Showing papers on "Hydrogen atom abstraction published in 1993"

Oxidation of free amino acids and amino acid residues in proteins by radiolysis and by metal-catalyzed reactions

Abstract: Basic mechanisms that underlie the oxygen free radical-promoted oxidation of free amino acids and amino acid residues of proteins are derived from radiolysis studies. Results of these studies indicate that the most common pathway for the oxidation of simple aliphatic amino acids involves the hydroxyl radical-mediated abstraction of a hydrogen atom to form a carbon-centered radical at the alpha-position of the amino acid or amino acid residue in the polypeptide chain. Addition of O2 to the carbon-centered radicals leads to formation of peroxy radical derivatives, which upon decomposition lead to production of NH3 and alpha-ketoacids, or to production of NH3, CO2, and aldehydes or carboxylic acids containing one less carbon atom. As the number of carbon atoms in the amino acid is increased, hydrogen abstraction at other positions in the carbon chain becomes more important and leads either to the formation of hydroxy derivatives, or to amino acid cross-linked products as a consequence of carbon-centered radical recombination processes. alpha-Hydrogen abstraction plays a minor role in the oxidation of aromatic amino acids by radiolysis. Instead, the aromatic ring is the primary site of attack leading to hydroxy derivatives, to ring scission, and in the case of tyrosine to the formation of Tyr-Tyr cross-linked dimers. The basic pattern for the oxidation of amino acids by metal ion-catalyzed reactions (Fenton chemistry) is similar to the alpha-hydrogen abstraction pathway. But unlike the case of oxidation by radiolysis, this Fenton pathway is the major mechanism for the oxidation of all aliphatic amino acids, regardless of chain length, as well as for the oxidation of aromatic amino acids. Curiously, the Fe(III)-catalyzed oxidation of free amino acids is almost completely dependent upon the presence of bicarbonate ion, and is greatly stimulated by iron chelators at chelator/Fe(III) ratios less than 1.0, and is inhibited at chelator/Fe(III) ratios greater than 1.0. It is deduced that the most active catalytic complex is composed of two equivalents of HCO3-, an amino acid, and at least one equivalent of iron; however, two forms of iron, an iron-chelate and another form, must somehow be involved. In contrast to the situation with radiolysis, the aromatic rings of aromatic amino acids are only minor targets for metal-catalyzed reactions. All amino acid residues in proteins are subject to attack by hydroxyl radicals generated by ionizing radiation; however, the aromatic amino acids and sulfur-containing amino acids are most sensitive to oxidation.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular recognition in cytochrome P-450 : mechanism for the control of uncoupling reactions

Abstract: The pathway for utilization of pyridine nucleotide derived reducing equivalents in the cytochrome P-450 monooxygenase systems has three major branch points. The first is a partitioning between autoxidation of a ferrous, oxygenated heme adduct and input of the second reducing equivalent required for monooxygenase stoichiometry. The second is between dioxygen bond scission and release of two-electron-reduced O2 as hydrogen peroxide. The third is between substrate hydrogen abstraction initiated by a putative higher valent iron-oxo species and reduction of this intermediate by two additional electrons to produce water in an overall oxidase stoichiometry. For all substrates investigated, the direct release of superoxide at the first branch point never competes with second electron input. In order to elucidate the aspects of molecular recognition of a substrate-P-450 complex which affect these individual branch points in the catalytic cycle, we have measured the NADH-derived reducing equivalents recovered in hydroxylated substrate, hydrogen peroxide, and water for a series of active-site mutants designed to alter the coupling of ethylbenzene hydroxylation. We find that the reaction specificity at the second and third branch points is affected by site-directed mutations that alter the topology of the binding pocket. The increased commitment to catalysis observed for all mutants suggests that active-site hydration is important in the uncoupling to form hydrogen peroxide at the second branch point. The liberation of hydrogen peroxide does not correlate with the location of the mutation in the pocket, as expected if the two-electron-reduced dioxygen-bound intermediate is not directly participating in the substrate activation step.(ABSTRACT TRUNCATED AT 250 WORDS)

Solvent effects on the competitive .beta.-scission and hydrogen atom abstraction reactions of the cumyloxyl radical. Resolution of a long-standing problem

