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Martin Newcomb

Publications and source records attributed to Martin Newcomb.

31 records · Page 2Linked to original sources
PubMed2003

Carboxylate-substituted radicals from phenylselenide derivatives. Designs on models for coenzyme B12-dependent enzyme-catalyzed rearrangements.

Laser flash photolysis (266 nm) of alpha- and beta-phenylselenyl esters, carboxylic acids, and carboxylates in aqueous acetonitrile media gave the corresponding radicals by homolytic cleavage of the phenylselenyl groups. In the beta-substituted systems, acid and carboxylate radicals reacted in intramolecular reporter reactions with approximately equal rate constants. For the alpha-substituted systems, an ester- and carboxylic acid-substituted radical reacted in an intramolecular reporter reaction with the same rate constants, but the analogous alpha-carboxylate radical, a radical anion, reacted an order of magnitude less rapidly and with an activation energy that is 3 kcal/mol greater than that found for analogues. A kinetic titration of the equilibrating alpha-acid and alpha-carboxylate radicals gave pKa = 4.6. The results indicate that alpha-ester and alpha-carboxylic acid radicals are unlikely to be appropriate models for alpha-carboxylate radicals, the intermediates formed in a large subset of coenzyme B12-dependent enzyme-catalyzed reactions.

Carboxylic Acids↗
PubMed2003

Kinetic results implicating a polar radical reaction pathway in the rearrangement catalyzed by alpha-methyleneglutarate mutase.

alpha-Methyleneglutarate mutase (MGM) catalyzes the rearrangement of 2-methyleneglutarate to 3-methylitaconate (2-methylene-3-methylsuccinate). A putative mechanism for the MGM-catalyzed reaction involves 3-exo cyclization of the 2-methyleneglutaric acid-4-yl radical to a cyclopropylcarbinyl radical intermediate that ring opens to the 3-hydroxycarbonyl-2-methylenebutanoic acid-4-yl radical (3-methylitaconic acid radical). Model reactions for this mechanism were studied by laser flash photolysis kinetic methods. alpha-Ester radicals were produced by 266 nm photolysis of alpha-phenylselenyl ester derivatives. Rate constants for cyclizations of the (Z)-1-ethoxycarbonyl-4-(2,2-diphenylcyclopropyl)-3-buten-1-yl radical ((Z)-8a) and (E)- and (Z)-1,3-di(ethoxycarbonyl)-4-(2,2-diphenylcyclopropyl)-3-buten-1-yl radicals ((E)- and (Z)-8b) were determined. The ester group in (Z)-8a accelerates the 3-exo cyclization in comparison to the parent radical lacking an ester group by a factor of 3, an effect ascribed to a polarized transition state. The ester groups at C3 in radicals 8b slow the 3-exo cyclization reaction by a factor of 50. The rate constant for cyclization of the 2-methyleneglutaric acid-4-yl radical is estimated to be k approximately 2000 s(-1) at ambient temperature. When coupled with the estimated partitioning of the intermediate cyclopropylcarbinyl radical, the overall rate constant for the conversion is estimated to be k approximately equal to 1 x 10(-3) s(-1), which is much too small for any radical reaction and several orders of magnitude too small for kinetic competence for the MGM-catalyzed process. The possibility that the radical reaction in nature involves an unusual mechanism in which polar effects are important is discussed.

Catalysis↗
PubMed2003

Efficient production of enol ether radical cations by heterolytic cleavage of beta-mesylate radicals.

[reaction: see text] alpha-Methoxy-beta-mesyloxy radicals were produced in laser flash photolysis reactions, and yields of enol ether radical cations formed by heterolytic fragmentation of the mesylate group were determined. The mesylate heterolysis reaction is faster than heterolyses of phosphate and bromide groups in analogous radicals and highly efficient in medium-polarity solvents.

Journal Article↗
PubMed2003

Multiple mechanisms and multiple oxidants in P450-catalyzed hydroxylations.

