Showing papers on "Reaction rate constant published in 1996"

Reduction of Nitro Aromatic Compounds by Zero-Valent Iron Metal

Abstract: The properties of iron metal that make it useful in remediation of chlorinated solvents may also lead to reduction of other groundwater contaminants such as nitro aromatic compounds (NACs). Nitrobenzene is reduced by iron under anaerobic conditions to aniline with nitrosobenzene as an intermediate product. Coupling products such as azobenzene and azoxybenzene were not detected. First-order reduction rates are similar for nitrobenzene and nitrosobenzene, but aniline appearance occurs more slowly (typical pseudo-first-order rate constants 3.5 × 10-2, 3.4 × 10-2, and 8.8 × 10-3 min-1, respectively, in the presence of 33 g/L acid-washed, 18−20 mesh Fluka iron turnings). The nitro reduction rate increased linearly with concentration of iron surface area, giving a specific reaction rate constant (3.9 ± 0.2 × 10-2 min-1 m-2 L). The minimal effects of solution pH or ring substitution on nitro reduction rates, and the linear correlation between nitrobenzene reduction rate constants and the square-root of mixing ra...

Dechlorination of Trichloroethene in Aqueous Solution Using Fe0

Abstract: Flow-through column tests were conducted to investigate the products of degradation of aqueous trichloroethene (TCE) in contact with granular iron metal. The results indicated the degradation process to be pseudo-first-order and the rate constant to be relatively insensitive to the initial concentration of TCE over the range from about 1.3 to 61 mg/L. The principal degradation product was ethene, followed by ethane with substantially smaller amounts of other C1−C4 hydrocarbons. About 3.0−3.5% of the initial TCE appeared as chlorinated degradation products, including the three dichloroethene isomers and vinyl chloride. Although the chloride mass balance was generally between 98 and 102%, a maximum of 73% of the carbon could be accounted for in the identified products. Based on the low concentrations of chlorinated degradation products in the solution phase, it is proposed that most of the TCE remains sorbed to the iron surface until complete dechlorination is achieved.

Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route

Abstract: Despite the fact that H-terminated, HF-etched Si crystals are the starting point for construction of most contemporary electronic devices,1 little is known about the chemical reactions of H-terminated Si surfaces under ambient temperature and pressure.2,3 Functionalization of Si without partial oxidation and/or formation of electrical defects is potentially important in fabricating improved electronic devices4,5 as well as in measurement of charge transfer rate constants at semiconductor/ liquid contacts.6 One recently described approach involves the reaction of HF-etched Si(111) with olefins and organic diacyl peroxides, in which formation of a self-assembled (near)monolayer of Si-alkyls was hypothesized.2 We report here an alternative strategy to functionalize HF-etched Si surfaces involving halogenation and subsequent reaction with alkyl Grignard or alkyl lithium reagents. We report vibrational spectroscopic and temperature programmed desorption data which confirm that the alkyl groups are bonded covalently to the Si surface, and we demonstrate that such overlayer formation can impede the undesirable oxidation of Si in a variety of environments while providing surfaces of high electrical quality. The H-terminated Si surface7 was first exposed to PCl5 for 20-60 min at 80-100 °C, in chlorobenzene with benzoyl peroxide as the radical initiator.8,9 Upon chlorination, the XP survey spectra (Figure 1) showed peaks at 270.2 ( 0.4 binding electron volts, BeV, (Cl 2s) and 199.3 ( 0.4 BeV (Cl 2p), indicating that this procedure yielded Cl on the surface. The high-resolution XP spectrum of the Si 2p peak of this surface displayed, in addition to the substrate Si signal, an additional peak located at 0.98 ( 0.12 BeV higher in binding energy (Figure 2) whose position and intensity was consistent with the formation of a surface Si-Cl bond.10 Auger electron spectra

An efficient TiO2 thin-film photocatalyst: photocatalytic properties in gas-phase acetaldehyde degradation

