Photochemistry of 6-chloro and 6-bromopicolinic acids in water. Heterolytic vs. homolytic photodehalogenation.

The photochemistry of 6-chloro and 6-bromopicolinate ions ( and , respectively) was investigated by product studies and ns laser flash photolysis (LFP). In deoxygenated pH 5.4 water, yields 6-hydroxypicolinic acid (70%) and a substituted pyrrole. In 2-propanol-water (1 : 1) mixture, the reaction yields, very unselectively, 6-hydroxypicolinic acid, 2-carboxypyridine, pyridine and bipyridines. Photolysis of aqueous leads to 6-hydroxypicolinic acid (78%) and hydroxybipyridines. Oxygen suppresses the photolysis of but does not affect that of . By LFP, we detected a short-lived transient at the pulse end from (lambda(max)= 305 nm, k=(2.8 +/- 0.2)x 10(5) s(-1), epsilonphi= 2200 +/- 200 dm3 mol(-1) cm(-1)). This is quenched either by oxygen or methyl acrylate and thus assigned to the triplet excited state. The triplet excited state of is detected at pH 1 only (lambda(max)= 320 nm, k > 3 x 10(7) s(-1)). The radical ion Cl2- could be successfully detected by photolysing in 2-propanol-water (1 : 1) in the presence of Cl-. Similarly, Br2- could be detected by irradiating aqueous in the presence of Br-. These results show that the photodehalogenation of is heterolytic in water and mainly homolytic in 2-propanol-water mixtures while that of is both heterolytic and homolytic in water. A mechanism in which the triplet excited state undergoes homolysis of the C-X bond and subsequent electron transfer from the carboxypyridyl radical to the halogen atom to form an ion pair may account for these observations.     

Stepwise two-color laser photolysis studies of alpha-cleavage in highly excited triplet states of alpha-acyl-4-phenylphenols.

The photochemical properties of alpha-cleavage of C-O bond in highly excited triplet states (T(n) with n>2) of p-biphenyl acetate and p-biphenyl benzoate (Me-OBP and Ph-OBP) in solution were investigated in comparison with those in the lowest excited singlet and triplet states by using single laser and sequential two-color two-laser photolysis techniques. Upon 266 nm laser photolysis of Me-OBP and Ph-OBP, occurrence of C-O bond cleavage in the excited singlet state was recognized from the observation of the formation of p-phenylphenoxy radical (PPR) in the transient absorption. The quantum yields (Phi(rad)) of the PPR formation were determined to be 0.29 and 0.24 for Me-OBP and Ph-OBP, respectively. Triplet sensitization using acetone (Ac) provided efficient formation of the lowest triplet states (T(1)) of Me-OBP and Ph-OBP, and the molar absorption coefficients of the triplet-triplet absorption were determined. No photochemical reactions were found in the T(1) state. Upon 355 nm laser flash photolysis of the T(1) states of Me-OBP and Ph-OBP, formation of PPR accompanied with decomposition of the triplet state was confirmed in the transient absorption. This observation indicated that alpha-cleavage proceeds in the highly excited triplet state. The quantum yields (Phi(dec)) of the decomposition in the dissociative highly excited triplet state (T(R)) were determined to be 0.25 and 0.15 for Me-OBP and Ph-OBP, respectively. The reaction mechanism for alpha-bond cleavage in the T(R) state was discussed.     

Oxygen uptake induced by electron transfer from donors to the triplet state of methylene blue and xanthene dyes in air-saturated aqueous solution.

The effects of oxygen in the photolysis of rose bengal, eosin, erythrosin and methylene blue were studied in the presence of formate and electron donors, such as ascorbic acid, aromatic amino acids or aliphatic amines, e.g. triethylamine (TEA). The overall reaction is conversion of oxygen via the hydroperoxyl/superoxide ion radical into hydrogen peroxide. The quantum yield of oxygen uptake (Phi(-O2)) increases with the donor concentration. The photoinduced formation of H2O2 is initiated by quenching of the triplet state of the dye by the donor and subsequent reactions of both the dye and donor radicals with oxygen. For methylene blue and the xanthene dyes in the presence of 10 mM ascorbic acid or 0.1 M TEA Phi(-O2)=0.07-0.25. The spectral and kinetic properties of the specific dye transients, including the radicals involved and the pH and concentration dependences, are discussed.     

