Product and mechanistic analysis of the reactivity of a C6-pyrimidine radical in RNA

Nucleobase radicals are the major reactive intermediates produced when hydroxyl radical reacts with nucleic acids. 5,6-Dihydrouridin-6-yl radical (1) was independently generated from a ketone precursor via Norrish Type I photocleavage in a dinucleotide, single-stranded, and double-stranded RNA. This radical is a model of the major hydroxyl radical adduct of uridine. Tandem lesions resulting from addition of the peroxyl radical derived from 1 to the 5'-adjacent nucleotide are observed by ESI-MS. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and at the initial site of generation. The preference for cleavage at these two positions depends upon the secondary structure of the RNA and whether O(2) is present or not. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. In general, strand scission is significantly more efficient under anaerobic conditions than when O(2) is present. Strand scission is more than twice as efficient in double-stranded RNA than in a single-stranded oligonucleotide under anaerobic conditions. Internucleotidyl strand scission occurs via β-fragmentation following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical ultimately yields products containing 3'-phosphate or 3'-deoxy-2'-ketouridine termini. These end groups are proposed to result from competing deprotonation pathways. The dependence of strand scission efficiency from 1 on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions.     

Characterization and mechanism of formation of tandem lesions in DNA by a nucleobase peroxyl radical

5,6-Dihydro-2'-deoxyuridin-6-yl (1) was independently generated via photolysis of 3. The radical is an analogue of the major reactive species produced from thymidine upon reaction with hydroxyl radical, which is the dominant DNA-damaging agent produced by the indirect effect of gamma-radiolysis. Under aerobic conditions, the peroxyl radical (2) derived from 1 reacts approximately 82% of the time with either the 5'- or 3'-adjacent nucleotide to produce two contiguously damaged nucleotides, known as tandem lesions. The structures and distribution of tandem lesions were investigated using probes that selectively detect abasic sites, ESI-MS/MS, and competition kinetics. In addition to 2-deoxyribonolactone, nonoxidized abasic sites were detected. 18O-Labeling verified that H2O was the source of oxygen in the abasic sites, but that O2 was the source of the oxygen in the 5,6-dihydro-6-hydroxy-2'-deoxyuridine derived from 2. ESI-MS/MS experiments, in conjunction with isotopic labeling, identified several products and provided direct evidence for peroxyl radical addition to the adjacent thymine bases. Kinetic studies revealed that peroxyl radical addition to the 5'-thymine was favored by approximately 4-5-fold over C1'-hydrogen atom abstraction from the respective deoxyribose ring, and that 2-deoxyribonolactone formation accounts for approximately 11% of the total amount of tandem lesions produced. These results suggest that tandem lesions, whose biochemical effects are largely unknown, constitute a major family of DNA damage products produced by the indirect effect of gamma-radiolysis.     

Evidence for glycosidic bond rotation in a nucleobase peroxyl radical and its effect on tandem lesion formation

Nucleobase peroxyl radicals are the major reactive intermediates formed in DNA when the biopolymer is exposed to gamma-radiolysis under aerobic conditions. The major reaction pathways for the peroxyl radical (1) derived from 5,6-dihydro-2'-deoxyuridin-6-yl involve pi-bond addition to or hydrogen atom abstraction from the adjacent nucleotides to produce tandem lesions. The ability to independently generate 1 at a defined site in DNA enabled us to probe its reactivity by varying the local DNA structure. The effect of DNA structure variation reveals that 1 reacts from its syn- and anti-conformations in competition with trapping by thiol. These experiments also reveal that tandem lesions will be produced as a mixture of diastereomers, which could impact their biological effects.     

Independent generation and study of 5,6-Dihydro-2'-deoxyuridin-6-yl,a member of the major family of reactive intermediates formed in DNA from the effects of gamma-radiolysis

