Alkyltelluro Substitution Improves the Radical‐Trapping Capacity of Aromatic Amines

The synthesis of a variety of aromatic amines carrying an ortho‐alkyltelluro group is described. The new antioxidants quenched lipidperoxyl radicals much more efficiently than α‐tocopherol and were regenerable by aqueous‐phase N‐acetylcysteine in a two‐phase peroxidation system. The inhibition time for diaryl amine 9 b was four‐fold longer than recorded with α‐tocopherol. Thiol consumption in the aqueous phase was found to correlate inversely to the inhibition time and the availability of thiol is the limiting factor for the duration of antioxidant protection. The proposed mechanism for quenching of peroxyl radicals involves O‐atom transfer from peroxyl to Te followed by H‐atom transfer from amine to alkoxyl radical in a solvent cage.     

Proton‐Coupled Electron Transfer of Unsaturated Fatty Acids and Mechanistic Insight into Lipoxygenase

A proton‐coupled electron transfer (PCET) process plays an important role in the initial step of lipoxygenases to produce lipid radicals which can be oxygenated by reaction with O₂ to yield the hydroperoxides stereoselectively. The EPR spectroscopic detection of free lipid radicals and the oxygenated radicals (peroxyl radicals) together with the analysis of the EPR spectra has revealed the origin of the stereo‐ and regiochemistry of the reaction between O₂ and linoleyl (= (2Z)‐10‐carboxy‐1‐[(1Z)‐hept‐1‐enyl]dec‐2‐enyl) radical in lipoxygenases. The direct determination of the absolute rates of H‐atom‐transfer reactions from a series of unsaturated fatty acids to the cumylperoxyl (= (1‐methyl‐1‐phenylethyl)dioxy) radical by use of time‐resolved EPR at low temperatures together with detailed kinetic investigations on both photoinduced and thermal electron‐transfer oxidation of unsaturated fatty acids provides the solid energetic basis for the postulated PCET process in lipoxygenases. A strong interaction between linoleic acid (= (9Z,12Z)‐octadeca‐9,12‐dienoic acid) and the reactive center of the lipoxygenases (Fe^III? OH) is suggested to be involved to make a PCET process to occur efficiently, when an inner‐sphere electron transfer from linoleic acid to the Fe^III state is strongly coupled with the proton transfer to the OH group.     

Solvent‐Free Photooxidation of Alkanes by Dioxygen with 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone via Photoinduced Electron Transfer

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) which acts as a super photooxidant. Solvent‐free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser‐induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.     

A Mechanistic Study of the Effects of Antioxidants on the Formation of Malondialdehyde‐Like Products in the Reaction of Hydroxyl Radicals with Deoxyribose

The reactions of ^.OH radicals with deoxyribose, DR, form five different DR^. radicals, only one of which is transformed into malondialdehyde (MDA)‐like products. The radiolytic yield of the MDA‐like products increases with the increase in the DR concentration indicating that some of the initially formed “unproductive” radicals react with DR to form the “productive” radicals. The yield of the MDA‐like products also increases with the dose rate delivered to the solution suggesting that the formation of the MDA‐like products involves the reaction of the “productive” radicals with a radical. The addition of ascorbate, AH^?, to the solution decreases the yield of the MDA‐like products as expected from the relative rates of the reaction of DR and AH^? with ^.OH radicals. On the other hand the addition of the exogenous thiol, N‐acetylcysteine (NAC), to the solutions decreases the yield of the MDA‐like products considerably more than expected from the rate constants of the reaction with ^.OH radicals. The addition of the endogenous thiol, glutathione (GSH), to the solutions affects the yield of the MDA‐like products at low concentration less than expected and at “high” concentrations more than expected from the rate constant of the reaction. Addition of low concentration of AH^? to solutions containing GSH increases considerably its antioxidant activity whereas addition of small concentrations of AH^? to solutions containing NAC has no effect on its antioxidant activity. The results point out that the DR^. radicals react differently with NAC and GSH and that the GS^. and NAC^. radicals react differently with DR, the GS^. radical being considerably more active than the NAC^. radical. Thus it has to be concluded that the relative activity of antioxidants depends also on the rate constants of many secondary reactions and on the concentrations of all the solutes present in the system.     