Photoinduced oxygen uptake for 9,10-anthraquinone in air-saturated aqueous acetonitrile in the presence of formate, alcohols, ascorbic acid or amines.

The photolysis of 9,10-anthraquinone (AQ), 2-methyl- and 2,3-dimethyl-AQ was studied in air-saturated acetonitrile-water in the presence of various donors: formate, ascorbic acid, alcohols, e.g. 2-propanol or methanol, and amines, e.g. ethylenediaminetetraacetate (EDTA). The photoreaction is initiated by H-atom or electron transfer from the donor to the AQ triplet state. The conversion of oxygen into hydrogen peroxide occurs via the superoxide radical and its conjugate acid. The quantum yield of oxygen uptake (Phi(-O2)) increases with increasing donor concentration. Phi(-O2) = 0.3-0.6 in the presence of 1 M 2-propanol and 3-10 mM ascorbic acid or EDTA. The properties of the quinone and donor radicals involved and the pH and concentration dependences of Phi(-O2) are described.     

Intramolecular and intermolecular hydrogen-bonding effects on photophysical properties of 2'-aminoacetophenone and its derivatives in solution.

Effects of intra- and intermolecular hydrogen-bonds on the photophysical properties of 2'-aminoacetophenone derivatives (X-C6H4-COCH3) having a substituted amino group (X) with different hydrogen-bonding ability to the carbonyl oxygen (X: NH2(AAP), NHCH3(MAAP), N(CH3)2(DMAAP), NHCOCH3(AAAP), NHCOCF3(TFAAP)) are investigated by means of steady-state and time-resolved fluorescence spectroscopy and time-resolved thermal lensing. Based on the photophysical parameters obtained in aprotic solvents with different polarity and protic solvents with different hydrogen-bonding ability, the characteristic photophysical behavior of the 2'-aminoacetophenone derivatives is discussed in terms of hydrogen-bonding and n,pi*-pi,pi* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k(ic)= 1.0 x 10(11) s(-1) for AAP and 3.9 x 10(10) s(-1) for MAAP in n-hexane). The internal conversion rates of both compounds decrease markedly with increasing solvent polarity, suggesting that vibronic interactions between close-lying S1(pi,pi*) and S2(n,pi*) states lead to the large increase in the non-radiative decay rate of the lowest excited singlet state. It is also suggested that for MAAP, which has a stronger hydrogen-bond as compared to AAP, an intramolecular hydrogen-bonding induced deactivation is involved in the dissipation of the S1 state. For DMAAP, which cannot possess an intramolecular hydrogen-bond, the primary relaxation mechanism of the S1 state in nonpolar aprotic solvents is the intersystem crossing to the triplet state, whereas in protic solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. In contrast, the fluorescence spectra of AAAP and TFAAP, which have an amino group with a much stronger hydrogen-bonding ability, give strongly Stokes-shifted fluorescence, indicating that these compounds undergo excited-state intramolecular proton transfer reaction upon electronic excitation.     

Triplet and ground state potential energy surfaces of 1,4-diphenyl-1,3-butadiene: theory and experiment.