Abstract: A semitransparent TiO2 film with extraordinarily high photocatalytic activity was prepared on a glass substrate by sintering a TiO2 sol at 450 °C. Crystallographic analysis by X-ray diffraction and Raman spectroscopy showed that the film was purely anatase. The photocatalytic properties of the film were investigated by measuring the photodegradative oxidation of gaseous acetaldehyde at various concentrations under strong and weak UV light irradiation conditions. The photocatalytic activity of the film was higher than that of one of the most active commercial TiO2 powders, Degussa P-25. The kinetics of acetaldehyde degradation as catalyzed by the TiO2 film as well as by P-25 powder were analyzed in terms of the Langmuir-Hinshelwood model. It is shown that the number of adsorption sites per unit true surface area is larger with the TiO2 film, as analyzed in the powder form, than with P-25 powder. Meanwhile, the first-order reaction rate constant is also much larger with the film than with P-25 powder. Moreover, under most experimental conditions, particularly with high concentrations of acetaldehyde and weak UV illumination intensity, the quantum efficiency was found to exceed 100% on an absorbed-photon basis, assuming that only photo-generated holes play a major role in the reaction. This leads to the conclusion that the photodegradative oxidation of acetaldehyde is not mediated solely by hydroxyl radicals, generated via hole capture by surface hydroxyl ions or water molecules, but also by photocatalytically generated superoxide ion, which can be generated by the reduction of adsorbed oxygen with photogenerated electrons.

Kinetics of chromate reduction by ferrous iron

Abstract: Reduction of Cr(VI) to Cr(III) is environmentally favorable as the latter species is not toxic to most living organisms and also has a low mobility and bioavailability. Ferrous iron is one possible reductant implicated as a major contributor to the removal of Cr(VI) from suboxic and anoxic waters and soils. Despite the importance of this redox reaction, no mechanistic or kinetic information are available, which are needed to determine the rate of Cr(VI) reduction and to assess the role of oxygen in limiting this reaction. In this study we used a stopped-flow kinetic technique monitored by UV−VIS spectroscopy and an initial rate method to ascertain the rate constant and the rate dependence of each reactant. We observed that the rate of Cr(VI) removal conformed to −d[Cr(VI)]/dt = kcr[Fe(II)]0.6[Cr(VI)]1 where kcr = 56.3 (±3.7) mmol-0.6 min-1 L0.6. Based on this rate expression and that for the oxygenation of Fe(II), Cr(VI) reduction should be unaffected by oxygen except at pH values in excess of 8 even at m...

Blackbody Infrared Radiative Dissociation of Bradykinin and Its Analogues: Energetics, Dynamics, and Evidence for Salt-Bridge Structures in the Gas Phase

Abstract: Blackbody infrared radiative dissociation (BIRD) spectra of singly and doubly protonated bradykinin and its analogues are measured in a Fourier-transform mass spectrometer. Rate constants for dissociation are measured as a function of temperature with reaction delays up to 600 s. From these data, Arrhenius activation parameters in the zero-pressure limit are obtained. The activation parameters and dissociation products for the singly protonated ions are highly sensitive to small changes in ion structure. The Arrhenius activation energy (Ea) and pre-exponential (or frequency factor, A) of the singly protonated ions investigated here range from 0.6 to 1.4 eV and 105 to 1012 s-1, respectively. For bradykinin and its analogues differing by modification of the residues between the two arginine groups on either end of the molecule, the singly and doubly protonated ions have average activation energies of 1.2 and 0.8 eV, respectively, and average A values of 108 and 1012 s-1, respectively, i.e., the presence of ...

Effects of Fulvic Acid on Fe(II) Oxidation by Hydrogen Peroxide

Abstract: Iron redox cycling can catalyze the oxidation of humic substances and increase the rate of oxygen consumption in surface waters rich in iron and organic carbon. This study examines the role of Fenton`s reaction [oxidation of Fe(II) by hydrogen peroxide] in this catalytic cycle. A number of competing processes were observed in model systems containing dissolved Fe, hydrogen peroxide, and Suwannee River fulvic acid. First, the effective rate constant of Fenton`s reaction increased with increasing fulvic acid concentration, indicating the formation Fe(II)-fulvate complexes that react more rapidly with hydrogen peroxide than Fe(II)-aquo complexes. This effect was significant at pH 5 but negligible at pH 3. A second effect was scavenging of the HO{sup .} radical produced in Fenton`s reaction by fulvic acid, forming an organic radical. The organic radical reduced oxygen to HO{sub 2}{sup .}/O{sub 2}{sup .-}, which then regenerated hydrogen peroxide by reaction with Fe(II). Finally, Fe(III) was reduced by a dark reaction with fulvic acid, characterized by an initially fast reduction followed by slower processes. The behavior of Fe(II) and hydrogen peroxide over time in the presence of fulvic acid and oxygen could be described by a kinetic model taking all of these reactions into account. The netmore » result was an iron redox cycle in which hydrogen peroxide as well as oxygen were consumed (even though direct oxidation of Fe(II) by oxygen was not significant), and the oxidation of fulvic acid was accelerated. 56 refs., 7 figs., 1 tab.« less

