In order to find ways to characterize oxygen-saturated aqueous TiO 2 suspensions, the formation of photo-induced free radicals was followed by EPR spectroscopy, using as indicators N-oxide and nitrone spin trapping agents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), α-(4-pyridyl-1-oxide)-N- tert -butylnitrone (POB N), 4-(N-methylpyridyl)-N- tert -butylnitrone (MePyBN), as well as semi-stable free radicals, 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (TEMPOL), cation radical of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), diammonium salt (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH). DMPO and TMPO are efficiently oxidized to the EPR-silent products via radical in termediates. Conversely, the nitrone spin traps (POBN and MePyBN) showed selective formation of hydroxyl radical spin adducts upon continuous irradiation of oxygenated TiO 2 suspensions. Their concentrations increased proportionally with the amount of photocatalyst and irradiation time. The EPR spectrum of the semi-stable free radicals TEMPOL, ABTS ·+ or DPPH is gradually eliminated during irradiation, and this system represents a simple technique for the evaluation of TiO 2 activity.

The gamma-radiation treatment of ground black pepper samples resulted in the production of three paramagnetic species (GI-GIII) which arise from a different origin and have different thermal behavior and stability. The axially symmetric spectra can be characterized by the spin Hamiltonian parameters: GI (g(perpendicular to) = 2.0060, g(parallel to) = 2.0032; A(perpendicular to) = 0.85 mT, A(parallel to) = 0.70 mT) and GII (g(perpendicular to) = 2.0060, g(parallel to) = 2.0050; A(perpendicular to) = 0.50 mT, A(parallel to) = 0.40 mT) assigned to carbohydrate radical structures. The parameters of EPR signal GIII (g(perpendicular to) = 2.0029, g(parallel to) = 2.0014; A(perpendicular to) = 3.00 mT, A(parallel to) = 1.80 mT) possessed features characteristic of cellulose radical species. The activation energies, evaluated by Arrhenius analysis, are in order E-a(GI)<E-a(GIII)<E-a(GII). The EPR measurements performed 20 weeks after radiation process confirmed that a temperature increase from 298 to 353 K, caused a significant decrease of integral EPR signal intensity for gamma-irradiated samples (similar to 40%), compared to the reference (non-irradiated) ground black pepper, where a decrease of similar to 13% was found. The influence of gamma-radiation treatment on the radical-scavenging activities of aqueous and ethanol extracts of black pepper were investigated by both an EPR spin trapping technique and DPPH assay. No changes were detected in either the water or ethanol extracts for a gamma-irradiation dose of 10 kGy. (C) 2005 Elsevier Ltd. All rights reserved.

Wines are increasingly considered to be beneficial to health, so we expanded our previous investigations of red and white wines to Tokay wines, employing electron paramagnetic resonance spectroscopy (EPR). Four sources of free radicals were used in scavenging experiments: thermally decomposed radical initiators, K2S2O8 and 2,2′-azo-bis(2-methylpropionamidine) hydrochloride (AAPH), as well as the cation radical of 2,2′-azino-bis(3-ethylbenthiazoline-6-sulfonic acid) salt (ABTS), and the stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH). The radical-scavenging abilities of 30 samples of Tokay wines from the Slovak region were compared with 10 samples of red and 10 samples of white wines originating from various regions. Tokay wines show a very good scavenging ability, positioned between white and red wines expressed as trolox equivalent antioxidant capacities (TEAC) with 14.8±1.5 for red, 8.1±3.4 Tokay and 3.3±1.6 for white wines in mmol dm−3 wine.

Commercial sunscreen products containing titanium dioxide were irradiated with λ > 300 nm and the formation of oxygen- (OH, O2-/OOH) and carbon-centered radicals was monitored by EPR spectroscopy and spin trapping technique using 5,5-dimethyl-1-pyrroline N-oxide, α-phenyl-N-tert-butylnitrone (PBN), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone as spin traps, and free nitroxide radical 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl. The photoinduced production of singlet oxygen was shown by 4-hydroxy-2,2,6,6-piperidine. The generation of reactive oxygen radical species upon irradiation of sunscreens significantly depends on their composition, as the additives present (antioxidants, radical-scavengers, solvents) can transform the reactive radicals formed to less harmful products. The continuous in situ irradiation of titanium dioxide powder, recommended for cosmetic application, investigated in different solvents (water, dimethyl sulfoxide, isopropyl myristate) resulted in the generation of oxygen-centered reactive radical species (superoxide anion radical, hydroxyl and alkoxyl radicals).