Relative energies of the ground state isomers of 1,4-diphenyl-1,3-butadiene (DPB) are determined from the temperature dependence of equilibrium isomer compositions obtained with the use of diphenyl diselenide as catalyst. Temperature and concentration effects on photostationary states and isomerization quantum yields with biacetyl or fluorenone as triplet sensitizers with or without the presence of O(2), lead to significant modification of the proposed DPB triplet potential energy surface. Quantum yields for ct-DPB formation from tt-DPB increase with [tt-DPB] revealing a quantum chain process in the tt --> ct direction, as had been observed for the ct --> tt direction, and suggesting an energy minimum at the (3)ct* geometry. They confirm the presence of planar and twisted isomeric triplets in equilibrium (K), with energy transfer from planar or quasi-planar geometries (quantum chain events from tt and ct triplets) and unimolecular decay (k(d)) from twisted geometries. Starting from cc-DPB, varphi(cc-->tt) increases with increasing [cc-DPB] whereas varphi(cc-->ct) is relatively insensitive to concentration changes. The concentration and temperature dependencies of the decay rate constants of DPB triplets in cyclohexane are consistent with the mechanism deduced from the photoisomerization quantum yields. The experimental DeltaH between (3)tt-DPB* and (3)tp-DPB*, 2.7 kcal mol(-1), is compared with the calculated energy difference [DFT with B3LYP/6-31+G(d,p) basis set]. Use of the calculated DeltaS = 4.04 eu between the two triplets gives k(d) = (2.4-6.4) x 10(7) s(-1), close to 1.70 x 10(7) s(-1), the value for twisted stilbene triplet decay. Experimental and calculated relative energies of DPB isomers on the ground and triplet state surfaces agree and theory is relied upon to deduce structural characteristics of the equilibrated conformers in the DPB triplet state.     

Reactivity of 2',7'-dichlorodihydrofluorescein and dihydrorhodamine 123 and their oxidized forms toward carbonate, nitrogen dioxide, and hydroxyl radicals

The aim of this study was to investigate the oxidation of two common fluorescent probes, dichlorodihydrofluorescein (DCFH2) and dihydrorhodamine (DHR), and their oxidized forms, dichlorofluorescein and rhodamine, by the radical products of peroxynitrite chemistry, *OH, NO2*, and CO3*-. At pH 8.0-8.2, rate constants for the interaction of carbonate radical with probes were estimated to be 2.6 x 10(8) x M(-1) s(-1) for DCFH2 and 6.7 x 10(8) M(-1) s(-1) for DHR. Nitrogen dioxide interacted more slowly than carbonate radical with these probes: the rate constant for the interaction between NO2* and DCFH2 was estimated as 1.3 x 10(7) M(-1) s(-1). Oxidation of DHR by nitrogen dioxide led to the production of rhodamine, but the kinetics of these reactions were complex. Hydroxyl radical interacted with both probes with rate constants close to the diffusion-controlled limit. We also found that oxidized forms of these fluorescent probes reacted rapidly with carbonate, nitrogen dioxide, and hydroxyl radicals. These data suggest that probe oxidation may often be in competition with reaction of the radicals with cellular antioxidants.     

Electrochemiluminescence determination of pipemidic acid using sulfite as energy transfer mediator

An electrochemiluminescence (ECL) based on energy transfer from electro-generated triplet sulfur dioxide to pipemidic acid (PPA) was studied. A weak ECL from triplet sulfur dioxide (3)SO2 * was observed when sulfite was electrochemically oxidized in sulfuric acid solution on a Pt electrode. When PPA was present, the weak ECL was enhanced. The enhanced ECL was attributed to energy transfer from (3)SO2 * to PPA. Based on the enhanced ECL, a flow-injection (FIA) ECL method for the determination of PPA was proposed. The proposed method allowed the measurement of PPA over the range of 1.0x10(-7) to 2.0x10(-5)moll(-1). The detection limit was 3.9x10(-8)moll(-1), and the relative standard deviation for 1.0x10(-6)moll(-1) PPA (n=9) was 1.3%. This method was evaluated by the analysis of PPA in pharmaceutical preparations and urine samples.     