Rate constants and transition‐state geometry of reactions of alkyl,alkoxyl, and peroxyl radicals with thiols

Enthalpy, activation energy, and rate constant of 9 alkyl, 3 acyl, 3 alkoxyl, and 9 peroxyl radicals with alkanethiols, benzenethiol, and L ‐cysteine are calculated. The intersection parabolas model is used for activation energy calculations. Depending on the structure of attacking radical, the activation energy of reactions with alkylthiols varies from 3 to 43 kJ mol^?1 for alkyl radicals, from 7 to 9 kJ mol^?1 for alkoxyl, and from 18 to 35 kJ mol^?1 for peroxyl radicals. The influence of adjacent π‐bonds on activation energy is estimated. The polar effect is found in reactions of hydroxyalkyl and acyl radicals with alkylthiols. The steric effect is observed in reactions of alkyl radicals with tert‐alkylthiols. All these factors are characterized via increments of activation energy. Quantum chemical calculations of activation energy and geometry of transition state were performed for model reactions: C^?H₃ + CH₃SH, CH₃O^? + CH₃SH, and HO₂^? + CH₃SH with using density functional theory and Gaussian‐98. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 284–293, 2009     

Effect of Molecular Interactions on Electron‐Transfer and Antioxidant Activity of Bis(alkanol)selenides: A Radiation Chemical Study

Understanding electron‐transfer processes is crucial for developing organoselenium compounds as antioxidants and anti‐inflammatory agents. To find new redox‐active selenium antioxidants, we have investigated one‐electron‐transfer reactions between hydroxyl (^.OH) radical and three bis(alkanol)selenides (SeROH) of varying alkyl chain length, using nanosecond pulse radiolysis. ^.OH radical reacts with SeROH to form radical adduct, which is converted primarily into a dimer radical cation (>Se∴Se<)⁺ and α‐{bis(hydroxyl alkyl)}‐selenomethine radical along with a minor quantity of an intramolecularly stabilized radical cation. Some of these radicals have been subsequently converted to their corresponding selenoxide, and formaldehyde. Estimated yield of these products showed alkyl chain length dependency and correlated well with their antioxidant ability. Quantum chemical calculations suggested that compounds that formed more stable (>Se∴Se<)⁺, produced higher selenoxide and lower formaldehyde. Comparing these results with those for sulfur analogues confirmed for the first time the distinctive role of selenium in making such compounds better antioxidants.     

The Reaction of Sulfenic Acids with Peroxyl Radicals: Insights into the Radical‐Trapping Antioxidant Activity of Plant‐Derived Thiosulfinates

Sulfenic acids play a prominent role in biology as key participants in cellular signaling relating to redox homeostasis, in the formation of protein‐disulfide linkages, and as the central players in the fascinating organosulfur chemistry of the Allium species (e.g., garlic). Despite their relevance, direct measurements of their reaction kinetics have proven difficult owing to their high reactivity. Herein, we describe the results of hydrocarbon autoxidations inhibited by the persistent 9‐triptycenesulfenic acid, which yields a second order rate constant of 3.0×10⁶ M ^?1 s^?1 for its reaction with peroxyl radicals in PhCl at 30 °C. This rate constant drops 19‐fold in CH₃CN, and is subject to a significant primary deuterium kinetic isotope effect, k_H/k_D=6.1, supporting a formal H‐atom transfer (HAT) mechanism. Analogous autoxidations inhibited by the Allium‐derived (S)‐benzyl phenylmethanethiosulfinate and a corresponding deuterium‐labeled derivative unequivocally demonstrate the role of sulfenic acids in the radical‐trapping antioxidant activity of thiosulfinates, through the rate‐determining Cope elimination of phenylmethanesulfenic acid (k_H/k_D≈4.5) and its subsequent formal HAT reaction with peroxyl radicals (k_H/k_D≈3.5). The rate constant that we derived from these experiments for the reaction of phenylmethanesulfenic acid with peroxyl radicals was 2.8×10⁷ M ^?1 s^?1; a value 10‐fold larger than that we measured for the reaction of 9‐triptycenesulfenic acid with peroxyl radicals. We propose that whereas phenylmethanesulfenic acid can adopt the optimal syn geometry for a 5‐centre proton‐coupled electron‐transfer reaction with a peroxyl radical, the 9‐triptycenesulfenic is too sterically hindered, and undergoes the reaction instead through the less‐energetically favorable anti geometry, which is reminiscent of a conventional HAT.     