Nucleobase radicals are the major family of reactive intermediates formed when nucleic acids are exposed to gamma-radiolysis. Elucidation of their reactivity is complicated by the formation of multiple species randomly throughout the biopolymers. 5,6-Dihydro-2'-deoxyuridin-6-yl (1) was generated upon photolysis (350 nm) of the respective tert-butyl ketone (2). The radical abstracts hydrogen atoms from beta-mercaptoethanol (k = 8.8 +/- 0.5 x 10(6) M(-)(1) s(-)(1)) and 2,5-dimethyltetrahydrofuran (k = 31 +/- 2.5 M(-)(1) s(-)(1)). The latter was used as a model for the 2-deoxyribose component of DNA. The major product formed in the presence of O(2) was 6-hydroxy-5,6-dihydro-2'-deoxyuridine (11), which is believed to be formed directly from the peroxy precursor and not via elimination of superoxide. Small amounts of 2-deoxyribonolactone (13) were also formed under aerobic conditions. This product is believed to result from intramolecular hydrogen atom abstraction by the C6-peroxyl radical (14) and suggests that gamma-radiolysis may indirectly result in oxidation of the C1'-position of nucleotides, despite the inaccessibility of this hydrogen in duplex DNA.     

Acid catalyzed multiple chain termination by quinone monoanilide in oxidizing hydrocarbons

A novel ternary system that causes multiple chain termination in oxidizing hydrocarbons is suggested. The system involvesN-phenylquinone imine, hydrogen peroxide, and citric acid. The inhibiting effect of the system is studied for the initiated oxidation of methyl oleate and ethylbenzene. The rate of the inhibiting oxidation of the hydrocarbon is proportional to the initiation rate and inversely proportional to the product of the concentrations of quinone imine, hydrogen peroxide, and the acid. The mechanism proposed involves the protonation of quinone imine, the abstraction of an H atom from quinone imine by the peroxyl radical, the reduction of the resulting radical cation by hydrogen peroxide to form the semiquinone radical, and the reaction of the latter with RO₂.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 79–82, January, 1995.     

Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines(1)

Hydrogen abstraction from diarylamines (4-X-C(6)H(4))(2)NH [X = H, CH(3), C(8)H(17), CH(3)O, and Br] by the 2-methyl-2-phenylpropyl radical in n-dodecane solution was investigated by thermolysis of 3-methyl-3-phenylbutanoyl peroxide in the presence of various concentrations of the amines. The reaction is a non-chain process in which the 2-methyl-2-phenylpropyl radical and its rearrangement product, the 2-benzylpropan-2-yl radical, abstract hydrogen from both the solvent and the amine. Cross-disproportionation reactions of the rearranged radical led to the formation of significant amounts of beta,beta-dimethylstyrene. Rate constants for hydrogen abstraction by the unrearranged, primary alkyl radical from n-dodecane (k(373K) = 3.5 x 10(3) M(-)(1) s(-)(1)), diphenylamine (k(373K) = 1.3 x 10(6) M(-)(1) s(-)(1)), and the substituted diarylamines were determined from the product yields and the known rate constant for the radical rearrangement. From kinetic experiments with N-deuteriodiphenylamine the deuterium kinetic isotope effect,k(NH)/k(ND), was found to be 2.3 at 373 K.     

Activation of the sulfhydryl group by Mo centers: kinetics of reaction of benzyl radical with a binuclear Mo(micro-SH)Mo complex and with arene and alkane thiols

This paper provides evidence from kinetic experiments and electronic structure calculations of a significantly reduced S-H bond strength in the Mo(micro-SH)Mo function in the homogeneous catalyst model, CpMo(micro-S)(2)(micro-SH)(2)MoCp (1, Cp = eta(5)-cyclopentadienyl). The reactivity of 1 was explored by determination of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(k(abs)/M(-)(1) s(-)(1)) = (9.07 +/- 0.38) - (3.62 +/- 0.58)/theta) for comparison with expressions for CH(3)(CH(2))(7)SH, log(k(abs)/M(-)(1) s(-)(1)) = (7.88 +/- 0.35) - (4.64 +/- 0.54)/theta, and for 2-mercaptonaphthalene, log(k(abs)/M(-)(1) s(-)(1)) = (8.21 +/- 0.17) - (4.24 +/- 0.26)/theta (theta = 2.303RT kcal/mol, 2sigma error). The rate constant for hydrogen atom abstraction at 298 K by benzyl radical from 1 is 2 orders of magnitude greater than that from 1-octanethiol, resulting from the predicted (DFT) S-H bond strength of 1 of 73 kcal/mol. The radical CpMo(micro-S)(3)(micro-SH)MoCp, 2, is revealed, from the properties of slow self-reaction, and exclusive cross-combination with reactive benzyl radical, to be a persistent free radical.     

The effects of secondary structure and O2 on the formation of direct strand breaks upon UV irradiation of 5-bromodeoxyuridine-containing oligonucleotides.