Antioxidant properties of melatonin: a pulse radiolysis study

Various one-electron oxidants such as OH*, tert-BuO*, CCl3OO*, Br2*- and N3*, generated pulse radiolytically in aqueous solutions at pH 7, were scavenged by melatonin to form two main absorption bands with lambda(max) = 335 nm and 500 nm. The assignment of the spectra and determination of extinction coefficients of the transients have been reported. Rate constants for the formation of these species ranged from 0.6-12.5x10(9) dm3 mol(-1) s(-1). These transients decayed by second order, as observed in the case of Br2*- and N3* radical reactions. Both the NO2* and NO* radicals react with the substrate with k = 0.37x10(7) and 3x10(7) dm3 mol(-1) s(-1), respectively. At pH approximately 2.5, the protonated form of the transient is formed due to the reaction of Br2*- radical with melatonin, pKa ( MelH* <=> Mel* + H+) = 4.7+/-0.1. Reduction potential of the couple (Mel*/MelH), determined both by cyclic voltammetric and pulse-radiolytic techniques, gave a value E(1)7 = 0.95+/-0.02 V vs. NHE. Repair of guanosine radical and regeneration of melatonin radicals by ascorbate and urate ions at pH 7 have been reported. Reactions of the reducing radicals e(aq)- and H* atoms with melatonin have been shown to occur at near diffusion rates.     

Forward (singlet-singlet) and backward (triplet-triplet) energy transfer in a dendrimer with peripheral naphthalene units and a benzophenone core.

The photochemical and photophysical behaviour of two dendrimers consisting of a benzophenone core and branches that contain four (4) and eight (5) naphthalene units at the periphery has been investigated in CH(2)Cl(2) solution (298 K) and in CH(2)Cl(2)/CHCl(3) 1:1 v/v rigid matrix (77 K). For comparison purposes, the photophysical properties of dimethoxybenzophenone (1), 2-methylnaphthalene (2) and of a dendron containing four naphthalene units (3) have also been studied. In both dendrimers 4 and 5, excitation of the peripheral naphthalene units is followed by fast (1.1 x 10(9) s(-1) at 298 K, > 2.5 x 10(9) s(-1) at 77 K for 5; 2.9 x 10(8) s(-1) at 298 K, 7 x 10(5) s(-1) at 77 K for 5) singlet-singlet energy transfer to the benzophenone core. On a longer time scale (>1 x 10(6) s(-1) at 298 K, >6 x 10(3) s(-1) at 77 K for 4; 3.1 x 10(7) s(-1) at 298 K, ca. 3 x 10(2) s(-1) at 77 K for 5) a back energy transfer process takes place from the triplet state of the benzophenone core to the triplet state of the peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet-triplet energy transfer to the peripheral naphthalene units. In hydrogen donating solvents, the benzophenone core is protected from degradation by the presence of the naphthalene units. In solutions containing Tb(CF(3)SO(3))(3), sensitization of the green Tb(3+) luminescence is observed on excitation of both the peripheral naphthalene units and the benzophenone core of 5. Upon excitation of the naphthalene absorption band (266 nm) with a laser source, intradendrimer triplet-triplet annihilation of naphthalene excited states leads to delayed naphthalene fluorescence (lambda(max)= 335 nm), that can also be obtained upon excitation at 355 nm (benzophenone absorption band). The results obtained show that preorganization of photoactive units in a dendritic structure can be exploited for a variety of useful functions, including photosensitized emission, protection from undesired photoreactions, and energy up-conversion.     

Evaluation of absolute configuration of naphthylphenyl-substituted oligosilanes by CD exciton chirality method

Absolute configurations of methylnaphthylphenyl-substituted oligosilanes, MeNpPhSi*SiMeR(1)R(2) [2 (R(1), R(2)=Me), 3 (R(1)=Me, R(2)=Ph), 4 (R(1), R(2)=Ph), and 5 (R(1)=Me, R(2)=SiMe(3))] were predicted by circular dichroism (CD) exciton chirality method. The sigma-pi conjugation effect of oligosilylene units (sigma-linkage) with pi-electron systems caused an intense red-shift of (1)L(a,Ph) transition band of the oligosilanes as shown in UV/VIS and made it possible to observe clear CD exciton chirality between the two aromatic chromophores on chiral silicon atom.     