H‐atom abstraction from selected C?H bonds in 2,3‐dimethylpentanal, 1,4‐cyclohexadiene,and 1,3,5‐cycloheptatriene

The gas‐phase reactions of OH radicals with 1,4‐cyclohexadiene, 1,3,5‐cycloheptatriene, and 2,3‐dimethylpentanal have been investigated to determine the importance of H‐atom abstraction at specific positions in these molecules. Benzene was observed as a product of the reaction of OH radicals with 1,4‐cyclohexadiene in 12.5 ± 1.2% yield, in good agreement with a previous study and indicating that this is the fraction of the reaction proceeding by H‐atom abstraction from the allylic C? H bonds. In contrast, no formation of tropone from 1,3,5‐cycloheptatriene was observed, suggesting that in this case H‐atom abstraction is not important. For the reaction of OH radicals with 2,3‐dimethylpentanal, formation of 3‐methyl‐2‐pentanone was observed in 5.4 ± 1.0% yield (after correction for reaction of 3‐methyl‐2‐pentanone with OH radicals), and this product is predicted to be formed after initial H‐atom abstraction from the 2‐position CH group. Acetaldehyde and 2‐butanone were also observed as products, with initial yields of ～90% and ～26%, respectively, and their formation appeared to involve, at least in part, an intermediary acyl peroxy radical. Using a relative rate method, the measured rate constants for the reactions of OH radicals with 2,3‐dimethylpentanal, 3‐methyl‐2‐pentanone, and tropone are (in units of 10^?12 cm³ molecule^?1 s^?1) 2,3‐dimethylpentanal, 42 ± 7; 3‐methyl‐2‐pentanone, 6.87 ± 0.08; and tropone, 42 ± 6. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 415–426, 2003     

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ ^{. ?} produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.     

Kinetics and Products of the Reactions of Ethyl and n‐Propyl Nitrates with OH Radicals

The kinetics of the reactions of ethyl (1) and n‐propyl (2) nitrates with OH radicals has been studied using a low‐pressure flow tube reactor combined with a quadrupole mass spectrometer. The rate constants of the title reactions were determined under pseudo–first‐order conditions from kinetics of OH consumption in high excess of nitrates. The overall rate constants, k₁ = 1.14 × 10^?13 (T/298)^2.45 exp(193/T) and k₂ = 3.00 × 10^?13 (T/298)^2.50 exp(205/T) cm³ molecule^?1 s^?1 (with conservative 15% uncertainty), were determined at a total pressure of 1 Torr of helium over the temperature range (248–500) and (263–500) K, respectively. The yields of the carbonyl compounds, acetaldehyde and propanal, resulting from the abstraction by OH of an α‐hydrogen atom in ethyl and n‐propyl nitrates, followed by α‐substituted alkyl radical decomposition, were determined at T = 300 K to be 0.77 ± 0.12 and 0.22 ± 0.04, respectively.     

Rate constants of the gas‐phase reactions of OH radicals with trans‐2‐hexenal,trans‐2‐octenal,and trans‐2‐nonenal

The rate constants of the gas‐phase reaction of OH radicals with trans‐2‐hexenal, trans‐2‐octenal, and trans‐2‐nonenal were determined at 298 ± 2 K and atmospheric pressure using the relative rate technique. Two reference compounds were selected for each rate constant determination. The relative rates of OH + trans‐2‐hexenal versus OH + 2‐methyl‐2‐butene and β‐pinene were 0.452 ± 0.054 and 0.530 ± 0.036, respectively. These results yielded an average rate constant for OH + trans‐2‐hexenal of (39.3 ± 1.7) × 10^?12 cm³ molecule^?1 s^?1. The relative rates of OH+trans‐2‐octenal versus the OH reaction with butanal and β‐pinene were 1.65 ± 0.08 and 0.527 ± 0.032, yielding an average rate constant for OH + trans‐2‐octenal of (40.5 ± 2.5) × 10^?12 cm³ molecule^?1 s^?1. The relative rates of OH+trans‐2‐nonenal versus OH+ butanal and OH + trans‐2‐hexenal were 1.77 ± 0.08 and 1.09 ± 0.06, resulting in an average rate constant for OH + trans‐2‐nonenal of (43.5 ± 3.0) × 10^?12 cm³ molecule^?1 s^?1. In all cases, the errors represent 2σ (95% confidential level) and the calculated rate constants do not include the error associated with the rate constant of the OH reaction with the reference compounds. The rate constants for the hydroxyl radical reactions of a series of trans‐2‐aldehydes were compared with the values estimated using the structure activity relationship. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 483–489, 2009     