BACKGROUND: 5-Bromodeoxyuridine is a radiosensitizing agent that is currently being evaluated in clinical trials as an adjuvant in the treatment of a variety of cancers. gamma-Radiolysis and UV irradiation of oligonucleotides containing 5-bromodeoxyuridine result in the formation of direct strand breaks at the 5'-adjacent nucleotide by oxidation of the respective deoxyribose. We investigated the effects of DNA secondary structure and O2 on the induction of direct strand breaks in 5-bromodeoxyuridine-containing oligonucleotides. RESULTS: The efficiency of direct strand break formation in duplex DNA is dependent upon O2 and results in fragments containing 3'-phosphate and the labile 3'-ketodeoxyadenosine termini. The ratio of the 3'-termini is also dependent upon O2 and structure. Deuterium product isotope effects and tritium-transfer studies indicate that hydrogen-atom abstraction from the C1'- and C2'-positions occurs in an O2- and structure-dependent manner. CONCLUSIONS: The reaction mechanisms by which DNA containing 5-bromodeoxyuridine is sensitized to damage by UV irradiation are dependent upon whether the substrate is hybridized and upon the presence or absence of O2. Oxygen reduces the efficiency of direct strand break formation in duplex DNA, but does not affect the overall strand damage. It is proposed that the sigma radical abstracts hydrogen atoms from the C1'- and C2'-positions of the 5'-adjacent deoxyribose moiety, whereas the nucleobase peroxyl radical selectively abstracts the C1'-hydrogen atom from this site. This is the second example of DNA damage amplification by a nucleobase peroxyl radical, and might be indicative of a general reaction pattern for this family of reactive intermediates.     

A new method to study antioxidant capability: hydrogen transfer from phenols to a prefluorescent nitroxide

[reaction: see text]. Rate constants for hydrogen abstraction from phenols by a prefluorescent-TEMPO probe are reported. The nitroxide is employed as a potential model of peroxyl radicals. The probe works by nitroxide suppression of the fluorescence of the chromophore. The fluorescence is restored when the nitroxide abstracts a hydrogen atom to produce the diamagnetic hydroxylamine. The phenols studied in this project exhibited rate constants between 0.003 and 0.2 M(-1) s(-1). A deuterium isotope effect of 10 for TROLOX confirms that the mechanism is dominated by hydrogen transfer.     

Synchrotron photoionization measurements of OH-initiated cyclohexene oxidation: ring-preserving products in OH + cyclohexene and hydroxycyclohexyl + O2 reactions

Earlier synchrotron photoionization mass spectrometry experiments suggested a prominent ring-opening channel in the OH-initiated oxidation of cyclohexene, based on comparison of product photoionization spectra with calculated spectra of possible isomers. The present work re-examines the OH + cyclohexene reaction, measuring the isomeric products of OH-initiated oxidation of partially and fully deuterated cyclohexene. In particular, the directly measured photoionization spectrum of 2-cyclohexen-1-ol differs substantially from the previously calculated Franck-Condon envelope, and the product spectrum can be fit with no contribution from ring-opening. Measurements of H(2)O(2) photolysis in the presence of C(6)D(10) establish that the addition-elimination product incorporates the hydrogen atom from the hydroxyl radical reactant and loses a hydrogen (a D atom in this case) from the ring. Investigation of OH + cyclohexene-4,4,5,5-d(4) confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-d(10) reaction with O(2) are observed upon adding a large excess of O(2) to the OH + C(6)D(10) system.     

Peroxyl radical clocks

A series of peroxyl radical clocks has been developed and calibrated based on the competition between the unimolecular beta-fragmentation (k(beta)) of a peroxyl radical and its bimolecular reaction with a hydrogen atom donor (k(H)). These clocks are based on either methyl linoleate or allylbenzene and were calibrated directly with alpha-tocopherol or methyl linoleate, which have well-established rate constants for reaction with peroxyl radicals (k(H-tocopherol) = 3.5 x 10(6) M(-1) s(-1), k(H-linoleate) = 62 M(-1) s(-1)). This peroxyl radical clock methodology has been successfully applied to determine inhibition and propagation rate constants ranging from 10(0) to 10(7) M(-1) s(-1).     

