- Evidence for a transient peroxynitro acid in the reaction catalyzed by nitronate monooxygenase with propionate 3-nitronate
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Nitronate monooxygenase is a flavin-dependent enzyme that catalyzes the denitrification of propionate 3-nitronate (P3N) and other alkyl nitronates. The enzyme was previously known as 2-nitropropane dioxygenase, until its reclassification in 2010 by the IUBMB. Physiologically, the monooxygenase from fungi protects the organism from the environmental occurrence of P3N, which shuts down the Krebs cycle by inactivating succinate dehydrogenase and fumarase. The inhibition of these enzymes yields severe neurological disorders or death. Here, we have used for the first time steady-state and rapid kinetics, viscosity and pH effects, and time-resolved absorbance spectroscopy of the enzyme in turnover with P3N and the substrate analogue ethyl nitronate (EN) to elucidate the mechanism of the reaction. A transient increase in absorbance at ~300 nm, never reported before, was seen during steady-state turnover of the enzyme with P3N and oxygen, with no concomitant changes between 400 and 600 nm. The transient species was not detected when oxygen was absent. Anaerobic reduction of the enzyme with P3N yielded anionic flavosemiquinone and was fast (e.g., ≥1900 s-1). Steady-state kinetics demonstrated that oxygen reacts before the release of the product of P3N oxidation from the enzyme. No pH effects were seen with P3N on kcat/Km, k cat/Koxygen, and kcat; in contrast, with EN, the kcat/Km and kcat decreased with increasing pH defining two plateaus and a pKa ~ 8.0. Solvent viscosity at the pH optima suggested product release as being partially controlling the overall rate of turnover with the physiological substrate and its analogue. A mechanism that satisfies the kinetic results is proposed.
- Smitherman, Crystal,Gadda, Giovanni
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- Relative Reactivity of Amines and Oxyanions toward Proton Abstraction from Nitroethane. Electrostatic Effects and the Reactivity-Selectivity Principle
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Tertiary amines are more effective than primary and secondary amines which are more effective than oxyanions toward proton abstraction from nitroethane.The four homologous series of bases give rise to catalytic rate constants for nitroethane ionization that fall on three Broensted plots: for tertiary amines β=0.45, for primary and secondary amines β=0.60, and for oxyanions β=0.71.The increase in the Broensted β value with decreasing reactivity is in accord with the reactivity-selectivity principle.In nitroethane ionization a tertiary amine with conjugate acid of pKa=10.0 is 13-fold and one of pKa=6.2 is 130-fold more reactive than an oxyanion with a conjugate acid of the same pKa.These are much smaller rate ratios than were observed in the enolization of oxaloacetate where tertiary amines were found to be 800-fold and 2200-fold more reactive than oxyanions of pKa=10.0 and 6.2, respectively, giving further support to the previously proposed nucleophilic addition-elimination mechanism for tertiary amine catalyzed enolization of oxaloacetic acid.The greater reactivity of tertiary amines compared to oxyanions is ascribed to energy-lowering and energy-raising electrostatic interactions between the catalyst and the anionic charge developing on the substrate in the transition state.With primary and secondary amines the degree of transition-state electrostatic stabilization is decreased by solvation.Electrostatic stabilization can also be affected by certain functional groups in the amine.
- Bruice, Paula Yurkanis
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- Deprotonation of Nitroalkanes by Bicyclic Amidine and Guanidine Bases; Evidence for Molecular Recognition within a Catalytic Cycle for C-C Bond Formation
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Evidence from 1H NMR spectroscopy, supported by X-ray crystallography, suggests that the bicyclic amidine and guanidine bases 2 and 3 react with nitroalkanes in non-polar organic solvents to give tightly-bound ion pairs 5; it is argued that homochiral analogues of these complexes may prove valuable as intermediates in enantioselective catalytic C-C bond-forming reactions.