Reaction of Aromatic Peroxyl Radicals with Alkynes: A Mass Spectrometric and Computational Study Using the Distonic Radical Ion Approach

The reaction of the aromatic distonic peroxyl radical cations N‐methyl pyridinium‐4‐peroxyl (PyrOO^.+) and 4‐(N,N,N‐trimethyl ammonium)‐phenyl peroxyl (AnOO^.+), with symmetrical dialkyl alkynes 10a – c was studied in the gas phase by mass spectrometry. PyrOO^.+ and AnOO^.+ were produced through reaction of the respective distonic aryl radical cations Pyr^.+ and An^.+ with oxygen, O₂. For the reaction of Pyr^.+ with O₂ an absolute rate coefficient of k₁=7.1×10^?12 cm³ molecule^?1 s^?1 and a collision efficiency of 1.2 % was determined at 298 K. The strongly electrophilic PyrOO^.+ reacts with 3‐hexyne and 4‐octyne with absolute rate coefficients of k_hexyne=1.5×10^?10 cm³ molecule^?1 s^?1 and k_octyne=2.8×10^?10 cm³ molecule^?1 s^?1, respectively, at 298 K. The reaction of both PyrOO^.+ and AnOO^.+ proceeds by radical addition to the alkyne, whereas propargylic hydrogen abstraction was observed as a very minor pathway only in the reactions involving PyrOO^.+. A major reaction pathway of the vinyl radicals 11 formed upon PyrOO^.+ addition to the alkynes involves γ‐fragmentation of the peroxy O? O bond and formation of PyrO^.+. The PyrO^.+ is rapidly trapped by intermolecular hydrogen abstraction, presumably from a propargylic methylene group in the alkyne. The reaction of the less electrophilic AnOO^.+ with alkynes is considerably slower and resulted in formation of AnO^.+ as the only charged product. These findings suggest that electrophilic aromatic peroxyl radicals act as oxygen atom donors, which can be used to generate α‐oxo carbenes 13 (or isomeric species) from alkynes in a single step. Besides γ‐fragmentation, a number of competing unimolecular dissociative reactions also occur in vinyl radicals 11 . The potential energy diagrams of these reactions were explored with density functional theory and ab initio methods, which enabled identification of the chemical structures of the most important products.     

OH‐Initiated Photooxidations of 1‐Pentene and 2‐Methyl‐2‐propen‐1‐ol: Mechanism and Yields of the Primary Carbonyl Products

The products of the gas‐phase reactions of OH radicals with 1‐pentene and 2‐methyl‐2‐propen‐1‐ol (221MPO) at T=298±2 K and atmospheric pressure were investigated by using a 4500 L atmospheric simulation chamber that was built especially for this work. The molar yield of butyraldehyde was 0.74±0.12 mol for the reaction of 1‐pentene. This work provides the first product molar yield determination of formaldehyde (0.82±0.12 mol), 1‐hydroxypropan‐2‐one (0.84±0.13 mol), and methacrolein (0.078±0.012 mol) from the reaction of 221MPO with OH radicals. The mechanism of this reaction is discussed in relation to the experimental results. Additionally, taking into consideration the complex mechanism, the rate coefficients of the reactions of OH with formaldehyde, 1‐hydroxypropan‐2‐one, and methacrolein were derived at atmospheric pressure and T=298±2 K.; the obtained values were (8.9±1.6)×10^?12, (2.4±1.4)×10^?12, and (22.9±2.3)×10^?12 cm³ molecule^?1 s^?1, respectively.     

Reactivity of natural phenols in radical reactions

Activation enthalpies and energies and the rate constants of reactions with peroxyl, alkyl, and thiyl radicals (76 reactions) were calculated for a group of natural antioxidants (19 monohydroxy and polyhydroxy phenols). The calculation was performed with the use of the model of a radical abstraction reaction as the intersection of two parabolic potential curves. The results of the calculation were compared with experimental data: the average discrepancy in the activation energies of the reactions RO ₂ ^? + ArOH was 0.8 kJ/mol. Interatomic distances in the reaction centers of the transition states of the test reactions were calculated. Factors affecting the reactivity of these compounds are discussed.     