Abstract: Laser flash photolysis (LFP) techniques with detection in the infrared and in UV-visible regions of the spectrum have been used in combination with detailed product studies to assess solvent effects on the hydrogen abstraction and β-scission reactions of cumyloxyl radicals. The variation in the ratio of the products of these two competing processes in solvents with different polarities is due to the solvent's influence on β-scission

Synthesis of diamond from methane and nitrogen mixture

Abstract: We have found that diamond can be synthesized from a mixture of CH4 and N2 without adding any H2. This new synthesis is sharply different from the common practice of diamond growth by chemical vapor deposition, which uses a hydrogen‐rich mixture of CH4 and H2. In this new approach, nitrogen becomes an active component of microwave plasma leading to diamond growth. Nitrogen participates in abstraction of hydrogen from the diamond surface. We hypothesize that formation of HCN is an indication of hydrogen abstraction that allows diamond to grow from CH4+N2 mixtures. As a consequence of surface processes, the crystal structure of the grown diamond is distorted. The sequence of tetrahedral layers is mixed (cubic and hexagonal) and it suffers from turbostatic disorder. Diamond films were characterized by x‐ray diffraction, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and Raman spectroscopy.

Vibrational distribution of CH3 produced by the reaction of O(1D2) atom with CH4

Abstract: The nascent vibrational distribution of CH3 produced by the reaction O(1D2)+CH4→OH+CH3 (ΔH0=−182 kJ mol−1) has been investigated by infrared diode laser kinetic spectroscopy. The reaction was initiated by the generation of O(1D2) atoms by excimer laser photolysis of N2O or O3 at a total pressure of 200 mTorr, and the ν2 (out‐of‐plane bending) bands of CH3, v2=1←0 up to 4←3, were measured as functions of time. The vibrational distribution of ν2 (v≤3) was found to be noninverted and much less excited than a prior distribution. The fraction of the available energy released to the ν2 vibration,

Cytochrome P450 hydroxylation of hydrocarbons: variation in the rate of oxygen rebound using cyclopropyl radical clocks including two new ultrafast probes.

Abstract: The oxidation of eight methyl-substituted and three alkyl-substituted cyclopropanes by rat liver microsomal cytochrome P450 and pure reconstituted rabbit P450 2B4 was studied. Alkane hydroxylation catalyzed by P450 is generally believed to proceed by hydrogen abstraction followed by reaction of the carbon-centered radical with an iron-bound hydroxyl radical, a process called oxygen rebound. Hydrogen abstraction from methylcyclopropanes generates cyclopropylcarbinyl radicals whose solution rate constants for ring opening are known [Bowry, V.W., et al. (1991) J. Am. Chem. Soc. 113, 5687-5698]. Rearranged products were only observed with the five substrates which, upon hydrogen abstraction, would generate a cyclopropylcarbinyl radical that undergoes ring opening with a rate constant > or = 2.0 x 10(9) s-1 in solution. Values of the rate constants for oxygen rebound (kOH) were calculated by determining the ratio of unrearranged products (cyclopropylmethanols) to rearranged products (alkenols). For each substrate this ratio was generally about the same for the oxidations catalyzed by microsomal P450 and by P450 2B4. It is concluded that all of the substrates are oxidized via an intermediate cyclopropylcarbinyl radical. Two ultrafast probes, trans-1-methyl-2-phenylcyclopropane and 1,1-diphenyl-2-methylcyclopropane, gave alcohol product ratios which yielded unreasonably high values for kOH, viz., ca. 1.5 x 10(12) and ca. 7 x 10(12) s-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Deuterium kinetic isotope effect and stopped-flow kinetic studies of the quinoprotein methylamine dehydrogenase.

Abstract: Stopped-flow kinetic studies of the reductive half-reaction of methylamine dehydrogenase from Paracoccus denitrificans yielded kinetic constants for the reversible formation of the imine intermediate formed between the substrate and the tryptophan tryptophylquinone (TTQ) prosthetic group and for the hydrogen abstraction step which occurs concomitantly with TTQ reduction When CD3NH2 was used as a substrate, deuterium kinetic isotope effects of 42 and 38, respectively, were measured for the rate constants that correspond to the formation and dissociation of the iminoquinone intermediate A deuterium kinetic isotope effect of 172 was measured for the hydrogen abstraction step The maximum deuterium kinetic isotope effect which was measured in steady-state kinetic experiments was 30 These data are discussed in relation to the reaction mechanism of methylamine dehydrogenase and the similar large deuterium kinetic isotope effect for hydrogen abstraction which has been observed for another quinoprotein, plasma amine oxidase