Remarkably stereoselective photoinduced electron-transfer reaction between zinc myoglobin and optically active binaphthyl bisviologen

We have designed and synthesized new optically active bisviologens ([BNMV](4+)) containing a binaphthyl moiety to examine the stereoselective photoinduced electron-transfer (ET) reactions with zinc-substituted myoglobin (ZnMb) by flash photolysis. The photoexcited triplet state of ZnMb, (3)(ZnMb)*, was successfully quenched by [BNMV](4+) ions to form the radical pair of a ZnMb cation (ZnMb(.+)) and a reduced viologen ([BNMV](.3+)), followed by a thermal ET reaction to the ground state. The rate constants ( k(q)) for the ET quenching at 25 degrees C were obtained as k(q)( R)=(2.9+/-0.2)x10(7) M(-1) s(-1) and k(q)( S)=(2.2+/-0.2)x10(7) M(-1) s(-1), respectively. The ratio of k(q)( R)/ k(q)( S)=1.3 indicates that the ( R)-isomer of the chiral viologen preferentially quenches (3)(ZnMb)*. On the other hand, the rate constants ( k) for the thermal ET reaction from [BNMV](.3+) to ZnMb(-+) at 25 degrees C were k( R)=(1.2+/-0.1)x10(8) M(-1) s(-1) and k( S)=(0.47+/-0.03)x10(8) M(-1) s(-1), respectively, and the ratio remarkably increased to k( R)/ k( S)=2.6. The activation parameters, Delta H(not equal) and Delta S(not equal), were determined from the kinetic measurements at various temperatures (10-30 degrees C) to understand the ET mechanisms. In the quenching reaction, the energy differences of Delta Delta H*(R- S) and T Delta Delta S*( R- S) at 25 degrees C were calculated to be -3.9+/-1.6 and -3.3+/-0.2 kJ mol(-1), respectively, whereas Delta Delta H*( R-S)=7.7+/-1.9 kJ mol(-1 )and T Delta Delta S*( R-S)=9.9+/-0.5 kJ mol(-1 )were found for the thermal ET reaction. Therefore, the thermal ET reaction to the ground state was proved to be dominated by the entropy term, and the large stereoselectivity may arise from the decrease in charge repulsion between donor and acceptor.     

Reaction cavities of liquid and solid phases of a long n-alkane, n-nonadecane, as probed by the regio- and stereo-chemical fates of two singlet radical pairs.

The regio- and stereo-chemistries of photo-Fries reactions of 1-naphthyl (R)-2-phenylpropanoate ((R)-1) and photo-Claisen reactions of 1-naphthyl (R)-1-phenylethyl ether ((R)-2) have been investigated in the liquid and solid phases of n-nonadecane. The results are compared with those obtained in a lower and higher viscosity liquid n-alkane. Irradiation of (R)-1 yields a prochiral singlet 1-naphthoxy-1-phenylethyl radical pair (radical pair B) indirectly, after decarbonylation of the initially formed chiral singlet 1-naphthoxy-2-phenylpropanoyl radical pair (radical pair A). Radical pair B is generated directly upon lysis of the excited singlet state of (R)-2. The restricted environments of the solid phases enable both the directly and indirectly formed radical pairs to maintain much (but not all) of the memory of their initial orientations. Thus, the in-cage combination products from indirectly formed radical pair B contain up to 33% enantiomeric excess. However, the combinations of directly formed radical pair B lack complete stereo-specificity due to their disturbing influence on their local environments in the solid phases. A discussion of the dynamics of the radical pair motions in the different environments is presented.     

Pulse radiolysis studies of beta-carotene in oxygenated DMSO solution. Formation of beta-carotene radical cation

The spectroscopic and kinetic characteristics of beta-carotene radical cation (beta-carotene(.+)) were studied by pulse radiolysis in aerated DMSO solution. The buildup of beta-carotene(.+) with k(1) = (4.8 +/- 0.2) x 10(8) dm(3) mol(-1) s(-1) [lambda(max) = 942 nm, epsilon = (1.6 +/- 0.1) x 10(4) dm(3) mol(-1) cm(-1)] results from an electron transfer from beta-carotene to DMSO(.+). The beta-carotene(.+) species decays exclusively by first-order reaction, k = (2.1 +/- 0.1) x 10(3) s(-1), probably by two processes: (1) at low substrate concentration by hydrolysis and (2) at high concentrations also by formation of dimer radical cation (beta-carotene)(2)(.+). Under the experimental conditions, a small additional beta-carotene triplet-state absorption ((3)beta-carotene) in the range of 525 to 660 nm was observed. This triplet absorption is quenched by oxygen (k = 7 x 10(4) s(-1)), resulting in singlet oxygen ((1)O(2)), whose reactions can also lead to additional formation of beta-carotene(.+).     