Photodecomposition of iodopentanes in air: Product distributions from the self‐reactions of n‐pentyl peroxyl radicals

Product distributions from the 254‐nm photooxidation of the three iodopentane isomers were explored as a technique for studying the self‐reactions of individual pentyl peroxyl radicals (in air at ambient temperature and pressure). Pentanols and the associated carbonyl compounds (pentanal or pentanones) were major products as expected. Other major products resulted from the isomerization of pentan‐1‐oxyl and pentan‐2‐oxyl radicals, but their nature could not be identified. Minor products were alcohols and carbonyl compounds arising from the decomposition of pentoxyl radicals. Diols and mixed hydroxycarbonyl compounds from cross‐combination reactions were essentially absent, in contrast to expectation. The observed product distributions were evaluated to derive branching ratios for the radical‐preserving pathways of the self‐reactions, 0.42 ±0.17, 0.46 ± 0.10, 0.39 ± 0.08, for pentan‐1‐yl peroxyl, pentan‐2‐yl peroxyl, and pentan‐3‐yl peroxyl, respectively. Rate coefficients derived for the decomposition of the corresponding pentoxyl radicals, relative to their reaction with oxygen, are (5.1 ± 0.5) × 10¹⁸, (1.0 ± 0.2) × 10¹⁸, and (3.2 ± 0.3) × 10¹⁸ molecule cm^?3, respectively. Rate constants for the isomerization of pentan‐1‐oxyl and pentan‐2‐oxyl were estimated from the contributions of isomerization products to the total amounts of products as (4.0 ± 1.1) × 10⁵ s^?1 and (1.0 ± 2.0) × 10⁵ s^?1, respectively. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 126–138, 2002     

Atmospheric chemistry of isopropyl formate and tert‐butyl formate

Formates are produced in the atmosphere as a result of the oxidation of a number of species, notably dialkyl ethers and vinyl ethers. This work describes experiments to define the oxidation mechanisms of isopropyl formate, HC(O)OCH(CH₃)₂, and tert‐butyl formate, HC(O)OC(CH₃)₃. Product distributions are reported from both Cl‐ and OH‐initiated oxidation, and reaction mechanisms are proposed to account for the observed products. The proposed mechanisms include examples of the α‐ester rearrangement reaction, novel isomerization pathways, and chemically activated intermediates. The atmospheric oxidation of isopropyl formate by OH radicals gives the following products (molar yields): acetic formic anhydride (43%), acetone (43%), and HCOOH (15–20%). The OH radical initiated oxidation of tert‐butyl formate gives acetone, formaldehyde, and CO₂ as major products. IR absorption cross sections were derived for two acylperoxy nitrates derived from the title compounds. Rate coefficients are derived for the kinetics of the reactions of isopropyl formate with OH (2.4 ± 0.6) × 10^?12, and with Cl (1.75 ± 0.35) × 10^?11, and for tert‐butyl formate with Cl (1.45 ± 0.30) × 10^?11 cm³ molecule^?1 s^?1. Simple group additivity rules fail to explain the observed distribution of sites of H‐atom abstraction for simple formates. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 479–498, 2010     

The Role of One‐Electron Reduction of Lipid Hydroperoxides in Causing DNA Damage

The in vivo metabolism of plasma lipids generates lipid hydroperoxides that, upon one‐electron reduction, give rise to a wide spectrum of genotoxic unsaturated aldehydes and epoxides. These metabolites react with cellular DNA to form a variety of pre‐mutagenic DNA lesions. The mechanisms of action of the radical precursors of these genotoxic electrophiles are poorly understood. In this work we investigated the nature of DNA products formed by a one‐electron reduction of (13S)‐hydroperoxy‐(9Z,11E)‐octadecadienoic acid (13S‐HPODE), a typical lipid molecule, and the reactions of the free radicals thus generated with neutral guanine radicals, G(?H)^.. A novel approach was devised to generate these intermediates in solution. The two‐photon‐induced ionization of 2‐aminopurine (2AP) within the 2′‐deoxyoligonucleotide 5′‐d(CC[2AP]TCGCTACC) by intense nanosecond 308 nm excimer laser pulses was employed to simultaneously generate hydrated electrons and radical cations 2AP^.+. The latter radicals either in cationic or neutral forms, rapidly oxidize the nearby G base to form G(?H)^.. In deoxygenated buffer solutions (pH 7.5), the hydrated electrons rapidly reduce 13S‐HPODE and the highly unstable alkoxyl radicals formed undergo a prompt β‐scission to pentyl radicals that readily combine with G(?H)^.. Two novel guanine products in these oligonucleotides, 8‐pentyl‐ and N²‐pentylguanine, were identified. It is shown that the DNA secondary structure significantly affects the ratio of 8‐pentyl‐ and N²‐pentylguanine lesions that changes from 0.9:1 in single‐stranded, to 1:0.2 in double‐stranded oligonucleotides. The alkylation of guanine by alkyl radicals derived from lipid hydroperoxides might contribute to the genotoxic modification of cellular DNA under hypoxic conditions. Thus, further research is warranted on the detection of pentylguanine lesions and other alkylguanines in vivo.     