Radical-scavenging activity of the water-soluble derivative obtained from cell wall of the baker's yeast Saccharomyces cerevisiae was investigated using the technique of electron paramagnetic resonance. The experiments involved a study of the scavenging activity of carboxymethyl (1 -> 3)-beta-D-glucan (CMG) towards the radicals formed in the thermally initiated decomposition of potassium persulfate, hydrogen peroxide, or 2,2'-azo-bis(2-amidinopropane)-dihydrochloride in aqueous solutions using spin trapping as an indicative technique. In the absence of glucan, high intensity spectra of generated free radicals in the form of their adducts with 5,5-dimethylpyrroline-N-oxide were observed.Addition of CMG resulted in concentration-dependent substantial decrease of spectral intensities of adducts as a result of competition of CMG in the scavenging of reactive radicals formed.In the in vivo experiments involving administration of CMG to rats with experimentally induced adjuvant arthritis (AA) a substantial decline of the level of plasmatic carbonyls, a parameter indicating oxidative tissue damage during the progress of arthritic diseases, was observed. We assume that radical-scavenging properties of CMG can be responsible for its antioxidant activity in the AA model, suggesting possible application of the yeast glucan derivatives in the treatment of arthritis. (c) 2005 Elsevier Ltd. All rights reserved.

Redox reactions of C-60, C-120, C120O and C120OS in tetrabutylammonium tetrafluoroborate/o-dichlorobenzene solutions of high purity are studied by simultaneous measurement of UV-vis-NIR and ESR spectra during electrochemical reduction in dependence on the dimeric fullerene structure. It was found that C-120 shows ESR behavior similar to C120O in the first electron transfer at 260 K characterized by the single ESR line of R2C60- fullierdes. C120OS anions exhibit a sharp ESR line (linewidth 0.04 mT) with the characteristic feature of R4C60 anions indicating a strongly perturbed icosahedral symmetry of both C-60 cages in the dimer. The stability of radical ions of the C-60 dimers under study increases with the number of chemical bonds between both fullerene cages (C-120 < C120O < C120OS). NIR absorption of the fullerene anions generated in the first voltammetric double peak (lambda(max) = 1020 nm for C-120, lambda(max) = 940 nm for C120O, lambda(max) = 1020 nm for C120OS) is dominated by both the mono- and dianionic structures comparable to R2C60 monoanions (C-120 and C120O) and R4C60 (C120OS) fullerene anion spectra, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.

The antioxidant properties of green, black and mixed (fruit) tea samples of different origin were investigated by means of EPR spectroscopy. A six line EPR spectrum of solid tea samples indicates the presence of Mn(II) ions and it is superimposed with a sharp singlet line attributed to semiquinone radical species (ΔHpp=1 mT; g=2.0022). Antioxidant properties of aqueous tea extracts in H2O2/NaOH/dimethylsulfoxide system generating reactive radicals (OH, O2−, CH3) were followed by spin trapping technique. In addition, antioxidant capacity of these samples was assessed using stable radicals 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL). Typically, the highest antioxidant potential to terminate superoxide radicals was found in green teas, followed by black and fruity teas. The pro-oxidant activity of green teas evidenced by spin traps was promoted in samples with higher Mn(II) and ascorbic acid concentrations. Various sources of free radicals used in the antioxidant tests due to their specific action show different termination rates in the presence of the individual tea samples.

The radical intermediates produced upon UV irradiation of deoxygenated alcoholic titanium dioxide suspensions of nitrosobenzene, nitrobenzene, 2-nitrosotoluene, 2,3,5,6-tetramethylnitrosobenzene, 3,5-di-bromo-4-nitrosobenzenesulfonate (sodium salt), 2,4,6-tri-t-butyl-nitroso-benzene, and 2-methyl-2-nitrosopropane were investigated using in situ EPR technique. Nitrosobenzene is efficiently photoreduced in TiO2 suspensions (toluene/alcohol, 1:1 (v/v)) forming exclusively one stable radical intermediate corresponding to C6H5NOH species. The formation of this radical species is consistent with the proposed photocatalytic reduction mechanism, occurring from the primary generated nitrosobenzene mono-anion by the hydrogen abstraction from surroundings. The origin of hydrogen added to the nitroso group was demonstrated by the photocatalytic experiments using deuterated methanol, where the production of C6H5NOD was established. Additionally, an identical radical C6H5NOH was detected, when nitrobenzene was reduced under analogous experimental conditions. The photoinduced electron transfer from TiO2 to nitroso compounds is accompanied by alcohol oxidation via the photogenerated titanium dioxide valance band holes forming alkoxy and hydroxyalkyl radicals. Production of hydroxyalkyl radicals (CH2OH, CH(OH)CH3, C(OH)(CH3)2) with redox potentials suitable for a direct electron transfer to nitroso compounds represents an alternative reaction pathway for their reduction. On the other hand, the investigated nitroso derivatives are efficient spin-trapping agents, therefore, formation of nitroxyl radical spin adducts was observed in the photocatalytic experiments. The EPR spectra monitored upon irradiation of substituted nitrosobenzene derivatives in alcoholic TiO2 suspensions reveal the correlation between nitrosobenzene derivative first step reduction potentials and yield of radical species produced.

