrosis markers, creatine kinase-MB and lactate dehydrogenase, in blood plasma at the end of reperfusion. G1, G2, and Gal reduced formation of the spin adducts of hydroxyl radicals in the interstitium of the area at risk during reperfusion, moreover, G2 and Gal also reduced formation of the secondary products of lipid peroxidation in the reperfused myocardium. It was shown in the in vivo experiments and in the in vitro model system that the ability of galanin peptides to reduce formation of ROS and attenuate lipid peroxidation during myocardial reperfusion injury was not associated directly with their effects on activities of the antioxidant enzymes of the heart: Cu,Zn-superoxide dismutase, catalase, and glutathione peroxidase. The peptides G1, G2, and Gal at concentrations of 0.01 and 0.1 mM inhibited Cu2+-induced free radical oxidation of human low-density lipoproteins in vitro. The results of oxidative stress modeling demonstrated that the natural and synthetic agonists of galanin receptors reduced formation of the short-lived ROS in the reperfused myocardium, as well as of lipid radicals in blood plasma. Thus, galanin receptors could be a promising therapeutic target for cardiovascular diseases.

Microwave‐induced free radicals production in red wine and model wine by electron paramagnetic resonance spin trapping

Microwave can directly induce the production of 1-hydroxyethyl free radical in red wine and model wine. Free radicals are considered as key intermediates in the wine aging processing by microwave, but their properties have not been determined. Electron paramagnetic resonance (EPR) spin trapping with 5,5-dimethyl-1-pyrrolin N-oxide (DMPO) was used to detect the type of free radicals. After microwave irradiation, spin adducts of hydroxyl free radical were detected in DMPO solution, whereas 1-hydroxyethyl free radical spin adducts were captured in red wine and model wine, because 1-hydroxyethyl free radicals arose from the oxidation of ethanol by hydroxyl free radical in red wine induced by microwave irradiation. Compared with the processing of natural oxidation, the intensity of free radicals induced by microwave in red wine became more strong, and the effects of microwave temperature, time, and power on the intensity of 1-hydroxyethyl free radical spin adducts in model wines were also studied. The intensity of free radical spin adducts increased with the increasing of microwave power and time. The intensity of DMPO/1-hydroxyethyl free radical spin adducts increases at low temperature, but decreased at higher temperatures. Practical applications This study investigated the existence of free radical in the processing of wine aging, and the possible mechanism of microwave irradiation accelerated aging of red wine was studied with model wine. Results revealed that the intensity of 1-hydroxyethyl free radical induced by microwave in red wine and model wine. Therefore, microwave irradiation was considered to be a promising aging technology, so microwave radiation technology can be used in many aging products that sped up the aging processing.

Active carbon/TiO2 composites for photocatalytic decomposition of benzoic acid in water and toluene in air

The present work describes the preparations of active carbon (AC) – titania composites with 2 different AC/TiO2 ratios, their characterization using SEM, BET, determination of particle size, and evaluation of adsorption ability and photocatalytic activity using benzoic acid (BA) in the aqueous phase and toluene in the gaseous phase. Composite materials based on AC/TiO2 were prepared from two types of active carbon (having a BET surface area of 1737 m2 g−1 and 890 m2 g−1, respectively) and TiO2 (P25, BET surface area 45 m2 g−1) by mixing. The main studied parameter was milling of AC. Immobilized photocatalyst films were prepared by drop-casting of aqueous composite suspensions on glass substrates and drying. The composite films prepared from milled AC have much better adhesion to substrate in aqueous media than those from non-milled AC, which is due to a decrease of particle size by about 1 order during milling. The ability of composites prepared from milled AC to remove BA was more than 2.5 times higher than that of composites from non-milled AC. In comparison with TiO2, prepared composites showed similar or higher photocatalytic activity and the irradiation of AC/TiO2 composites in aqueous suspensions resulted in the generation of a comparable concentration of the hydroxyl radical spin-adduct (DMPO-OH) as with TiO2.