Direct ESR detection of pentadienyl radicals and peroxyl radicals in lipid peroxidation: mechanistic insight into regioselective oxygenation in lipoxygenases

Well-resolved ESR spectra of free pentadienyl radicals have been observed under photoirradiation of di-tert-butylperoxide (Bu(t)OOBu(t)) and polyunsaturated fatty acids in the absence of O(2), allowing us to determine the hfc values. The hfc values of linoleyl radical indicate that the spin density is the largest at the C-11 position. The linoleyl radical is readily trapped by O(2) to produce the peroxyl radical (11-HPO.) in which O(2) is added mainly at the C-11 position of the pentadienyl radical as indicated by the comparison of the ESR spectra of peroxyl radicals derived from linoleic acid and [11,11-(2)H(2)]linoleic acid. The peroxyl radical (13-HPO.), which is initially formed by the hydrogen abstraction from 13-(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid (13-HPOD) by Bu(t)O., is found to isomerize to 11-HPO. via removal of O(2) from 13-HPO. and addition of O(2) to linoleyl radical to produce 11-HPO. . This finding supports an idea of O(2) entering via a specific protein channel, which determines the stereo- and regiochemistry of the biradical combination between O(2) and linoleyl radical in lipoxygenases.     

Flash vacuum pyrolysis of methoxy-substituted lignin model compounds

The flash vacuum pyrolysis (FVP) of methoxy-substituted beta-O-4 lignin model compounds has been studied at 500 degrees C to provide mechanistic insight into the primary reaction pathways that occur under conditions of fast pyrolysis. FVP of PhCH(2)CH(2)OPh (PPE), a model of the dominant beta-O-4 linkage in lignin, proceeds by C-O and C-C cleavage, in a 37:1 ratio, to produce styrene plus phenol as the dominant products and minor amounts of toluene, bibenzyl, and benzaldehyde. From the deuterium isotope effect in the FVP of PhCD(2)CH(2)OPh, it was shown that C-O cleavage occurs by homolysis and by 1,2-elimination in a ratio of 1.4:1, respectively. Methoxy substituents enhance the homolysis of the beta-O-4 linkage, relative to PPE, in o-CH(3)O-C(6)H(4)OCH(2)CH(2)Ph (o-CH(3)O-PPE) and (o-CH(3)O)(2)-C(6)H(3)OCH(2)CH(2)Ph ((o-CH(3)O)(2)-PPE) by a factor of 7.4 and 21, respectively. The methoxy-substituted phenoxy radicals undergo a complex series of reactions, which are dominated by 1,5-, 1,6-, and 1,4-intramolecular hydrogen abstraction, rearrangement, and beta-scission reactions. In the FVP of o-CH(3)O-PPE, the dominant product, salicylaldehyde, forms from the methoxyphenoxy radical by a 1,5-hydrogen shift to form 2-hydroxyphenoxymethyl radical, 1,2-phenyl shift, and beta-scission of a hydrogen atom. The 2-hydroxyphenoxymethyl radical can also cleave to form formaldehyde and phenol in which the ratio of 1, 2-phenyl shift to beta-scission is ca. 4:1. In the FVP of o-CH(3)O-PPE and (o-CH(3)O)(2)-PPE, products (ca. 20 mol %) are also formed by C-O homolysis of the methoxy group. The resulting phenoxy radicals undergo 1,5- and 1,6-hydrogen shifts in a ratio of ca. 2:1 to the aliphatic or benzylic carbon, respectively, of the phenethyl chain. In the FVP of (o-CH(3)O)(2)-PPE, o-cresol was the dominant product. It was formed by decomposition of 2-hydroxy-3-hydroxymethylbenzaldehyde and 2-hydroxybenzyl alcohol, which are formed from a complex series of reactions from the 2, 6-dimethoxyphenoxy radical. The key step in this reaction sequence was the rapid 1,5-hydrogen shift from 2-hydroxy-3-methoxybenzyloxy radical to 2-hydroxymethyl-6-methoxyphenoxy radical before beta-scission of a hydrogen atom to give the substituted benzaldehyde. The 2-hydroxybenzyl alcohols rapidly decompose under the reaction conditions to o-benzoquinone methide and pick up hydrogen from the reactor walls to form o-cresol.     