Antioxidant effect of probucol on RO2*/O2(*-)-induced peroxidation of human low-density lipoproteins

This study was designed to evaluate the antioxidant effect of probucol on peroxidation of low-density lipoproteins (LDLs) initiated by oxygenated free radicals (O2*-) and ethanol-derived peroxyl radicals (RO2*) generated by gamma radiolysis. Initial radiolytic yields related to the markers of lipid peroxidation [i.e. decrease in endogenous alpha-tocopherol, formation of thiobarbituric acid-reactive substances (TBARS) and conjugated dienes] were determined as a function of LDL concentration (1.5 and 3 g l(-1), expressed as total LDL) and in the absence or the presence of probucol at different concentrations (2.3 x 10(-6), 3.5 x 10(-6), 9 x 10(-6) and 20.5 x 10(-6) mol l(-1)). Our results showed that probucol was able to decrease not only the yields of TBARS and conjugated dienes but also the levels of these peroxidation products obtained at high doses (2500 Gy) compared to LDLs without probucol. Under our conditions, probucol displayed an optimal antioxidant effect for an initial concentration in LDLs equivalent to 15 probucol molecules per LDL particle, which corresponded to a pharmacologically relevant concentration of probucol. Moreover, our data showed that probucol was unable to react with RO2* and thus did not protect LDL vitamin E from free radical attack. In addition, the scavenging capacity of probucol on O2*- appeared to be very poor, and probucol more likely reacted with LDL intermediate radical products. Finally, a very significant steady-state level of probucol remained in LDLs at high doses (up to 2500 Gy), equivalent to at least 40% of the initial concentration of probucol. This addressed the question of a mechanism for regeneration of probucol in LDLs. Our results as a whole suggested that the antioxidant effect of probucol in vivo could not be explained by its scavenging capacity with regard to RO2*/O2*- free radicals.     

Reactions of desferrioxamine with peroxynitrite-derived carbonate and nitrogen dioxide radicals

The iron chelating agent desferrioxamine inhibits peroxynitrite-mediated oxidations and attenuates nitric oxide and oxygen radical-dependent oxidative damage both in vitro and in vivo. The mechanism of protection is independent of iron chelation and has remained elusive over the past decade. Herein, stopped-flow studies revealed that desferrioxamine does not react directly with peroxynitrite. However, addition of peroxynitrite to desferrioxamine in both the absence and the presence of physiological concentrations of CO2 and under excess nitrite led to the formation of a one-electron oxidation product, the desferrioxamine nitroxide radical, consistent with desferrioxamine reacting with the peroxynitrite-derived species carbonate (CO3*-) and nitrogen dioxide (*NO2) radicals. Desferrioxamine inhibited peroxynitrite-dependent free radical-mediated processes, including tyrosine dimerization and nitration, oxyhemoglobin oxidation in the presence of CO2, and peroxynitrite plus carbonate-dependent chemiluminescence. The direct two-electron oxidation of glutathione by peroxynitrite was unaffected by desferrioxamine. The reactions of desferrioxamine with CO3*- and *NO2 were unambiguously confirmed by pulse radiolysis studies, which yielded second-order rate constants of 1.7 x 10(9) and 7.6 x 10(6) M(-1) s(-1), respectively. Desferrioxamine also reacts with tyrosyl radicals with k = 6.3 x 10(6) M(-1) s(-1). However, radical/radical combination reactions between tyrosyl radicals or of tyrosyl radical with *NO2 outcompete the reaction with desferrioxamine and computer-assisted simulations indicate that the inhibition of tyrosine oxidation can be fully explained by scavenging of the peroxynitrite-derived radicals. The results shown herein provide an alternative mechanism to account for some of the biochemical and pharmacological actions of desferrioxamine via reactions with CO3*- and *NO2 radicals.     