Protein Folding Triggered by Electron Transfer

Abstract: Rapid photochemical electron injection into unfolded ferricytochrome c titrated with 2.3 to 4.6 M guanidine hydrochloride (GuHCl) at pH 7 and 40°C produced unfolded ferrocytochrome, which then converted to the folded protein. Two folding phases were observed: a fast process with a time constant of 40 microseconds (4.6 M GuHCl), and a slower phase with a rate constant of 90 ± 20 per second (2.3 M GuHCl). The activation free energy for the slow step varied linearly with GuHCl concentration; the rate constant, extrapolated to aqueous solution, was 7600 per second. Electron-transfer methods can bridge the nanosecond to millisecond measurement time gap for protein folding.

Cross-Sections, Rate Constants and Transport Coefficients in Silane Plasma Chemistry

Abstract: This paper presents a critical review of the basic data concerning the physics and chemistry of low pressure SiH 4 glow discharges used to deposit hydrogenated amorphous silicon films (a-Si:H). Starting with an updated table of thermochemical data, we analyze the gas-phase elementary processes consisting of i) electron-molecule collisions, ii) ion-molecule collisions, iii) neutral-neutral collisions, iv) other electron and ion collisions involving electron-ion and ion-ion recombination, electron attachment on radicals and detachment of anions, and v) cluster growth kinetics in dusty plasmas. Experimental data or theoretical estimates are given and discussed in terms of cross-sections. collision and reaction rate constants, and transport coefficients. We also analyze the surface processes and reaction probabilities of ions, radicals and molecules.

Mechanism of carbon dioxide-catalyzed oxidation of tyrosine by peroxynitrite

Abstract: Peroxynitrite ion (ONO2-) reacted rapidly with CO2 to form a short-lived intermediate provisionally identified as the ONO2CO2- adduct. This adduct was more reactive in tyrosine oxidation than ONO2- itself and produced 3-nitrotyrosine and 3,3'-dityrosine as the major oxidation products. With tyrosine in excess, the rate of 3-nitrotyrosine formation was independent of the tyrosine concentration and was determined by the rate of formation of the ONO2CO2- adduct. The overall yield of oxidation products was also independent of the concentration of tyrosine and medium acidity; approximately 19% of the added ONO2- was converted to products under all reaction conditions. However, the 3-nitrotyrosine/3,3'-dityrosine product ratio depended upon the pH, tyrosine concentration, and absolute reaction rate. These data are in quantitative agreement with a reaction mechanism in which the one-electron oxidation of tyrosine by ONO2CO2- generates tyrosyl and NO2 radicals as intermediary species, but are inconsistent with mechanisms that invoke direct electrophilic attack on the tyrosine aromatic ring by the adduct. Based upon its reactivity characteristics, ONO2CO2- has a lifetime shorter than 3 ms and a redox potential in excess of 1 V, and oxidizes tyrosine with a bimolecular rate constant greater than 2 x 10(5) M-1 s-1. In comparison, in CO2-free solutions, oxidation of tyrosine by peroxynitrite was much slower and gave significantly lower yields (approximately 8%) of the same products. When tyrosine was the limiting reactant, 3,5-dinitrotyrosine was found among the reaction products of the CO2-catalyzed reaction, but this compound was not detected in the uncatalyzed reaction.

Fast one-phase oil-rich processes for the preparation of vegetable oil methyl esters

Abstract: A re-evaluation of kinetic data shows that the methoxide base-catalyzed methanolysis of soybean oil at 40°C (6:1 methanol:oil molar ratio) to form methyl esters proceeds approximately 15 times more slowly than butanolysis at 30°C. This is interpreted to be the result of a two-phase reaction in which methanolysis occurs only in the methanol phase. Low oil concentration in methanol causes the slow reaction rate; a slow dissolving rate of the oil in the methanol causes an initiation period. Intermediate mono- and diglycerides preferentially remain in the methanol, and react further, thus explaining the deviation from second-order kinetics. The same explanations apply for hydroxide ion catalyzed methanolysis. At the 6:1 methanol:oil molar ratio the addition of a cosolvent, such as 1.25 volumes of tetrahydrofuran (THF) per volume of methanol, produces an oil-dominant one-phase system in which methanolysis speeds up dramatically and occurs as fast as butanolysis. The critical separation of the glycerol-rich phase still occurs and does so faster than in the cosolvent-free system. For THF, recycle of solvent is simplified because of the similar boiling points of THF (67°C) and methanol (65°C). Possible explanations for the abnormal slowing of the methanolysis reactions are presented in terms of (1) lower rate constants for mono- and diglyceride reactions due to the formation of cyclic intermediates, (2) a fall in the polarity of the reaction mixture due to either methanol depletion or mixing of the oil, methanol and cosolvent, and (3) depletion of hydroxide ion when this is present.