Composite materials based on active carbon/TiO2 for photocatalytic water purification

The present work describes the preparations of active carbon (AC) – titania composites with different AC/TiO2 ratio, their characterization using XRD, BET and SEM and evaluation of adsorption capacity and photocatalytic activity in aqueous solution using azo-dye Acid Orange 7 (AO7). Composite materials based on AC/TiO2 were prepared from commercial active carbon (1737 m2 g-1) and TiO2 (P25, 45 m2 g-1) materials by the mixing method. The studied parameters were the ratio of AC/TiO2, hydrothermal treatment and milling of as-prepared composites.The composites show increased BET surface area proportional to the content of AC in the material. The disappearance of AO7 is due to the combination of two processes, e. g. adsorption and photocatalytic oxidation which take place in parallel. Thus the overall removal of AO7 has been corrected on adsorption (measured in dark) to obtain the rate of AO7 disappearance due to photocatalytic oxidation (PO). The optimum ratio of AC/TiO2 was found to be 0.23 where PO rate is only slightly lower than that for P25 and overall AO7 removal is twice higher than for P25 itself. The less intensive (low-frequency) milling of the composite results in the decrease of carbon particle size, increase in AO7 adsorption and decline in the PO of AO7. More intensive milling (high-frequency) results in the decrease of the anatase content, appearance of the small amount of brookite and almost negligible PO rate of AO7. Irradiation of low-frequency milled AC/TiO2 composite in aqueous suspensions resulted in the generation of a comparable concentration of hydroxyl radical spin-adduct (DMPO−OH) as non-milled composite, high-frequency milled composite revealed substantially lower DMPO−OH concentration which can be explained by the increased concentration of carbon-centered radicals in AC acting as scavengers of photogenerated electrons.

Accuracy and Validity of AREC (Alkoxy Radical Elimination Capacity) Assay in Evaluating the Antioxidant Abilities of Various Biosubstances

Abstract The AREC (alkoxy radical elimination capacity) assay was developed to observe the DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin adducts of free radicals produced by thermal decomposition of AAPH (2,2′-azobis(2,4-amidinopropane) dihydrochloride) using a flow-injection ESR (FI-ESR) system. The γ50 value is defined as [DMPO]0/ID50 = kS/k1, where k1 is the rate constant of spin-trapping of the alkoxy radical by DMPO, kS, that of alkoxy radical-elimination by the substrate, and ID50, 50% inhibition of the alkoxy radical by the substrate, and the AREC value is the ratio of γ50 value of the substrate to that of Trolox (6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylic acid, TRX). The γ50 and AREC values show fairly wide variations and can be determined for most biosubstances. The highest AREC values were observed for sinapic acid, l-glutathione, caffeic acid, and chlorogenic acid, followed by TRX, syringic acid, trans-ferulic acid, and homogentisic acid. Little correlation is observed between the AREC values and the hydroxy and superoxide radical-elimination abilities. The AREC values of 4-hydroxycinnamic acid derivatives (HCAs) are linearly related to the aryloxy radical-elimination abilities, which indicate that the alkoxy radical-elimination by HCAs is mainly caused by hydrogen-atom transfer. The newly defined AREC value is applicable for various biosubstances, and is far superior and a more reliable indicator than the oxygen radical absorption capacity (ORAC) value determined by the ORAC-fluorescein assay. Thus, the AREC value is an excellent indicator to characterize the antioxidant activities of a wide range of biologically important antioxidants present in fruits, vegetables, and beverages.

Oxidative stress and DNA damages induced by 1-nitropyrene in human lung fibroblasts: New insights into the mechanisms of genotoxicity and EPR-spin trapping direct monitoring of free radicals at subcellular levels

Polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs are ubiquitous contaminants released in the atmosphere from the incomplete combustion of organic matter and pesticide intermediates nitration. Many studies have established that 1-nitropyrene (1-NP), one the most abundant nitro-PAH pollutant in diesel exhaust, is associated with increased risk of cancer and inflammatory diseases via possible metabolic pathways, including ring oxidation, nitroreduction or conjugation reactions. In addition, genotoxicity tests (e.g. comet assay and Ames test), demonstrated that 1-NP is a potent mutagen and genotoxic agent. However, the mechanisms underlying the promotion of DNA damages remain to be fully elucidated in relation with the progression of oxidative stress due to the stimulation of cellular free radical sources. Methods: On human lung fibroblasts exposed to environmentally relevant doses of 1-NP (1-10 microM) for 24–48 h, we performed: (i) cytokinesis-blocked micronucleus (CBMN) assay in association with immunofluorescence staining of centromere protein A in micronuclei to distinguish between induction of structural or numerical chromosome changes; (ii) EPR experiments using a set of spin trapping agents (i.e. the phosphorylated nitrone DEPMPO (Culcasi et al., 2012), the new stereoselective superoxide probe Me4CyDEPMPO (Gosset et al., 2011)) and mitochondrial-targeted nitrones; and (iii) the evaluation of oxidative stress from protein carbonyls, lipid peroxidation and antioxidant depletion. Major results: One advantage of the CBMN assay is its ability to elucidate the genotoxic mechanism of agents with clastogenic or aneugenic modes of action. The data provide the first evidence for a superoxide-related aneugenic mode of action at low 1-NP doses and a dominant clastogenic mechanism as the 1-NP doses and exposure time increased. This two-steps mechanism differs from that induced by CeO2-nanoparticles, an oxidant present in diesel exhausts (Benameur et al., 2014). From the EPR detection of superoxide and hydroxyl radical spin adducts, two free radical cellular sources activated by 1-NP exposure were hypothetized, i.e. NADPH oxidase and mitochondria, this latter being likely involved in oxidative stress damages development. References Culcasi, et al., 2012. Chem. Biol. Interact.. Gosset, et al., 2011. Bioorg. Med. Chem.. Benameur, et al., 2014. Nanotoxicology.

Free radical generation induced by ultrasound in red wine and model wine: An EPR spin-trapping study

Direct evidence for the formation of 1-hydroxylethyl radicals by ultrasound in red wine and air-saturated model wine is presented in this paper. Free radicals are thought to be the key intermediates in the ultrasound processing of wine, but their nature has not been established yet. Electron paramagnetic resonance (EPR) spin trapping with 5,5-dimethyl-l-pyrrolin N-oxide (DMPO) was used for the detection of hydroxyl free radicals and 1-hydroxylethyl free radicals. Spin adducts of hydroxyl free radicals were detected in DMPO aqueous solution after sonication while 1-hydroxylethyl free radical adducts were observed in ultrasound-processed red wine and model wine. The latter radical arose from ethanol oxidation via the hydroxyl radical generated by ultrasound in water, thus providing the first direct evidence of the formation of 1-hydroxylethyl free radical in red wine exposed to ultrasound. Finally, the effects of ultrasound frequency, ultrasound power, temperature and ultrasound exposure time were assessed on the intensity of 1-hydroxylethyl radical spin adducts in model wine.

Highly photoactive anatase foams prepared from lyophilized aqueous colloids of peroxo-polytitanic acid

Peroxo-polytitanic acid foams annealed at temperature above 500°C lose volatile components and excess of oxygen providing anatase in the form of thin leaves consisting of intergrown nanocrystalline anatase particles which transform at temperatures above 850°C to rutile. The size and shape of the initial leaflets forming the foam is preserved up to ∼900°C. We observed that the annealed material is highly photoactive, owing to highly anisotropic shape of anatase aggregates and their perfect crystallinity. Outstanding photocatalytic activity determined by measuring the kinetics of degradation of Methylene Blue (MB) and 4-chlorphenol (4-CP) as well as measuring the formation of hydroxyl radical spin adducts by EPR spectroscopy was observed. All three methods demonstrated significantly higher activity in comparison with the Degussa P25 photocatalyst. Both methods used for photoactivity tests provided similar results, the activity increased with increasing annealing temperature in order 500°C<Degussa P25<600°C<850–950°C. Results of measurement of formation of OH radicals by EPR also confirmed the unusually high activity of our materials. In comparison with the Degussa P25, the sample annealed at 950°C showed significantly higher production of OH radicals.