Transient radical diffusion in photoinduced hydrogen abstraction reactions of benzophenone probed by the transient grating method

Abstract: The transient grating method is used for investigating the diffusion dynamics of the transient radicals created by photoinduced hydrogen abstraction reactions of benzophenone (BP) in various solvents, acetone in 2-propanol, and acetoaldehyde in ethanol. The diffusion coefficients (D) of the BP kethyl radical and BP are obtained simultaneously. In acetone/2-propanol and acetoaldehyde/ethanol systems, only the signals due to intermediate radicals are observed. The D values of the transient radicals are two to three times smaller than those of BP

Intramolecular hydrogen transfer as the key step in the dissociation of hydroxyl radical adducts of (alkylthio)ethanol derivatives

Abstract: The reaction of the hydroxyl radical with dimethyl sulfide (DMS), 2-(methylthio)ethanol (2-MTE), 2,2'-dihydroxydiethyl sulfide (2,2'-DHE), and 3,3'-dihydroxydipropyl sulfide (3,3'-DHP) has been investigated in H[sub 2]O and D[sub 2]O. As an initial step hydroxyl radicals add to the sulfur moiety. These hydroxyl radical adducts subsequently decay via a thioether concentration-dependent and a thioether concentration-independent pathway. The hydroxyl radical adduct of DMS dissociates into a sulfur radical cation and HO[minus] in the thioether concentration-independent pathway (k[sub H]/k[sub D] = 2.09), whereas a rate-limiting proton transfer from water operates in the thioether concentration-dependent mechanism (k[sub H]/k[sub D] = 5.40), as deduced from the measured solvent kinetic isotope effects. In contrast the hydroxyl radical adducts of 2-MTE and 2,2'-DHE decompose via elimination of water, formed through a rapid intramolecular hydrogen transfer from the adjacent hydroxyl groups. This mechanism leads to the formation of (alkylthio)ethoxy radicals. The latter undergo [alpha],[beta]-fragmentation into formaldehyde and [alpha]-thioether radicals as well as hydrogen abstraction from a [delta]-methylene group, analogous to a hydrogen transfer in the Barton reaction, leading to [alpha]-thioether radicals. 37 refs., 5 figs., 2 tabs.

Kinetic study of the prooxidant effect of tocopherol. Hydrogen abstraction from lipid hydroperoxides by tocopheroxyls in solution

Abstract: A kinetic study of the prooxidant effect of vitamin E (tocopherol, TocH) has been carried out. The rates of hydrogen abstraction (k−1) from methyl linoleate hydroperoxide (ML-OOH) by α-tocopheroxyl (α-Toc.) (1) and eight types of alkyl substituted Toc. radicals, (2–9) in benzene solution have been determined spectrophotometrically. The results show that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of Toc. increases. The k−1 value (5.0×10−1M−1s−1) obtained for α-Toc. (1) was found to be about seven orders of magnitude lower than the k1 value (3.2×106M−1s−1) for the reaction of α-TocH with peroxyl radical, which is well known as the usual radical-scavenging reaction of α-TocH. The above reaction rates (k−1) obtained were compared with those (k3) of methyl linoleate with Toc. (1–9) in benzene solution. The rates (k−1) were found to be about six times larger than those (k3) of the corresponding Toc.. The results suggest that both reactions may relate, to the prooxidant effect of α-TocH at high concentrations in foods and oils. The effect of the phytyl side chain on the reaction rate, of Toc. in micellar dispersions has also been studied. We have measured the rate constant, k−1, for the reaction of phosphatidylcholine hydroperoxide with a Toc. radical in benzene,tert-butanol and in Triton X-100 micellar dispersions, and compared the observed k−1 values with the corresponding values for ML-OOH.

The identification of the allylic nitrite and nitro derivatives of methyl linoleate and methyl linolenate by negative chemical ionization mass spectroscopy.

