Abstract  Two modified poly(N-carbazole), poly(2-carbazolylethyl methacrylate) (PCEMA) and poly(3-carbazolylpropyl methacrylate) (PCPMA), were synthesized. Doped with electro-optic (EO) chromophore 1-n-butoxyl-2,5-dimethyl-4-(4'-nitrophenylazo)benzene and the photosensitizer 2,4,7-trinitro-9-fluorenone, the EO response and photorefractive (PR) effect were studied experimentally. Due to the shorter spacer between the carbazolyl and the main-chain and thus a higher glass-transition-temperature T-g, PCEMA-based composite exhibited an effective EO coefficient as only a. quarter as that in PCPMA-based composite. However, a slight higher two-beam coupling coefficient and much faster PR response were observed in the former. It was suggested that both the spacer length and the polymerization degree changed the T-g and the number density of traps, which decided the EO and PR responses.

Abstract  A catalyzed synthesis of cyclopropanes and cyclobutanes via radical chemistry is described. The method that generally proceeds in high yields uses epoxides as radical precursors and titanocene(III) complexes as the electron transfer catalysts (see scheme). The key to the success of the transformation is constituted by the chemoselectivity of radical reduction. Electrophilic enol radicals generated through cyclization are reduced rapidly whereas their precursors, nucleophilic alkyl radicals, remain unaffected.

Abstract  A new prenylated coumarin, S-6-[2-(hydroxymethyl)butoxy]-7-hydroxy-4-methyl-2 H-chromen-2-one ( 1), named pavietin, has been isolated from the leaves of an Aesculus pavia genotype along with three known flavonol glycosides, quercetin 3- O-alpha-rhamnoside (quercitrin, 2), quercetin 3- O-alpha-arabinoside ( 3), and isorhamnetin 3- O-alpha-arabinoside (distichin, 4). The chemical structure of compound 1 was determined by chemical and spectroscopic methods, inclusive of UV, MS, and 1D and 2D NMR experiments. It showed appreciable antimicrobial properties against several pathogens, displaying a significant antifungal activity toward one of the main fungal parasites of Aesculus species, Guignardia aesculi. The same biological tests performed with a mixture of flavonoids 2- 4 resulted in weak or no activity. Compound 1 was undetectable in Aesculus hippocastanum, a closely related species lacking resistance to fungal pathogens. The possible role of 1 in plant resistance is discussed.

Abstract  Several techniques have been used to probe polymer end groups. The nitroxide radical trapping technique has been used (i) to show that initiator-derived unsaturated end groups in polymethyl methacrylate can be minimized by using t-hexyl peroxypivalate as the initiator (ii) to predict the end and penultimate groups in acrylonitrile/ethyl vinyl ether copolymer produced by t-butoxyl initiation by analogy with the initiation mechanism (iii) to predict probable end groups in polyacrylonitrile and polystyrene produced by cyanoisopropyl initiation in the presence of adventitious oxygen. NMR techniques have been used to show that the end groups of functionalized oligomers, made from styrene and methacrylonitrile by the addition-fragmentation chain transfer technique with allylic sulphides, conform to the expected structures. (C) 1998 John Wiley & Sons, Ltd.

Abstract  In this paper two hypotheses are tested: (i) the active oxygen species is similar in energetics for all cytochrome P450 (CYP) enzymes and (ii) linear free-energy relationships can be used to evaluate the mechanism of the reaction of these enzymes. A series of intramolecular isotope effects were determined and compared for CYPs 1A2, 2B1, 2C9, 2E1, and P450cam. The results indicate that the isotope effects are very similar for each of these isoforms of P450 and that the first hypothesis is likely to be true. Attempts to establish a linear free-energy relationship were only moderately successful: log Vmax = 0.11sigma+p + 1.73; r2 = 0.588. It was determined, through the use of intermolecular isotope effects, that the rates of hydrogen atom abstraction are masked. Thus, the second hypothesis is found to be false. This is likely to be a general result for CYP reactions, and linear free-energy relationships can only be used to determine the mechanism under very special circumstances. In all cases, the rate-limiting step should be evaluated with isotope effect experiments before any mechanistic conclusions can be drawn. If the intermolecular isotope effects are found to be masked, no mechanistic conclusion can be drawn from the linear free-energy relationship study.