Substituted fullerenes RxC60 are responsible for "spikes" in the ESR spectra (as shown) recorded during the reduction of pure C-60 samples. This finding was drawn from in situ ESR/Vis/NIR spectroelectrochemical and MALDI-TOF mass spectrometric studies. The C60 derivatives RxC60.(-) are formed by the reaction of C-60 dianions with typical organic solvents used in C-60 electrochemistry[GRAPHICS]o-DCB = o-dichlorobenzene, AN = acetonitrile, tol = toluene, R = C6H3Cl2 or CH2CN).

The EPR spectroscopy and the measurement of AC magnetic susceptibility (χ), magnetization and of specific heat capacity were used to study magnetic properties of the copolymer (COP) of 3-dodecylthiophene (DDT) and 2,3-R,R-thieno[3,4-b]pyrazine (TTP). The copolymer exhibits antiferromagnetic properties at the Nèel temperature of ca. 38K. At maximum complex, magnetic susceptibility achieved value of 6.5×10−6m3kg−1. That behaviour was ascribed to the heat-activated transitions of bipolarons to polarons, which are stimulated by conformational changes in the polymer chains of semi-crystalline regions existing in the polymer.

Free radical scavenging activities of wine samples were investigated by means of electron paramagnetic resonance (EPR) spin trapping. 1,1-Diphenyl-2-picrylhydrazyl served as a source of free radicals, OH radicals were produced by the Fenton system, and in situ thermally generated SO 4 − radicals from K 2 S 2 O 8 and carbon-centered radicals from azo-compounds were also used. 5,5-Dimethylpyrroline-N-oxide (DMPO) was used as a spin trapping agent. Three white and four red wine samples from the Slovak wine regions, along with 12% (v/v) aqueous ethanol solution as a reference sample, were compared. In general, the lowest scavenger activity was found in the reference sample, a higher one in the white, and a considerably higher one in the red wines, except when the Fenton system was used as the radical source. Here, unexpectedly, unlike in the other radical sources, the wine samples increased the yield of DMPO-adducts by about 5 times, compared to the reference sample. This is probably due to an additional activation of the Fenton system by wine components. Higher concentrations of Mn 2+ ions as determined by EPR spectroscopy corresponded to an increased scavenging activity of wines.

The photoinduced formation of fullerene anion radicals from mono- and bis-N-methylfulleropyrrolidinium salts in the presence of TiO2 or triethylamine as electron donating substrates was investigated using in situ EPR spectroscopy in various solvent mixtures. The photoreduction yield and stability of anion radicals generated are affected by the solvent properties, such as relative permittivity and hydrogen bond donation ability.

La stabilite de la biere a ete evaluee a l'aide d'une nouvelle methode basee sur les mecanismes radicalaires du vieillissement. Ce dernier a ete accelere a 60° C et la diminution de la stabilite a ete suivie par l'accroissement rapide de la concentration des radicaux a l'aide de capteurs de radicaux au spectrometre RPE. La longueur de la periode d'induction pour la formation des radicaux est une mesure de la stabilite. La methode presentee permet une comparaison rapide et une prediction de la stabilite des echantillons de bieres examines.

Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron transfer between 3,3',5,5'-tetramethylbenzidine (TMB) and C70 in different solvent systems. The changes found in UV-vis spectra pointed at ground state charge transfer complex formation [C70-TMB] in benzonitrile. Upon selective excitation of C70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR singlets were observed, which were assigned to C70 mono- and di-anion. In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals were obtained. C70 mono-anion was investigated also in frozen 1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR spectrum of C70 mono-anion was simulated assuming an axially symmetric g-matrix with gparallel - gperpendicular = 0.00165.