Abstract: The autoxidation of polyunsaturated fatty acids is initiated bothin vivo andin vitro by nitrogen dioxide. The mechanism of the initiation process is believed to involve both addition reactions and hydrogen atom abstraction reactions, with hydrogen abstraction predominating at low levels of nitrogen dioxide. Therefore low levels of nitrogen dioxide should react with polyunsaturated fatty acids to give allylic derivatives; in an anaerobic system these derivations should be allylic nitro and nitrite compounds. Using negative methane chemical ionization mass spectrometry and other analytical techniques, we have identified these allylic nitrite and nitro compounds from the reactions of low levels of nitrogen dioxide with methyl linoleate and methyl linolenate in the absence of oxygen.

Reaction of deuterium atoms with cyclohexane on copper (111): hydrogen abstraction reactions by Eley-Rideal mechanisms

Abstract: Cyclohexane desorbs molecularly intact from Cu(111) and does not react with deuterium atoms that are preadsorbed on the surface By contrast, when deuterium atoms formed on a hot tungsten filament are impinged onto a Cu(111) surface precovered with cyclohexane, dehydrogenated products (cyclohexene, cyclohexadiene, and benzene) are evolved when the surface is heated in a subsequent temperature-programmed reaction (TPR) experiment These D-atom-induced dehydrogenation products provide strong evidence for an Eley-Rideal mechanism where D atoms abstract hydrogen from cyclohexane prior to thermal accommodation with the surface The kinetics of cyclohexene evolution indicate that both single and sequential H-atom abstractions occur by this mechanism

Thermochemistry and ion-molecule reactions of isomeric C3H2.bul.+ cations

Abstract: Ab initio molecular orbital calculations at the Gaussian-2 (G2) level of theory have been used to examine the heats of formation and ion-molecule reactions of isomeric forms of the C 3 H 2 .+ cation. The linear structure l-C 3 H 2 .+ is predicted to lie higher in energy than the cyclic form c-C 3 H 2 .+ by 28 kJ mol -1 at 0 K (or 31 kJ mol -1 at 298 K), energy differences significantly smaller than previous theoretical estimates (∼50 kJ mol -1 )

Mechanism of hydrogen abstraction reactions by free radicals: simple metathesis or involving intermediate complex?

Abstract: Hydrogen abstraction reactions, R+HX→RH+X (R=CF y H 3-y (y=0-3), C 2 H 5 ; X=F, Cl, Br, CH 3 ), have been investigated by ab initio methods. It is found that with reactants which are polarized with a net positive charge on the hydrogen of HX and a net negative charge on the carbon at the radical site, reactions proceed via a weakly hydrogen-bonded intermediate complex (R..-H-X) prior to the formation of the transition state (R-H-X). All geometries (reactants, intermediate complexes, transition-state structures) were optimized at the (U)MP2/6-31G * level

Theoretical studies of hydrogen-abstraction reactions from diamond and diamond-like surfaces

Abstract: Reaction probabilities, cross sections, rate coefficients, frequency factors, and activation energies for hydrogen‐atom abstraction from a hydrogen‐covered C(111) surface have been computed using quantum wave packet and classical trajectory methods on the empirical hydrocarbon ♯1 potential hypersurface developed by Brenner. Upper bounds for the abstraction rates, activation energies, and frequency factors have been obtained for six different chemisorbed moieties on a C(111) diamond surface using a classical variational transition‐state method. For the hydrogen‐covered surface, the results of the wave packet/trajectory calculations give k(T)=1.67×1014 exp(−0.46 eV/kbT) cm3/mol s, which is about a factor of 2.9 less than the gas‐phase abstraction rate from tertiary carbon atoms at 1200 K. The variational calculations show that the activation energies for hydrogen‐atom abstraction vary from 0.0 to 1.063 eV. Some sp2‐bonded hydrogen atoms can be removed in a barrierless process if adjacent to a carbon radical...

Catalytic dehydration of alcohols by hydroxyl: 2-propanol; an intermediate case

Abstract: We describe a laser photolysis/cw, laser-induced fluorescence kinetic study of the reaction between OH and 2-propanol, measured over the temperature range 293-745 K. The rate coefficient for hydrogen atom abstraction by OH from 2-propanol is best fit by the expression k(T)=1.044×10 -17 T 1.86 exp(736/T) cm 3 molecule -1 s -1 . Chain-catalytic dehydration of 2-propanol by OH is an important component of the reaction mechanism. By using isotopic substitution, we determine, as a function of temperature, the branching ratio for H atom attraction by OH from the β-sites of 2-propanol. Between 500 and 600 K, biexponential [OH] decays result from the unimolecular decomposition of the H 2 CCH(OH)CH 3 intermediate