Abstract  Phytochemical studies of the roots of Dorstenia excentrica afforded a diastereoisomer of prandiol having the 2 ' S,1 " S configuration, 4-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxy]-7H-furo[3,2-g][1] 1 benzopyran-7-one, psoralen and 7-hydroxycoumarin. The furocoumarins 5-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxyl]-7H-[3,2-g][1]benzopyran-7-one and bergapten were also present in the roots of D. drakena, while 7-hydroxycoumarin, psoralen and the psoralen dimer were isolated from D. lindeniana. The structure of the psoralen dimer was established by X-ray diffraction analysis. (C) 1999 Elsevier Science Ltd. All rights reserved.

Abstract  The photolysis of the pyridinethiones 1a-d releases oxyl (hydroxyl, isopropyloxyl, tert-butoxyl, and benzoyloxyl) radicals, which damage DNA through strand cleavage and guanine oxidation. Unexpectedly, the N-hydroxy derivative 1a is significantly less effective in the oxidation of cell-free DNA than the alkoxy and benzoyloxy ones 1b-d. A similar trend was observed for the photooxidative consumption of 2'-deoxyguanosine (dG) by these pyridinethiones. Detailed mechanistic investigations have revealed that the guanine oxidation by the pyridinethiones 1b-d is not caused by oxyl radicals but by a type-I photooxidation process through the novel betain intermediate 8, which is formed in small amounts (ca. 5%) during the photolysis of these thiones from the intermediary photoproduct disulfide 3. This photosensitized guanine oxidation is effectively inhibited by the N,N'-dioxide 7, which is produced only in the photolysis of the N-hydroxypyridine-2-thione (1a) and not from the N-alkoxy and N-benzoyloxy derivatives 1b-d. Thus, for the N-hydroxy derivative 1a hydroxyl radicals are the main DNA-damaging species. The N-alkoxy and N-benzoyloxy derivatives 1b-d are more effective DNA-photooxidizing reagents than the N-hydroxypyridinethione 1a, because 1b-d oxidize DNA photocatalytically through sensitization by the betain, while 1a oxidizes DNA mainly through the stoichiometrically photogenerated hydroxyl radicals.

Abstract  A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(•))/k(H)(CumO(•)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(•))/k(H)(CumO(•)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(•) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(•), BnO(•)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.

Abstract  Treatment of the porcine intestinal brush-border membranes with 100 microM ascorbic acid and 10 microM Fe2+ in the presence of various concentrations of tert-butyl hydroperoxide (t-BuOOH) resulted in a marked fluorescence development at 430 nm, depending on the hydroperoxide concentration. This fluorescence formation was closely related to lipid peroxidation of the membranes as assessed by formation of conjugated diene. However there is no linear relation between thiobarbituric acid-reactive substances (TBARS) and fluorescence formation. On the other hand, fluorescence formation in the membranes by treatment with ascorbic acid/Fe2+ or t-BuOOH alone was negligible. The results with antioxidants and radical scavengers suggest that ascorbic acid/Fe2+/t-BuOOH-induced lipid peroxidation of the membranes is mainly due to t-butoxyl and/or t-butyl peroxy radicals. Most TBARS produced during the peroxidation reaction were released from the membranes, but fluorescent products remained in the membrane components. The fluorescence properties of products formed by lipid peroxidation of the membranes were compared with those of products derived from the interaction of malondialdehyde (MDA) or acetaldehyde with the membranes. The fluorescence products in the acetaldehyde-modified membranes also exhibited the emission maximum at 430 nm, while the emission maximum of MDA-modified membranes was 470 nm. The fluorescence intensity of MDA-modified membranes was markedly decreased by treatment with 10 mM NaBH4 but that of the peroxidized or acetaldehyde-modified membranes was enhanced by about two-fold with the treatment. In addition, a pH dependence profile revealed that the fluorescence intensity of the peroxidized or acetaldehyde-modified membranes decreases with increasing pH of the medium, whereas that of MDA-modified ones did not change over the pH range from 5.4 to 8.0. On the basis of these results, the fluorescence properties of products formed in the intestinal brush-border membranes by lipid peroxidation are discussed.

Abstract  The C-H bond dissociation enthalpies (BDEs) of molecules related to naphthalene radical chemistry were investigated using quantum chemistry calculations (CBS-QB3 and CCSD(T*)-F12), and the C-H BDE in 1,2-dihydronaphthalene was determined using time-resolved photoacoustic calorimetry. A thermodynamically consistent data set of the energetics (C-H BDEs and enthalpies of formation) of the molecules involved (naphthalene, 1- and 2-hydronaphthyl radicals, 1,2- and 1,4-dihydronaphthalene, tetralyl radical, and tetralin) is presented. It was constructed using a thermodynamic cycle based on the simple notion that BDEs represent the difference between two states, bridging accurate experimental enthalpies of formation of the parent compounds with theoretical and experimental C-H BDEs leading to, and from, the radicals. (C) 2013 Elsevier Ltd. All rights reserved.