A quantum chemical study on the free radical scavenging activity of tyrosol and hydroxytyrosol

The free radical scavenging activity of hydroxytyrosol (HTyr) and tyrosol (Tyr) has been studied in aqueous and lipid solutions, using the density functional theory. Four mechanisms of reaction have been considered: single electron transfer (SET), sequential electron proton transfer (SEPT), hydrogen transfer (HT), and radical adduct formation. It was found that while SET and SEPT do not contribute to the overall reactivity of HTyr and Tyr toward ^·OOH and ^·OCH₃ radicals, they can be important for their reactions with ^·OH, ^·OCCl₃, and ^·OOCCl₃. The ^·OOH-scavenging activity of HTyr and Tyr was found to take place exclusively by HT, and it is also predicted to be the main mechanism for their reactions with ^·OCH₃. HT is proposed as the main mechanism for the scavenging activity of HTyr and Tyr when reacting with other ^·OR and ^·OOR radicals, provided that R is an alkyl or an alkenyl group. The major products of reaction are predicted to be the phenoxyl radicals. In addition, Tyr was found to be less efficient than HTyr as free radical scavenger. Moreover, while HTyr is predicted to be a good peroxyl scavenger, Tyr is predicted to be only moderately for that purpose.     

Photocatalytic Degradation of Ethyl tert‐Butyl Ether in Aqueous Solution Mediated by TiO2 Suspensions: Parameter and Reaction Pathway Investigations

Degradation of ethyl tert‐butyl ether (ETBE) with UV/TiO₂ was studied by solid‐phase microextraction and gas chromatography‐mass spectrometry. The complete removal of 0.1 g L^?1 of ETBE was achieved after 20 h of treatment. Factors such as pH of the system, catalyst and substrate concentration, and the presence of anions influenced the degradation rate. Establishment of the degradation pathway was made possible by a thorough analysis of the reaction mixture, which identified the main intermediate products generated. The possible degradation pathways were proposed and discussed in this research. The attack on the C–H bond in ETBE by ·OH forms an alkyl radical, which consequently produces a peroxyl radical upon reaction with oxygen. Peroxyl radicals react with one another and produce an alkoxy radical. The β‐bond fragmentation of the alkoxy radical produces different intermediates.     

Kinetics of the reactions of OH with 3‐methyl‐2‐cyclohexen‐1‐one and 3,5,5‐trimethyl‐2‐cyclohexen‐1‐one under simulated atmospheric conditions

Relative rate coefficients for the reactions of OH with 3‐methyl‐2‐cyclohexen‐1‐one and 3,5,5‐trimethyl‐2‐cyclohexen‐1‐one have been determined at 298 K and atmospheric pressure by the relative rate technique. OH radicals were generated by the photolysis of methyl nitrite in synthetic air mixtures containing ppm levels of nitric oxide together with the test and reference substrates. The concentrations of the test and reference substrates were followed by gas chromatography. Based on the value k(OH + cyclohexene) = (6.77 ± 1.35) × 10^?11 cm³ molecule^?1 s^?1, rate coefficients for k(OH + 3‐methyl‐2‐cyclohexen‐1‐one) = (3.1 ± 1.0) × 10^?11 and k(OH + 3,5,5‐trimethyl‐2‐cyclohexen‐1‐one) = (2.4 ± 0.7) × 10^?11 cm³ molecule^?1 s^?1 were determined. To test the system we also measured k(OH + isoprene) = (1.11 ± 0.23) × 10^?10 cm³ molecule^?1 s^?1, relative to the value k(OH + (E)‐2‐butene) = (6.4 ± 1.28) × 10^?11 cm³ molecule^?1 s^?1. The results are discussed in terms of structure–activity relationships, and the reactivities of cyclic ketones formed in the photo‐oxidation of monoterpene are estimated. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 7–11, 2002