Abilities of Antioxidants to Eliminate the Peroxyl Radical Derived from 2,2′-Azobis(2-Amidinopropane) Dihydrochloride (AAPH)

Electron spin resonance (ESR) of 5-(2,2-dimethyl-1,3-propoxy cyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) spin adducts of free radicals derived from the UV irradiation of 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) under rapid scanning condition was examined. The ESR signal obtained was the superposition of two spectra, the spin adduct of the alkoxyl radical [CYPMPO-ORa, Ra = C(CH3)2-C(+NH2Cl−)NH2] and the peroxyl radical (CYPMPO-OORa). The decay rate of CYPMPO-OORa was 8,400 times that of CYPMPO-ORa. The order of the k SB/k ST values (the slope of the Stern–Volmer’s plot) for the peroxyl radical (RaOO·) was l-ascorbic acid > caffeic acid > rutin ~ Trolox ~ (+)-catechin ~ glutathione (reduced), which was almost the same order as that for the alkoxyl radical (RaO·). Though the k SB/k ST value of each antioxidant for the peroxyl radical was about half of that for the alkoxyl radical, the ratios of the values of antioxidants to that of trolox (the relative ORAC values) were almost the same between the peroxyl and alkoxyl radicals. The relative oxygen radical absorbance capacity (ORAC) values of polyphenols estimated by the ORAC-ESR assay using both peroxyl and alkoxyl radicals were smaller than those by the ORAC-FL assay.

Reevaluation of Quantitative ESR Spin Trapping Analysis of Hydroxyl Radical by Applying Sonolysis of Water as a Model System

Abstract Electron spin resonance (ESR) spin trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is commonly applied for quantitative analysis of hydroxyl radical. For better understanding of the analysis, we investigated kinetics related to formation and decay of hydroxyl radical spin adduct of DMPO compared with that of 3,3,5,5-tetramethyl-1-pyrroline N-oxide (M4PO) and 5-(diphenylphosphinoyl)-5-methyl-1-pyrroline N-oxide (DPPMPO). In our study where hydroxyl radical was generated by sonolysis of water, we found that (1) DMPO-OH formation was saturated even though hydroxyl radical was generated continuously and (2) the concentration of DMPO-OH decreased in inverse proportion to time after cessation of ultrasound irradiation, suggesting that the decay is a second-order reaction. Similar results were obtained in an experiment of DMPO-OH generated by photolysis of H2O2. Other than DMPO, M4PO, and DPPMPO also trapped hydroxyl radical but the spin trap efficiency was less than that of DMPO. Furthermore, M4PO-OH and DPPMPO-OH decayed more quickly than DMPO-OH. From this, we conclude that DMPO is more suitable for quantification of hydroxyl radical than M4PO and DPPMPO but results from the quantitative analysis must be interpreted with consideration of the kinetics related to formation and decay of DMPO-OH, especially in quantification of large amount of hydroxyl radical generation.

ChemInform Abstract: The EPR Spectra of Thiyl Radical Spin Adducts Produced by Photolysis of Disulfides in the Presence of 2,4,6-Tri-tert-butylnitrosobenzene and 5,5-Dimethyl-1-pyrroline N-Oxide.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Using Cyclodextrins to Encapsulate Oxygen-Centered and Carbon-Centered Radical Adducts: The Case of DMPO, PBN, and MNP Spin Traps

We present electron spin resonance (ESR) experiments that describe the interaction of beta-cyclodextrin (beta-CD) with spin adducts of three spin traps: 5,5-dimethyl-1-pyrroline N-oxide (DMPO), N-tert-butyl-alpha-phenylnitrone (PBN), and 2-methyl-2-nitrosopropane (MNP). The focus was on spin adducts of oxygen-centered radicals trapped by DMPO and PBN and on carbon-centered radical adducts trapped by MNP. The radicals were generated by reaction with hydroxyl radicals and the spin adducts studied were DMPO/OH and PBN/OH, MNP/CH(2)COOH generated in CH(3)COOH, and MNP/CF(2)COOH in CF(2)HCOOH. Di-tert-butyl nitroxide ((CH(3))(3)C)(2)NO (DTBN) was also detected in experiments with MNP as the spin trap. A range of interactions of the spin adducts and DTBN with beta-CD was identified. The presence of beta-CD led to significant stabilization of DMPO/OH and PBN/OH but to a negligible effect on the (14)N hyperfine splitting of the adducts, a(N), indicating that the N-O group is outside the beta-CD cavity. An increase of a(N) was detected for DTBN and MNP/CH(2)COOH in CH(3)COOH in the presence of beta-CD, a result we assigned to bonding at the rim of the host. Experiments with methylated beta-CD (Me beta-CD) provided support for this conclusion. A different type of complexation was detected for DTBN and MNP/CF(2)COOH in CF(2)HCOOH: for specific host concentrations both "in" and "out" species were detected. We suggest that the hydrophobicity of the fluorinated adduct leads to insertion of the adduct inside the host cavity. Calculation of the association constant K(a) indicated the competition between DTBN and the adduct for inclusion in the host. For MNP as spin trap, the two nitroxide radicals (adduct and DTBN) have the same type of interaction with the host: at the rim in acetic acid, and inside the host cavity in CF(2)HCOOH. Experiments with DTBN in the absence of the spin trap and of adducts illuminated the effect of the local polarity and of the pH on the hyperfine splittings and indicated that the presence of acetic acid encourages rim complexation.