Cytochrome P450 enzymes catalyze a number of oxidations in nature including the difficult hydroxylations of unactivated positions in an alkyl group. The consensus view of the hydroxylation reaction 10 years ago was that a high valent iron-oxo species abstracts a hydrogen atom from the alkyl group to give a radical that subsequently displaces the hydroxy group from iron in a homolytic substitution reaction (hydrogen abstraction-oxygen rebound). More recent mechanistic studies, as summarized in this review, indicated that the cytochrome P450-catalyzed "hydroxylation reaction" is complex, involving multiple mechanisms and multiple oxidants. In addition to the iron-oxo species, another electrophilic oxidant apparently exists, either the hydroperoxo-iron intermediate that precedes iron-oxo or iron-complexed hydrogen peroxide formed by protonation of the hydroperoxo-iron species on the proximal oxygen. The other electrophilic oxidant appears to react by insertion of OH(+) into a C-H bond to give a protonated alcohol. Computational work has suggested that iron-oxo can react through multiple spin states, a low-spin ensemble that reacts by insertion of oxygen, and a high-spin ensemble that reacts by hydrogen atom abstraction to give a radical.

Biochemical Phenomena↗
PubMed2003

Induction of apoptosis in macrophages by Pseudomonas aeruginosa azurin: tumour-suppressor protein p53 and reactive oxygen species, but not redox activity, as critical elements in cytotoxicity.

Azurin is a copper-containing protein involved in electron transfer during denitrification. We reported recently that purified azurin demonstrates cytotoxicity to macrophages by forming a complex with the tumour-suppressor protein p53, thereby stabilizing it and enhancing its function as an inducer of proapoptotic activity (Yamada, T., Goto, M., Punj, V., Zaborina, O., Kimbara, K., Das Gupta, T. K., and Chakrabarty, A. M. 2002, Infect Immun70: 7054-7062). It is, however, not known whether the oxidoreductase (redox) activity of azurin or the involvement of copper is important for its cytotoxicity. We have isolated apo-azurin devoid of copper and site-directed mutants that are redox negative because of either replacement of a cysteine residue (Cys-112) involved in co-ordination with copper or mutational replacement of two methionine residues (Met-44 and Met-64) that are present in the hydrophobic patch of azurin and allow interaction of azurin with its redox partner cytochrome c551. We demonstrate that, although the wild type (wt) and the Cys-112 Asp mutant azurin can form complexes with the tumour-suppressor protein p53 and generate high levels of reactive oxygen species (ROS), the redox-negative Met-44LysMet-64Glu mutant azurin is defective in complex formation with p53, generates low levels of ROS and lacks appreciable cytotoxicity towards macrophages. Thus, complex formation with p53 and ROS generation, rather than azurin redox activity, are important in the cytotoxic action of azurin towards macrophages.

Amino Acid Sequence↗
PubMed2002

p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals.

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the beta-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH2O < MeCHO < Me2CO < PhCHO < Ph2CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the beta-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.

Journal Article↗
PubMed2002

Evaluation of norcarane as a probe for radicals in cytochome p450- and soluble methane monooxygenase-catalyzed hydroxylation reactions.

Norcarane was employed as a mechanistic probe in oxidations catalyzed by hepatic cytochome P450 enzymes and by the soluble methane monooxygenase (sMMO) enzyme from Methylococcuscapsulatus (Bath). In all cases, the major oxidation products (>75%) were endo- and exo-2-norcaranol. Small amounts of 3-norcaranols, 2-norcaranone, and 3-norcaranone also formed. In addition, the rearrangement products (2-cyclohexenyl)methanol and 3-cycloheptenol were detected in the reactions, the former possibly arising from a radical intermediate and the latter ascribed to a cationic intermediate. The formation of the cation-derived rearrangement product is consistent with one or more reaction pathways and is in accord with the results of previous probe studies with the same enzymes. The appearance of the putative radical-derived rearrangement product is in conflict with other mechanistic probe results with the same enzymes. The unique implication of a discrete radical intermediate in hydroxylations of norcarane may be the consequence of a minor reaction pathway for the enzymes that is not manifest in reactions with other probes. Alternatively, it might reflect a previously unappreciated reactivity of norcaranyl cationic intermediates, which can convert to (2-cyclohexenyl)methanol. We conclude that generalizations regarding the intermediacy of radicals in P450 and sMMO enzyme-catalyzed hydroxylations based on the norcarane results should be considered hypothetical until the origin of the unanticipated results can be determined.