Abstract  A time-resolved kinetic study on the reactions of the tert-butoxyl (t-BuO*), cumyloxyl (CumO*), and benzyloxyl (BnO*) radicals with alkylferrocenes has been carried out in MeCN solution. With all radicals, clear evidence for an electron transfer (ET) process has been obtained, and with the same ferrocene donor, the reactivity has been observed to increase in the order t-BuO* < CumO* < BnO*, with the difference in reactivity approaching 3 orders of magnitude on going from t-BuO* to BnO*. With BnO*, an excellent fit to the Marcus equation has been obtained, from which a value of the reduction potential of BnO* (E degrees(BnO*/BnO(-)) = 0.54 V/SCE) has been derived. The latter value appears, however, to be significantly higher than the previously determined reduction potential values for alkoxyl radicals and in contrast with the differences in the computed solution-phase electron affinities determined for t-BuO*, CumO*, and BnO*, indicating that the reaction of BnO* with ferrocene donors may not be described in terms of a straightforward outer sphere ET mechanism. From these data, and taking into account the available value of the reduction potential for CumO*, a value of E degrees (BnO*/BnO(-)) = -0.10 V/SCE has been estimated. On the basis of computational evidence for the formation of a pi-stacked prereaction complex in the reaction between BnO* and DcMFc, an alternative ET mechanism is proposed for the reactions of both CumO* and BnO*. In these cases, the delocalized nature of the unpaired electron allows for the aromatic ring to act as an electron relay by mediating the ET from the ferrocene donor to the formal oxygen radical center. This hypothesis is also in line with the observation that both BnO* and CumO* react with the ferrocene donors with rate constants that are in all cases at least 2 orders of magnitude higher than those measured for t-BuO*, wherein the radical is well-localized.

Abstract  The study was aimed at examining the effects of tert-butyl hydroperoxide (tBHP) on hepatic transcriptome expression patterns of the teleost fish Lithognathus mormyrus. tBHP is an organic hydro-peroxide, widely used as a model pro-oxidant. It generates the reactive oxygen species (ROS) tert-butoxyl and tert-butylperoxyl. Complementary DNAs of tBHP-treated vs control fish were applied onto a previously produced cDNA microarray of approximately 1500 unique sequences. The effects of the tBHP application were demonstrated by leukocyte infiltration into the liver and by differential expression of various genes, some already known to be involved in ROS-related responses. Indicator genes of putative ROS effects were: aldehyde dehydrogenase 3A2, Heme oxygenase and the hemopexin-like protein. Putative indicators of transendothelial leukocyte migration and function were: p22phox, Rac1 and CD63-like genes. Interestingly, 7-dehydrocholesterol reductase was significantly down-regulated in response to all treatments. Several non-annotated genes revealed uniform directions of differential expression in response to all treatments.

Abstract  Three novel compounds, together with five known ingredients, octacosanol, 3',4',5-trihydroxy-3,7-dimethoxyflavone, 3,4-dihydroxybenzaldehyde, isorhamnetin, and ent-kaurane-3beta,16beta,17-triol, were obtained from the leaves of Smallanthus sonchifolius (yacon), and their structures were elucidated as ent-kaurane-3beta,16beta,17,18-tertol (1), 3R,7E-9-butoxyl-megastigma-3-ol-3-O-beta-D-glucopyranoside (2), and 3S,5R,6Z-megastigma-6-en-3,5,8,9-tertol (3) on the basis of spectroscopic and chemical methods.