Bimolecular reactions of distonic ions: Proton transfer and hydrogen atom abstraction with ˙CH2OH2+

Abstract: The bimolecular reaction of . CH 2 OH 2 + with a variety of neutral molecules in the gas phase were studied by Fourier transform ion cyclotron resonance experiments. The distonic ion is found to be considerably more acidic than protonated methanol; bracketing experiments provide a value of 695 ± 8 kJ mol −1 for the proton affinity of the conjugate base, the hydroxymethyl radical. Efficient hydrogen atom abstraction is only observed from neutral species less basic than . CH 2 OH. Transfer of CH 2 +. to CS 2 , CH 3 I and C 6 H 2 F 4 is observed

A time-resolved electron spin resonance and laser flash spectroscopy investigation of the photolysis of benzaldehyde and benzoin in homogeneous solvents and micellar solutions

Abstract: Both photochemical a-cleavage of triplet benzoin (BZ) and hydrogen abstraction by triplet benzaldehyde (BA) from ground-state benzaldehyde produce geminate radical pairs of identical chemical structure. A search for “memory effects” in the chemically identical geminate radical pairs generated from different photochemical pathways was examined using the techniques of time-resolved electron spin resonance (TRESR) and timeresolved optical absorption spectroscopy. Photolysis of BZ in homogeneous organic solvents and in sodium dodecyl sulfate (SDS) micellar solutions leads to chemically induced dynamic electron polarization (CIDEP) of benzoyl and a-hydroxybenzyl radicals consisting of a strong emission (E) due to the triplet mechanism (TM) in the generation of electron polarization. Photolysis of BA in hydrogen-donating organic solvents results in a E/A (or E*/A) CIDEP pattern of a-hydroxybenzyl radicals due to the radical pair mechanism (RPM). In solvents which are poor hydrogen donors (benzene, acetonitrile) and/or at relatively high concentrations of BA, the photoreduction of triplet BA by ground-state BA generates benzoyl and a-hydroxybenzyl radicals, which manifest an E/A CIDEP spectrum assigned to RPM. Photoreduction of BA by KCNS in aqueous acetonitrile results in absorptive (A) CIDEP of a-hydroxybenzyl radicals, assigned to a rare case of RPM for which the g factor difference overwhelms the hyperfine interactions of the pertinent radical pair (the a-hydroxybenzyl and the (CNS)**- radical). Computer simulation allows the estimation of thegfactor of this inorganic polarized radical to be in the range 2.015 < g < 2.03. Under conditions of low occupancy number of BA, the photolysis of BA in SDS micellar solution displayed CIDEP spectra assigned to a spin-correlated geminate radical pair (SCRP) consisting of a-hydroxybenzyl and alkyl radicals of SDS. At a higher occupancy number, a different SCRP spectrum is observed and is assigned to a polarized a-hydroxybenzyl and benzoyl radical pair. Computer simulation confirms all of the proposed assignments. Measurements of the decay kinetics of the a-hydroxybenzyl radical, observed by time-resolved absorption spectroscopy, show that the application of an external magnetic field of 0.30 T leads to a decrease in the rate of micellized geminate recombination in the photoreduction of micellized BA and to an increase in the rate of radical escape. No significant magnetic field effect was found on the decay of the a-hydroxybenzyl radicals produced from BZ. These results imply that radicals formed by a-cleavage of triplet BZ escape from micelles faster than the chemically identical geminate pair generated by the photoreduction of triplet BA by ground-state BA.

Bond dissociation energies of 3,5,5- and 4,5,5-trimethyl-2-oxomorpholine by photoacoustic calorimetry. An assessment of the additivity of substituent effects

Abstract: Carbon-hydrogen bond dissociation energies (BDE) and absolute rate constants for hydrogen atom abstraction by the tert-butoxyl radical have been measured for a number of aliphatic amines, including two captodatively substituted compounds (4,5,5-trimethyl-2-oxomorpholine (1) and 3,5,5-trimethyl-2-oxomorpholine (2)). The bond energies, determined by photoacoustic calorimetry, cover a range of about 15 kcal mol -1 (ca. 90 kcal mol -1 for the aliphatic amines to 75 kcal mol -1 for 4,5,5-trimethyl-2-oxomorpholine)