Characterization of Free Radical Spin Adducts of 5-Diisopropyloxy-Phosphoryl-5-Methyl-1-Pyrroline-N-Oxide Using Mass Spectrometry and 31P Nuclear Magnetic Resonance

5-Diisopropyloxy-phosphoryl-5-methyl-1-pyrroline-N-oxide (DIPPMPO) was used to trap a variety of free radicals and the stable compounds generated by the natural decomposition of the initially formed spin adducts were characterized by (31)P nuclear magnetic resonance (NMR) and mass spectrometry. Initially, the starting spin trap DIPPMPO was completely characterized using GC-MS and its fragmentation pathway was studied in detail. Then, DIPPMPO was used to trap an oxygen-centered free radical (the hydroxyl radical *OH) and two carbon- centered free radicals (methyl *CH(3) and 1-phenyl-ethanol-1-yl *CCH(3)(OH)Ph radicals). The (31)P NMR signals were thus assigned and the structures of adducts were studied and confirmed by mass spectrometry. Overall, the fragmentation pathways of the radical adducts proceed mainly via the loss of the diisopropyloxy(oxido)phosphoranyl radical. For the specific case of trapping *OH radicals, it is possible to visualize the rearrangement of the nitroxide radical adduct to its nitrone form as invoked in the literature. This spin trapping technique, coupled with (31)P NMR and MS, provides a tool for the identification of short-lived and low molecular weight free radicals present in a variety of processes.

EPR Study of Cumene Hydroperoxide Decomposition Catalyzed by Tetra-(p-chlorophenyl)porphinatomanganese Chloride

The decomposition of hydroperoxide in the presence of a metalloporphyrin is an important metalloporphyrin-catalyzed hydrocarbon aerobic oxidation reaction. This process affects the oxidation product selectivity, the formation rate of oxidation products, and also the life of the metalloporphyrin catalysts. The decomposition of cumene hydroperoxide (CHP) catalyzed by tetra-(p-chlorophenyl) porphyrinatomanganese chloride (T(p-Cl)PPMnIIICl) was studied by EPR, and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used as an EPR spin trap. Experiments revealed that only the cumoxyl radical adduct was present during the initial reaction time at 25°C. Another kind of radical adduct, a rearrangement product of the cumylperoxyl radical spin adduct, was also detected by EPR when the concentration of CHP was increased. Results further show that CHP decomposition started by homolytic O–O bond cleavage because of the T(p-Cl)PPMnIIICl catalyst generating cumoxyl radicals in the initial part of the reaction and this led to 2-phenyl-2-propanol. The cumylperoxyl radicals were formed from the subsequent reaction of cumoxyl radicals with CHP when the initial CHP concentration increased. A free radical route for this decomposition reaction was also proposed.

Photoactivity of mechanochemically prepared nanoparticulate titanium dioxide investigated by EPR spectroscopy

Titanium dioxide nanopowders were prepared by mechanochemical synthesis in a high-energy ball mill using TiOSO4·xH2O and Na2CO3 followed by annealing in the temperature range 200–800°C. The UVA photonic efficiency of radical processes on synthesized TiO2 powders was determined by in situ EPR spectroscopy, using, as indicators, the N-oxide spin trapping agents (5,5-dimethyl-1-pyrroline N-oxide and 5-(diisopropoxy-phosphoryl)-5-methyl-1-pyrroline N-oxide) or the radical cation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate, ABTS+). The results obtained by monitoring the photoinduced generation of hydroxyl radical spin adducts correlated with those found by the investigation of the photoreduction of ABTS+. The presence of iron and chromium ions, which were evidenced in samples milled in steel, decreased the photonic efficiency of radical processes. The presence of a sulfate salt matrix during the annealing process distinctly inhibits the transformation of anatase to rutile. The highest photocatalytic activity was shown by anatase samples which were prepared by milling in corundum and annealed at 700°C. They were composed of crystallites with a mean size of 25–30nm and well developed crystal faces.