Direct strand scission in double stranded RNA via a C5-pyrimidine radical

Nucleobase radicals are the major family of reactive intermediates produced when nucleic acids are exposed to γ-radiolysis. The 5,6-dihydrouridin-5-yl radical (1), the formal product of hydrogen atom addition and a model for hydroxyl radical addition, was independently generated from a ketone precursor via Norrish Type I photocleavage in single and double stranded RNA. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and only minor amounts of strand scission are observed at the initial site of radical generation. Strand scission occurs preferentially in double stranded RNA and in the absence of O(2). The dependence of strand scission efficiency from the 5,6-dihydrouridin-5-yl radical (1) on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. Internucleotidyl strand scission occurs via β-elimination of the 3'-phosphate following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical yields RNA fragments containing 3'-phosphate or 3'-deoxy-2'-ketonucleotide termini from competing deprotonation pathways. The ketonucleotide end group is favored in the presence of low concentrations of thiol, presumably by reducing the cation radical to the enol. Competition studies with thiol show that strand scission from the 5,6-dihydrouridin-5-yl radical (1) is significantly faster than from the 5,6-dihydrouridin-6-yl radical (2) and is consistent with computational studies using the G3B3 approach that predict the latter to be more stable than 1 by 2.8 kcal/mol.     

Chain amplification in photoreactions of N-alkoxypyridinium salts with alcohols: mechanism and kinetics

Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting methoxy radical from an added alcohol generates an alpha-hydroxy radical that reduces another pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium) of approximately 15-20, at reactant concentrations of approximately 0.02-0.04 M. The reaction can also be sensitized with triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain propagation are imposed by the relationship between the oxidation potential of the alpha-hydroxy radical and the reduction potential of the pyridinium salt. The chain reactions proceed despite approximately 0.25 eV endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a combination of steady state and transient kinetics. The bimolecular rate constants for the chain propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from the diphenylketyl radical to P1 are approximately 6 x 10(6) and 1.1 x 10(6) M(-1) s(-1), respectively. The kinetic data indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d(3), is a dominant chain-terminating process. Because of a large deuterium isotope effect, approximately 7, the quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile.     

Ionizing radiation induced reactions of 2,6-dichloroaniline in dilute aqueous solution

In pulse radiolysis investigations the hydroxyl radical formed in water radiolysis reacts with 2,6-dichloroaniline in radical addition to the ring forming hydroxy-cyclohexadienyl radical and also in hydrogen atom abstraction from the amino group resulting in anilino radical. The hydroxy-cyclohexadienyl radical in the absence of dissolved O₂ partly transforms to anilino radical, when dissolved oxygen is present the radical transforms to peroxy radical. According to chemical oxygen demand measurements the reaction of one OH radical induces the incorporation of 0.6 O₂ into the products. It is a typical value for chlorine atom substituted aromatic molecules, and smaller than found for molecules without chlorine atom (1.0–2.0).     

Catalytic Mechanism of a Heme and Tyrosyl Radical-Containing Fatty Acid α-(Di)oxygenase

The steady-state catalytic mechanism of a fatty acid α-(di)oxygenase is examined, revealing that a persistent tyrosyl radical (Tyr379(?)) effects O(2) insertion into C(α)-H bonds of fatty acids. The initiating C(α)-H homolysis step is characterized by apparent rate constants and deuterium kinetic isotope effects (KIEs) that increase hyperbolically upon raising the concentration of O(2). These results are consistent with H(?) tunneling, transitioning from a reversible to an irreversible regime. The limiting deuterium KIEs increase from ～30 to 120 as the fatty acid chain is shortened from that of the native substrate. In addition, activation barriers increase in a manner that reflects decreased fatty acid binding affinities. Anaerobic isotope exchange experiments provide compelling evidence that Tyr379(?) initiates catalysis by H(?) abstraction. C(α)-H homolysis is kinetically driven by O(2) trapping of the α-carbon radical and reduction of a putative peroxyl radical intermediate to a 2(R)-hydroperoxide product. These findings add to a body of work which establishes large-scale hydrogen tunneling in proteins. This particular example is novel because it involves a protein-derived amino acid radical.     