Reaction kinetics of thermal denaturation of whey proteins in heated reconstituted whole milk

Abstract: Reconstituted whole milk was heated using pilot-scale heating equipment. Kinetic and thermodynamic parameters for the irreversible denaturation of β-lactoglobulins A and B and α-lactalbumin were determined. α-Lactalbumin denaturation was first order, whereas both β-lactoglobulin variants had a reaction order of 1.5. Arrhenius plots for all three proteins showed an abrupt change in temperature dependence. In the low-temperature range, the thermodynamic parameters were ascribed to typical denaturation processes in which the unfolding of the protein tertiary structure is the rate-determining step. At higher temperatures, these parameters were in the range expected for typical condensation reactions, suggesting that aggregation processes may be rate-determining in this temperature range. The rate constants for β-lactoglobulin denaturation were independent of the initial protein concentration at all temperatures. For α-lactalbumin at temperatures below 85 °C the rate constants may have been dependent on the in...

Factors Affecting Competitive Ion−Molecule Reactions: ClO- + C2H5Cl and C2D5Cl via E2 and SN2 Channels

Abstract: Dual-level generalized transition state theory and statistical calculations based on correlated electronic structure calculations with augmented correlated basis sets are used to predict rate constants and deuterium kinetic isotope effects for the competing SN2 and E2 reactions of ClO- with C2H5Cl. The E2 reaction is favored by zero point effects despite its higher barrier. Furthermore, the entropic contribution of low-frequency transition state modes promotes the E2 reaction by about an order of magnitude at 300 K, despite nearly equal combined electronic/zero point propensities for the two reactions. The kinetic isotope effect for the E2 reaction is predicted to be 8.3 at 100 K and 3.1 at room temperature. The kinetic isotope effect of the SN2 reaction is predicted to be surprisingly low at low temperatures (0.06 at 100 K) and 0.60 at room temperature. The activation energy for both reactions is predicted to be negative below about 900 K and positive above 1000 K. Variational effects on the location of ...

Large kinetic isotope effects in methane oxidation catalyzed by methane monooxygenase: evidence for C-H bond cleavage in a reaction cycle intermediate.

Abstract: The reduced hydroxylase component (MMOH) of soluble methane monooxygenase (MMO) from Methylosinus trichosporium OB3b reacts with O2 and CH4 to produce CH3OH and H2O in a single-turnover reaction. Transient kinetic analysis of this reaction has revealed at least five and probably six intermediates during the turnover [Lee, S.-K., Nesheim, J. C., & Lipscomb, J. D. (1993) J. Biol. Chem. 268, 21569-21577; Liu, Y., Nesheim, J. C., Lee, S.-K., & Lipscomb, J. D. (1995) J. Biol. Chem. 270, 24662-24665]. One intermediate, termed compound Q, reacts with CH4 to yield enzyme-bound product. It is shown here that the deuterium kinetic isotope effect (KIE) for the reaction of compound Q with CH4 is 50-100, which is one of the largest effects observed to date. The rate constants for the reactions of the deuterated homologs of methane decrease monotonically as the deuterium content increases, suggesting that a large primary isotope effect dominates. The KIEs determined by analyzing the products after a single turnover have the following values: 1:1 CH4:CD4 (19); CD3H (12); CD2H2 (9); and CH3D (4). The KIE values determined by directly observing the reactive intermediate and by monitoring product ratios are all large, consistent with complete C-H bond breaking in the oxygenation step of the reaction. However, the differences in the KIE values determined by these two methods suggest that the reaction is more complex than currently proposed. A modified mechanism introducing the possibility of hydrogen-atom reabstraction by an intermediate methyl radical is proposed.