Animals↗
PubMed2002

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations.

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.

Journal Article↗
PubMed1998

Absolute Rate Constants for alpha-Amide Radical Reactions.

The 1-(diethylcarbamoyl)-6,6-diphenyl-5-hexenyl radical (4a), the 1-(diethylcarbamoyl)-7,7-diphenyl-6-heptenyl radical (4b), and the 1-(diethylcarbamoyl)-1-methyl-6,6-diphenyl-5-hexenyl radical (4c) were produced from the corresponding PTOC esters (anhydrides of the carboxylic acid and N-hydroxypyridine-2-thione) by laser flash photolysis methods. The kinetics of cyclizations of radicals 4a and 4b were measured at various temperatures, and that of cyclization of 4c was measured at ambient temperature. Radicals 4a and 4b were employed as radical clocks in indirect kinetic studies to determine rate constants for reaction of secondary alpha-amide radicals with Bu(3)SnH. The calibrated tin hydride trapping reaction was then employed to determine rate constants for cyclization of the 1-(diethylcarbamoyl)-5-hexenyl radical (12). The rate constants for 5-exo cyclizations of secondary alpha-amide radicals are similar to those of their isostructural alkyl radical analogues. The rate constants for the 5-exo cyclization of tertiary alpha-amide radical 4c and the 6-exo cyclization of the secondary alpha-amide radical 4b are smaller than those of the analogous alkyl radicals and alpha-ester substituted radicals, apparently due to steric effects. The rate constants for tin hydride trapping of secondary alpha-amide radicals are similar to those for reactions with secondary alpha-ester radicals.

Journal Article↗
PubMed1997

Lewis Acid Activation and Catalysis of Dialkylaminyl Radical Reactions.

Lewis acid activation and catalysis of dialkylaminyl radical reactions is demonstrated both qualitatively and quantitatively. Cyclization of the N-butyl-4-pentenaminyl radical (1) in the presence of a wide range of Lewis acids was shown to be efficient with good to excellent yields of cyclic products obtained in reactions conducted even at -78 degrees C. Rate constants for fragmentation of the N-ethyl-2,2-diphenylethylaminyl radical (6), 6-exo cyclization of the N-methyl-6,6-diphenyl-5-hexenaminyl radical (7), and 5-exo cyclization of the N-methyl-5,5-diphenyl-4-pentenaminyl radical (8) in the presence of the Lewis acids LiBF(4), MgBr(2), and BF(3) were measured by laser flash photolysis (LFP) methods. The LFP studies demonstrated saturation kinetic behavior with respect to the Lewis acids. Equilibrium binding constants for the Lewis acids with the dialkylaminyl radicals and rate constants for reactions of the Lewis acid complexed dialkylaminyl radicals were obtained from nonlinear regression analysis of the observed rate constants.

Journal Article↗
PubMed1997

Absolute Rate Constants for Iminyl Radical Reactions.

The kinetics of cyclization of the 2-methyl-6,6-diphenyl-5-hexeniminyl radical (3) were measured by laser flash photolysis methods from 0 to 50 degrees C, and the kinetics of trapping of radical 3 by thiophenol and p-chlorothiophenol were determined between 4 and 83 degrees C by indirect methods using the unimolecular rate constants as the basis values. Radical 3 cyclizes with a rate constant of 2.2 x 10(6) s(-)(1) at 25 degrees C and reacts with thiophenol and p-chlorothiophenol at this temperature with rate constants of 0.6 and 1.4 x 10(7) M(-)(1) s(-)(1), respectively. Bu(3)SnH reacted with 3 too slowly to prevent nearly complete cyclization, but a crude rate constant for the trapping reaction was estimated. The cyclization and trapping reactions of iminyl radical 3 are slower than the corresponding reactions of alkyl radicals.

Journal Article↗