Model compound studies of the beta-O-4 linkage in lignin: absolute rate expressions for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl radical

Arrhenius rate expressions were determined for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl, PhC*(OH)CH2OPh (V). Ketyl radical V was competitively trapped by thiophenol to yield PhCH(OH)CH2OPh in competition with beta-scission to yield phenoxyl radical and acetophenone. A basis rate expression for hydrogen atom abstraction by sec-phenethyl alcohol, PhC*(OH)CH3, from thiophenol, log(k(abs)/M(-1) s(-1)) = (8.88 +/- 0.24) - (6.07 +/- 0.34)/theta, theta = 2.303RT, was determined by competing hydrogen atom abstraction with radical self-termination. Self-termination rates for PhC*(OH)CH3 were calculated using the Smoluchowski equation employing experimental diffusion coefficients of the parent alcohol, PhCH(OH)CH3, as a model for the radical. The hydrogen abstraction basis reaction was employed to determine the activation barrier for the beta-scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V): log(k beta)/s(-1)) = (12.85 +/- 0.22) - (15.06 +/- 0.38)/theta, k beta (298 K) ca. (64.0 s(-1) in benzene), and log(k beta /s(-1)) = (12.50 +/- 0.18) - (14.46 +/- 0.30)/theta, k beta (298 K) = 78.7 s(-1) in benzene containing 0.8 M 2-propanol. B3LYP/cc-PVTZ electronic structure calculations predict that intramolecular hydrogen bonding between the alpha-OH and the -OPh leaving group of ketyl radical (V) stabilizes both ground- and transition-state structures. The computed activation barrier, 14.9 kcal/mol, is in good agreement with the experimental activation barrier.     

The unusual reaction of semiquinone radicals with molecular oxygen

Hydroquinones (benzene-1,4-diols) are naturally occurring chain-breaking antioxidants, whose reactions with peroxyl radicals yield 1,4-semiquinone radicals. Unlike the 1,2-semiquinone radicals derived from catechols (benzene-1,2-diols), the 1,4-semiquinone radicals do not always trap another peroxyl radical, and instead the stoichiometric factor of hydroquinones varies widely between 0 and 2 as a function of ring-substitution and reaction conditions. This variable antioxidant behavior has been attributed to the competing reaction of the 1,4-semiquinone radical with molecular oxygen. Herein we report the results of experiments and theoretical calculations focused on understanding this key reaction. Our experiments, which include detailed kinetic and mechanistic investigations by laser flash photolysis and inhibited autoxidation studies, and our theoretical calculations, which include detailed studies of the reactions of both 1,4-semiquinones and 1,2-semiquinones with O2, provide many important insights. They show that the reaction of O2 with 2,5-di-tert-butyl-1,4-semiquinone radical (used as model compound) has a rate constant of 2.4 +/- 0.9 x 10(5) M-1 s-1 in acetonitrile and as high as 2.0 +/- 0.9 x 10(6) M-1 s-1 in chlorobenzene, i.e., similar to that previously reported in water at pH approximately 7. These results, considered alongside our theoretical calculations, suggest that the reaction occurs by an unusual hydrogen atom abstraction mechanism, taking place in a two-step process consisting first of addition of O2 to the semiquinone radical and second an intramolecular H-atom transfer concerted with elimination of hydroperoxyl to yield the quinone. This reaction appears to be much more facile for 1,4-semiquinones than for their 1,2-isomers.     

Formation and reactivity of a persistent radical in a dinuclear molybdenum complex

The reactivity of the S-H bond in Cp*Mo(mu-S) 2(mu-SMe)(mu-SH)MoCp* ( S 4 MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(mu-S) 3(mu-SMe)MoCp* ( S 4 Me*), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, Delta H (double dagger) and Delta S (double dagger) were determined to be 1.54 +/- 0.25 kcal/mol and -25.5 +/- 0.8 cal/mol K, respectively, giving k abs = 1.3 x 10 (6) M (-1) s (-1) at 25 degrees C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo 2S 4 core was found to be the benzyl cross-termination product, Cp*Mo(mu-S) 2(mu-SMe)(mu-SBz)MoCp* ( S 4 MeBz), consistent with the Fischer-Ingold persistent radical effect. S 4 Me* was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(mu-S) 2(mu-SMe)MoCp*) 2(mu-S 2) ( ( S 4 Me) 2 ). The radical-dimer equilibrium constant has been determined to be 5.7 x 10 (4) +/- 2.1 x 10 (4) M (-1) from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 x 10 (3) s (-1) at 25 degrees C, based on variable temperature (1)H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 x 10 (7) M (-1) s (-1) in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for ( S 4 Me) 2 , S 4 MeBz, and the cationic Cp*Mo(mu-S 2)(mu-S)(mu-SMe)MoCp*(OTf) ( S 4 Me ( + )), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S 4 core is presented.