Thermal rate constants of the N 2 +O→NO+N reaction using ab initio 3 A″ and 3 A' potential energy surfaces

Abstract: Theoretical determinations of the thermal rate constants and product energy distributions of the N2+O→NO+N reaction, which plays a crucial role in hydrocarbon air combustion and high temperature air chemistry, are carried out using a quasiclassical trajectory method. An analytical fit of the lowest 3A′ potential energy surface of this reaction based on the CCI ab initio data is obtained. The trajectory study is done on this surface and an analytical 3A″ surface proposed by Gilibert et al. [J. Chem. Phys. 97, 5542 (1992)]. The thermal rate constants computed from 3000 to 20 000 K are in good agreement with the available experimental data. In addition, the dependence of the rate constant on the N2 internal state is studied. It is found that a low vibrational excitation can reduce the rate constant of this reaction by a factor of 3. Also, we investigate the effect of the N2 vibrational state on the product NO vibrational distribution, and it is found that at low N2 vibrational states, the NO vibrational dist...

Inhibition of calcium channels in rat central and peripheral neurons by omega-conotoxin MVIIC.

Abstract: Inhibition of voltage-dependent calcium channels by omega-conotoxin MVIIC (omega-CTx-MVIIC) was studied in various types of rat neurons. When studied with 5 mM Ba2+ as charge carrier, omega-CTx-MVIIC block of N-type calcium channels in sympathetic neurons was potent, with half- block at 18 nM. Block of N-type channels had a rapid onset (tau approximately 1 sec at 1 microM omega-CTx-MVIIC) and quick reversibility (tau approximately 30 sec). The rate of block was proportional to toxin concentration, consistent with 1:1 binding of toxin to channels, with a rate constant (k on) of approximately 1 X 10(6) M-1. sec-1. Both potency and rate of block were reduced dramatically with increasing concentrations of extracellular Ba2+ omega- CTx-MVIIC also blocked P-type calcium channels in cerebellar Purkinje neurons, but both development and reversal of block were far slower than for N-type channels. The rate of block was proportional to toxin concentration, with k on -1.5 x 10(3) M-1. sec-1 at 5 mM Ba2+. From this value and an unblocking time constant of approximately 200 min, a dissociation constant of approximately 50 nM was estimated. Thus, block of P-type channels is potent but very slow. In hippocampal CA3 pyramidal neurons, omega-CTx-MVIIC blocked approximately 50% of the high-threshold calcium channel current; one component (approximately 20%) was blocked with the rapid kinetics expected for N-type channels, whereas the other component was blocked slowly. The component blocked slowly was reduced but not eliminated by preexposure to 200 nM or 1 microM omega-Aga-IVA.

Mechanism of Proton-Coupled Electron Transfer for Quinone (QB) Reduction in Reaction Centers of Rb. Sphaeroides

Abstract: The mechanism of the proton-coupled electron transfer reaction, + H+ → QA(QBH)- was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the QA site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the QB site. The measured rate constants, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled by (i) four possible two-step mechanisms in which electron transfer can precede or follow proton transfer and can be either the rate determining or fast step in the overall reaction and (ii) a one-step mechanism involving the concerted transfer of an electron and a proton. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The two stepwise mec...

OH Radical Reactions: The Major Removal Pathway for Polychlorinated Biphenyls from the Atmosphere

Abstract: Polychlorinated biphenyls (PCBs) are transported through the environment primarily in the atmosphere and may undergo chemical reactions, particularly with the OH radical, while in the vapor phase. Rate constants for the gas-phase reaction of 14 PCB congeners with the OH radical over the temperature range of 323−363 K were measured. The calculated temperature dependences of the reactions were used to estimate OH−PCB reaction rate constants at 298 K; see Table 4. These 298 K rate constants agree well with literature rate constants for PCBs containing zero to two chlorines and with OH−PCB reaction rate constants estimated from a structure−activity method. Calculated atmospheric lifetimes of PCBs due to OH-initiated reactions varied from 2 days for biphenyl to 34 days for a pentachlorobiphenyl. A simple model for the vertical concentration gradient of PCBs in the troposphere was developed and used to calculate the total global loss rate (8300 t yr-1) of PCBs from the atmosphere due to removal by OH. This path...

Self‐Assembly of Heterobimetallic Complexes and Reactive Nucleophiles: A General Strategy for the Activation of Asymmetric Reactions Promoted by Heterobimetallic Catalysts

Abstract: Heterobimetallic asymmetric catalysts, such as the lanthanum–lithium–binaphthol complex (La Li–BINOL), the aluminum–lithium–binaphthol complex (AlLi–BINOL), and a newly prepared gallium–sodium–binaphthol complex (Ga Na–BINOL), have been self-assembled with reactive nucleophiles, such as lithium nitronates and sodium malonates, to generate more efficient catalysts than the parent heterobimetallic catalysts. For example, by the combined use of La Li–BINOL (1 mol%; contains one H2O molecule) and BuLi (0.9 mol%) as the catalyst system, asymmetric nitroaldol reactions are greatly accelerated in all cases without a decrease in the optical purity of the nitroaldol products. Kinetic analyses have also been carried out on the Ga Na–BINOL-catalyzed Michael reaction of dibenzyl malonate with cyclohexenone, with or without NaOtBu. The calculated rate constants show that the combined use of Ga Na–BINOL and NaOtBu as the catalyst gives reaction rates that are about 50 times faster than with Ga Na–BINOL alone. This activation method should be useful for other asymmetric reactions catalyzed by heterobimetallic complexes.