The thermally initiated decomposition of K2S2O8 radical initiator coupled with EPR spin trapping represents a very powerful source of reactive free radicals which can be effectively used to initiate and to monitor various radical processes, as we were able to demonstrate in the characterization of the antioxidant properties of wine samples. The spin trapping agent alpha -phenyl-N-tert-butylnitrone (PBN) is a very effective trap, but with low selectivity. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), which has higher selectivity and a very pronounced dependence of adduct formation on the temperature (the formal activation energy of (DMPO)-D-.-OH formation is 81.1 kJ mol(-1)), offers more advantages. In pure aqueous solutions, formation of the (DMPO)-D-.-OH adduct dominates. In mixed water-ethanol solvents, carbon-centred DMPO radical adducts are increasingly evident with higher CH3CH2OH ratios, and at very high ethanol ratios the formation of (DMPO)-D-.-OOR adduct prevails. Copyright (C) 2000 John Wiley & Sons, Ltd.

The beer stability was investigated using a newly developed method based on the radical mechanisms of the deterioration processes. The deterioration is accelerated at 60 degrees C and the breakdown of the stability is indicated with a quick increase of radical concentration, which is followed by means of spin trapping agents in an EPR spectrometer. The induction period in radical formation is a criterion for the antioxidant activity and flavour stability. The method enables to obtain a rapid comparison and prediction of the stability of the investigated beer samples.

Fluorenylidene-linked triarylamines, potential hole-transporting materials, have been prepared by the palladium-catalyzed Hartwig-Buchwald amination. Their redox and spectral properties were investigated in solution, applying cyclic voltammetry, UV-VIS and EPR spectroscopy, and in situ spectroelectrochemical measurements. N,N,N',N'-Tetraphenylbenzidine (1), N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (2), and triphenylamine (3) served as model substances in the study of the synthesized complex compounds 4 and 5. In structure 4, two triphenylamine centres are linked with a non-conjugated fluorene bridge; in structure 5, two tetraarylbenzidine skeletons with two nitrogens are linked with a conjugated biphenyl-bridge system. In addition, structure 5 contains a non-conjugated fluorene bridge. The presence of the fluorene moiety in the molecular design has a significant influence on the investigated properties of the new materials. In the anodic oxidation of the tetraarylbenzidine-type compounds (1, 2, and 5), two well-defined reversible oxidation peaks were observed. However, the oxidation of the triphenylamine-type structures (3 and 4) is more complex, due to fast consecutive reactions. The dimer-like structures (4 and 5) are characterized by two independent oxidation centres that are simultaneously oxidized at approximately the same potentials. This was confirmed by quantitative cyclovoltammetric as well as UV-VIS investigations.

Pri charakterizácii stability vzoriek piva sa dobre osvedčila metóda spinového lapača PBN, ktorý po určitej indukčnej perióde pri vynútenom tepelnom namáhaní vzorky piva pri 60 °C, indikuje zlom v stabilite prudkým nárastom radikálového aduktu PBN-X. Dĺžka tejto indukčnej doby je mierou stability piva. Ukázalo sa, že dominantný vplyv na priebeh starnutia má prítomnosť kyslíka. Zohľadnením dátumu výroby, doby deklarovanej trvanlivosti, dátumu merania ako aj určenej doby trvania termickej stability sme vypracovali postup grafického zobrazenia, ktorý umožňuje vzájomné porovnanie a predpoveď stability jednotlivých vzoriek.

The photoinduced electron transfer to water-soluble mono-adducts and various bis-functionalized C60 derivatives I–VI (see Scheme 1 in the text) were investigated by EPR spectroscopy in heterogeneous water/2-propanol TiO2 system, and also in aqueous solutions containing l-ascorbic acid as an electron donating substrate. Irradiated TiO2 suspensions of C60C(COONa)2/γ-CD and C60(C4H10N+)/γ-CD yielded an EPR signal A, characterized by gA=2.0000 and peak-to-peak width, ppA=0.095 mT, assigned to the fullerenes' mono-anion. The latter transforms with continuing irradiation into radicals B and C (gB=2.0005, gC=2.0009). In contrast, the three water-soluble anionic bis-adducts (IV–VI) gave rise to only a single narrow mono-anion EPR line M (ppM<0.02 mT) positioned at gM=2.0007.