Involvement of both Type I and Type II mechanisms in Gram-positive and Gram-negative bacteria photosensitization by a meso-substituted cationic porphyrin

A meso-substituted cationic porphyrin (TMPyP) showed a photocytotoxicity against Gram-positive and Gram-negative bacteria. In order to determine the mechanism involved in the phototoxicity of this photosensitizer, electron paramagnetic resonance (EPR) experiments with 2,2,6,6-tetramethyl-4-piperidone (TEMP), a specific probe for singlet oxygen, and the spin-trap 5,5-dimethyl-1-pyrroline- N-oxide (DMPO) were carried out with illuminated TMPyP. An EPR signal characteristic of TEMP-singlet oxygen (TEMPO) adduct formation was observed, which could be ascribed to singlet oxygen ( 1O 2) generated by TMPyP photosensitization. The signal for the DMPO spin adduct of superoxide anion (DMPO-OOH) was observed in DMSO solution but not in aqueous conditions. However, an EPR spectrum characteristic of the DMPO-hydroxyl radical spin adduct (DMPO-OH) was observed in aqueous conditions. The obtained results testify a primary hydroxyl radical ( OH) generation probably from superoxide anion ( O 2 - ) via the Fenton reaction and/or via Haber-Weiss reaction. Gram-positive and Gram-negative bacteria inactivation by TMPyP photosensitization predominantly involved Type II reactions mediated by the formation of 1O 2, as demonstrated by the effect of quenchers for 1O 2 and scavengers for OH (sodium azide, thiourea, and dimethylsulphoxide). Participation of other active oxygen species cannot however be neglected since Type I reactions also had a significant effect, particularly for Gram-negative bacteria. For Gram-negative bacteria the photoinactivation rate was lower in the presence of superoxide dismutase, a specific O 2 - scavenger, and/or catalase, an enzyme which specifically eliminates H 2O 2, but was unchanged for Gram-positive bacteria. The generation of 1O 2, O 2 - and OH by TMPyP photosensitization indicated that TMPyP maintained a photodynamic activity in terms of Type I and Type II mechanisms.

Influence of the nature of ligands in iridium complexes with C60 fullerene and ·P(O)(OPri)2, ·CMe3, and ·CCl3 radicals on regioselectivity of addition and stability of spin-adducts formed: an EPR study

The addition of ·P(O)(OPri)2 (R1), ·CMe3 (R2), and ·CCl3 (R3) radicals to metallofullerenes (η2-C60)IrH(CO)(CNBut)2(o-HCB10H9CCH2PPh2-B,P) (1), (η2-C60)IrH(CO)(DIOP) (DIOP is (4R,5R)-(+)-4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane, 2), and (η2-C60)IrH(CO)(PPh3)2 (3) was studied by EPR spectroscopy. A stability study of spin adducts (SAs) of R1 radicals with complexes 1 and 2 showed that when the reactions are initiated by illumination with 366-nm light, the EPR spectra exhibit only signals of those isomers that are formed upon attack of the R1 radicals on the carbon atoms of the cis-1 and cis-2 bonds (i.e., carbon atoms of the fullerene hemisphere to which the metallofragment is attached). Investigations of the reactions of R2 and R3 radicals with complexes 1–3 initiated with 366-nm light made it possible to detect (i) regioisomers formed by adding these radicals to carbon atoms of the cis-n bonds and (ii) SAs formed by adding the radicals to carbon atoms of other bonds in complexes 1–3. The hyperfine structure of the EPR spectrum essentially depends on the spatial structure of substituents at the metal atom and allows individual regioisomers of not only phosphoryl radicals, but also carbon-centered radicals R2 and R3 with metallofullerenes 1–3 to be identified. The rate constants for addition of R2 and R3 radicals to complexes 2 and 3 were determined.