- Boyle, Peter H.,Convery, Maire A.,Davis, Anthony P.,Hosken, Gladys D.,Murray, Brian A.
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- Reactions of the simple nitroalkanes with hydroxide ion in water. Evidence for a complex mechanism
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Conventional kinetic analysis of the reactions of nitromethane (NM), nitroethane (NE) and 2-nitropropane (2-NP) with hydroxide ion in water revealed that the reactions are complex and involve kinetically significant intermediates. Kinetic experiments at the isosbestic points where changes in reactant and product absorbance cancel indicate the evolution and decay of absorbance characteristic of the formation of reactive intermediates. The deviations from 1st-order kinetics were observed to increase with increasing extent of reaction and in the reactant order: NM NE 2-NP. The apparent deuterium kinetic isotope effects for proton/deuteron transfer approach unity near zero time and increased with time toward plateau values as the reaction kinetics reach steady state. It is proposed that the initially formed preassociation complexes are transformed to more intimate reactant complexes which can give products by two possible pathways.
- Li, Zhao,Cheng, Jin-Pei,Parker, Vernon D.
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supporting information; experimental part
p. 4563 - 4569
(2011/07/09)
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- Radical and Ionic Reactions of (Benzoylmethyl)mercurials
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Photolysis of PhCOCH2HgCl or (PhCOCH2)2Hg yields benzoylmethyl radicals which can be trapped by anions such as Me2C=NO2-, RC(CO2Et)2-, RC(O-)=CH2 or by other electron-rich systems such as (RO)3P, N-methylpyrrole, enamines, or norbornene.Electron transfer from the adduct radicals to the mercurials yields PhCOCH2A from the anions A-, PhCOCH2P(O)(OR)2 from P(OR)3, and the phenacyl derivative from N-methylpyrrole or enamines.Easily oxidized anions such as PhCOCPh2- or PhC(CH3)=NO2- react with PhCOCH2* by electron transfer to yield the dimer derived from the anion.Addition of PhCOCH2* to norbornene yields a substituted 3-benzoylpropyl radical which cyclizes at the ortho position of the benzoyl group to give the α-tetralone derivative.
- Russell, Glen A.,Kulkarni, Shekhar V.,Khanna, Rajive K.
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p. 1080 - 1086
(2007/10/02)
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- Comparison of the Intrinsic Reactivities of Carbon and Oxygen Nucleophiles at the 1,3,5-Trinitro-substituted Aromatic Ring
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Kinetic and equilibrium data are reported for nucleophilic attack by nitroalkane anions at unsubstituted ring positions of 1,3,5-trinitrobenzene and of 2,4,6-trinitrotoluene.The results allow the calculation for this reaction type of values for intrinsic rate coefficients, k0' of 0.18 for CH2NO2(1-) and 0.22 for MeCHNO2(1-).The corresponding value for the malononitrile anion, CH(CN)2(1-), is 2.5E4, and for the methoxide ion the value is 1E3.The results are discussed in terms of the electronic-structural and solvational reorganisation occuring during reaction.
- Cox, Jonathan P. L.,Crampton, Michael R.,Wight, Paul
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- KINETICS AND MECHANISM OF THE REACTION OF TETRANITROMETHANE WITH NITROETHANE IN AN ALKALINE MEDIUM
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The kinetics of the reaction of tetranitromethane with nitroethane in alkaline aqueous buffer media were investigated by a spectrophotometric method.The process has radical-ion chain character, but there is a small contribution from a nonchain component.The kinetics of both reactions are described by equations of overall second order, i.e., first in the tetranitromethane and first in the nitrocarbanion.The proposed radical-anion chain mechanism involves electron transfer from the nitrocarbanion to tetranitromethane, addition of the NO2. radical which forms to the nitroalkane radical anion to tetranitromethane.A possible mechanism for the nonchain process is discussed.
- Kuznetsova, I. I.,Bazanov, A. G.,Tselinskii, I. V.
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p. 619 - 625
(2007/10/02)
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