Abstract  Theoretical models have been used to derive rate coefficients for the unimolecular reaction pathways of two prototypical alkoxyl radicals (1-butoxyl and 2-pentoxyl) which can undergo 1,5 H-shift isomerisation yielding the corresponding delta-hydroxy alkyl radical. Special emphasis has been given to the contribution of tunnelling in the isomerisation channels which has not been accounted for in previous theoretical studies. The combination of high level ab initio calculations with a fully coupled multiple channel master equation (ME) treatment resulted in a significant increase of the isomerisation rates by about a factor of 2.7 for the 1-butoxyl and 2.3 for the 2-pentoxyl radical, respectively, as compared to calculations in which tunnelling was neglected, even at 298 K. The corresponding Arrhenius energies in the temperature range from 200 up to 1000 K are significantly smaller when tunnelling is accounted for and differ from previous results which focused only on temperatures around 298 K. The rate expressions derived for the 1,5 H-shift isomerisation reactions are: k(iso,1but) = 1.58 x 10(12)(T/300 K)(-2.30) exp(-4679 K/T) s(-1) and k(iso,2pent) = 4.65 x 10(12)(T/300 K)(-3.22) exp(-4782 K/T) s(-1) valid for p = 1013 mbar and temperatures between 200 K < or = T < or = 1000 K. Our results are strongly supported by recent experiments of Cox and co-workers, (D. Johnson, P. Cassanelli, and R. A. Cox, J. Phys. Chem. A, , 2004, 108, 519 and P. Cassanelli, D. Johnson, and R. A. Cox, Phys. Chem. Chem. Phys. 2005, 7, 3702, 18) which are among the very few studies performed in a temperature regime (250 < or = T < or = 320 K) where tunnelling is thought to be effective. Since tunnelling has been neglected so far in the theoretical analysis of experimental data, the reliability of existing extrapolations to higher and lower temperatures is discussed in detail.

Abstract  Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap, alkoxyl radicals and peroxyl radicals produced from the reactions of tert-butyl hydroperoxide(tBuOOH) and cumene hydroperoxide (PhC(CH3)2OOH) with some copper(Cu)(II) complexes such as Cu(II) complexes of cimetidine (Cim), cyclo(L-histidyl-L-histidyl) (CyHH), L-histidylglycine (HG), and L-histidylglycylglycine (HGG) were detected by electron spin resonance (ESR) spectroscopy. However, Cu(II) complexes of glycyl-L-histidine (GH), glycyl-L-histidylglycine (GHG),glycylglycyl-L-histidine (GGH), and glycylglycyl-L-histidyl-glycine (GGHG) did not cause the generation of free radicals during the reaction with tert-butyl or cumene hydroperoxide. Addition of a biological reductant such as cysteine or glutathione to the system including these Cu(II) complexes and hydroperoxides gave tert-butoxyl and cumyl alkoxyl (RO.) radicals, respectively. These alkoxyl radicals underwent subsequent beta-scission reaction and generated the carbon-centered radical (R.). Although cysteine and glutathione are considered to be cellular antioxidants, our results suggest that these biological reductants facilitate Cu(II) complexes-dependent free radical generation.

Abstract  Solvent effects on the AIBN and ACVA forced degradation of cumene are explored. The degradant formation rates of the three cumene oxidative degradants, cumene hydroperoxide, acetophenone, and 2-phenyl-2-propanol are reported. The relative abundance and ratios of these three degradants provide insight into the fate of the peroxy radical oxidants generated by the forced stress system, and suggest that alkoxy radicals are actually a significant source of the observed reactivity. The presence of even 1% methanol in the forced stress solvent significantly quenches this alkoxy radical reactivity, dramatically reducing the overall degradation rate and leaving cumene hydroperoxide as the major product of the oxidation reaction. The origin of this significant solvent effect on the oxidation product distribution is shown to be related to the preferential H-atom abstraction from methanol and its trace impurities by any alkoxy radicals present in the reaction solution. The implications for these observations are explored with the intent of producing more predictive oxidative forced stress experiments.

Abstract  Laser flash photolysis and competitive techniques have been employed to study the reactions of alpha-tocopherol with various radicals and ketone triplets in solution. For example benzophenone triplets abstract hydrogen with rate constants of 5.1 x 10(9) and 3.7 x 10(9) M-1 s-1 in benzene and benzene/1.3 M methanol. Similar near-diffusion-controlled values were obtained for several other ketone triplets, as well as tert-butoxyl and 4-methoxybenzoyloxyl radicals. Deuterium kinetic isotope effects are frequently very small, reflecting the expected lack of selectivity of fast reactions. The reactivity of the 5-hexenyl radical toward alpha-tocopherol was examined by studying the competition of this process with the radical cyclization to the cyclopentylmethyl radical. The value of (1.7 +/- 0.2) x 10(6) M-1 s-1 (at 70-degrees-C in benzene) for this hydrogen atom abstraction from alpha-tocopherol makes this process exceptionally fast in comparison with the limited available rate data for reactions of carbon-centered radicals with other phenols.