Mechanistic insights into C-H activation from radical clock chemistry: oxidation of substituted methylcyclopropanes catalyzed by soluble methane monooxygenase from Methylosinus trichosporium OB3b

The soluble methane monooxygenase (MMO) system isolated from Methylosinus trichosporium OB3b catalyzes the adventitious oxidation of alkyl substituted methylcyclopropanes. If the chemical mechanism of C-H activation by MMO involves formation of a radical or carbocation intermediate at the methyl C-H of these 'radical clock' substrates, then cyclopropyl ring opened alcohols may appear in the product mixture due to rearrangement of the intermediate. The lifetime of radical intermediates can be determined from known rearrangement rate constants, k(r). Rearrangement was observed during the oxidation of 1,1,2,2-tetramethylcyclopropane (k(r)=1.7-17. 5x10(8) s(-1), 30 degrees C) but not for cis- or trans-1, 2-dimethylcyclopropane (k(r)=1.2-6.4x10(8) s(-1), 30 degrees C) or the very fast radical clock, trans-2-phenylmethylcyclopropane (k(r)=3.4x10(11) s(-1), 30 degrees C). The results show that the occurrence of rearranged products fails to correlate with either the chemical nature of the C-H bond being broken, which is very similar for all of the methylcyclopropanes studied here, or the magnitude of the radical k(r) value. This study suggests that the steric properties of the substrate play an important role in determining the outcome of the reaction. Substrates with bulky substituents near the C-H bond that is attacked appear to yield intermediates with sufficient lifetimes to rearrange. In contrast, substrates with less steric bulk are postulated to be able to approach the reactive oxygen species in the MMO active site more closely so that intermediates are either rapidly quenched or undergo subsequent interaction with the dinuclear iron cluster of MMO that prevents rearrangement.     

A theoretical study of the dioxygen activation by glucose oxidase and copper amine oxidase

Glucose oxidase (GO) and copper amine oxidase (CAO) catalyze the reduction of molecular oxygen to hydrogen peroxide. If a closed-shell cofactor (like FADH(2) in GO and topaquinone (TPQ) in CAO) is electron donor in dioxygen reduction, the formation of a closed-shell species (H(2)O(2)) is a spin forbidden process. Both in GO and CAO, formation of a superoxide ion that leads to the creation of a radical pair is experimentally suggested to be the rate-limiting step in the dioxygen reduction process. The present density functional theory (DFT) studies suggest that in GO, the creation of the radical pair induces a spin transition by spin orbit coupling (SOC) in O(2)(-)(rad), whereas in CAO, it is induced by exchange interaction with the paramagnetic metal ion (Cu(II)). In the rate-limiting step, this spin-transition is suggested to transform the O(2)(-)(rad)-FADH(2)(+)(rad) radical pair in GO and the Cu(II)-TPQ (triplet) species in CAO, from a triplet (T) to a singlet (S) state. For CAO, a mechanism for the O[bond]O cleavage step in the biogenesis of TPQ is also suggested.     

Photoreactions of p-benzo-, p-naphtho- and p-anthraquinones with ascorbic acid.

The photoreduction of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives, e.g. dimethylBQ, trimethylBQ, duroquinone, bromoNQ, methoxyNQ, methylAQ and dimethylAQ in acetonitrile-water by ascorbate was studied by time-resolved UV-vis spectroscopy using 20 ns laser pulses at 308 nm and continuous 254 nm irradiation. The semiquinone radical (*QH/Q*(-)) is formed after H-atom transfer from ascorbate to the quinone triplet state. The rate constant for quenching is k(q)=(2-9) x 10(9) M(-1) s(-1). Termination of the radicals takes place in the micros-ms range. The results are compared with those initiated by electron transfer from DABCO under similar conditions, where the k(q) values are similar, but the termination of Q*(-) takes place by electron back transfer not yielding hydroquinones. Specific properties of the quinone triplet state, e.g. self-quenching, nucleophilic water addition and the effects of structure are discussed.