Lewis acid catalysis of a Diels-Alder reaction in water

Abstract: Here we report the first detailed study of a Diels-Alder (DA) reaction that is catalyzed by Lewis acids in water. The effect of Co2+, Ni2+, Cu2+ and Zn2+ ions as Lewis acid catalysts on the rate and endo-exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-1-(2-pyridyl)-2-propen-1-ones (1a-e) and cyclo-pentadiene (2) in water has been studied. Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M CU(NO3)(2) in water accelerates the reaction by a factor of 79 300. The kinetics of the catalyzed reaction were analyzed in terms of equilibrium constants for complexation of the Lewis acid with 1a-e and rate constants for the reaction of the resulting complexes with 2. The rate enhancement imposed upon the uncatalyzed DA reaction of substrates 1 with 2 by water is much more pronounced than that for the catalyzed reaction. The increase of the endo-exo selectivity induced by water in the uncatalyzed process is completely absent for the Lewis acid catalyzed reaction. The modest solvent and substituent effects observed for the catalyzed reaction indicate that the change in charge separation during the activation process is not larger than the corresponding change For the uncatalyzed reaction.

The reaction of ebselen with peroxynitrite

Abstract: Ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)-one, rapidly reacts with peroxynitrite, the rate constant being of the order of 10(6) M-1 s-1; the reaction yields the selenoxide of the parent molecule, 2-phenyl-1,2-benzisoselenazol-3(2H)-one 1-oxide, as the sole selenium-containing product; a stoichiometry of 1 mol of ebselen reacted and of the selenoxide formed per mole of peroxynitrite was observed. The reaction was studied in detail at neutral and alkaline pH (pH 10-11). It also proceeds at acidic pH where peroxynitrous acid (ONOOH) is predominant, the yield of the selenoxide being lower because peroxynitrous acid (pKa = 6.8) decays rapidly. Reduction of the selenoxide in cells to regenerate ebselen would allow for a sustained defense against peroxynitrite. This novel reaction constitutes a potential cellular defense line against peroxynitrite, one of the important reactive species in inflammatory processes.

How Important Are Quantum Mechanical Nuclear Motions in Enzyme Catalysis

Abstract: The role of quantum mechanical nuclear motions in enzymatic reactions is examined by realistic simulations that take into account the fluctuations of an entire enzyme−substrate complex This is done by using the quantized classical path (QCP) approach which is based on Feynman's path integral formulation The calculations evaluate the quantum mechanical activation free energy and deuterium isotope effect for the proton transfer step in the catalytic reaction of carbonic anhydrase The calculated and observed isotope effects are in very good agreement, thus demonstrating the potential of our approach in extracting mechanistic information Furthermore, the value of the calculated quantum mechanical rate constant is in a good agreement with the corresponding observed value This is significant since the evaluation of the ratio between the quantum mechanical rate constants of the reaction in the protein and in aqueous solution does not involve any adjustable parameter The reliability of our calculations is b

Determination of the pKa of Ethyl Acetate: Brønsted Correlation for Deprotonation of a Simple Oxygen Ester in Aqueous Solution

Abstract: The rate constants for deprotonation of ethyl acetate by 3-substituted quinuclidines are correlated by β = 1.09 ± 0.05. The limits of kBH = 2−5 × 109 M-1 s-1 for the encounter-limited reaction of the simple oxygen ester enolate with protonated quinuclidine (pKBH = 11.5) were combined with kB = 2.4 × 10-5 M-1 s-1 for deprotonation of ethyl acetate by quinuclidine, to give pKaK = 25.6 ± 0.5 for ionization of ethyl acetate as a carbon acid in aqueous solution. A rate−equilibrium correlation for proton transfer from methyl and benzylic monocarbonyl compounds to hydroxide ion has been extended by 6 pK units in the thermodynamically unfavorable direction, and it is shown that the absence of curvature of this correlation is inconsistent with a constant Marcus intrinsic barrier for the enolization of simple carbonyl compounds.