Kinetic Hydrogen Isotope Effects Research Articles

A Mechanistic Study of the Oxidation of Substituted Benzyl Alcohols with Trichloroisocyanuric Acid

A systematic mechanistic study employing kobs kineticsmeasurements, linear free‐energy relationship, hydrogen kinetic isotope effect and rate law, as well as the Density Functional Theory (DFT) approach (M06‐2x/6‐311G(d,p)) for the oxidation of diverse substituted benzyl alcohols with trichloroisocyanuric acid (TCCA) / H2O in 50 % MeCN / CH2Cl2 at 25.0 °C is present. The kinetic results showed an autocatalytic behavior and a primary kinetic isotope effect (4.22). The linear free‐energy relationship (σ, ρ = ‐1.22) indicated a mechanism change for 4‐CF3, 3‐NO2 and 4‐NO2 substituted benzyl alcohols and a fractional rate law (3.25) for benzyl alcohol. The DFT results indicated Cl2, formed in situ from TCCA and the benzyl alcohol, as the actual oxidizing species. Furthermore, calculations support the kinetic results with high agreement through a transition state that performs a hydride abstraction by autocatalytic Cl2 in the induction zone. In the case of substrates bearing strong electron withdrawing groups, the mechanism changes to hydrogen abstraction from the corresponding benzyl hypochlorite as the main pathway.

Hydrogen isotope effect on thermodynamic and kinetic properties of Pd based ternary alloys: Pd–Ag-M (M = Au, Zr)

Owing to the importance of hydrogen and its isotopes in energy sector, the present study investigates thermodynamic and kinetic hydrogen isotope effects on desorption reaction of newly synthesized Pd-based ternary alloy-hydrides/deuterides viz., Pd0.89Ag0.05M0.06 (M = Au, Zr)–H/D. Alloys were prepared by arc melting method followed by characterization using PXRD, SEM, TXRF and EDX techniques. The effect of hydrogen isotopes on thermodynamics of H2/D2 desorption reaction was investigated by generating pressure-temperature isotherms (PCIs) at various temperatures using a Sieverts' type volumetric apparatus. The PCT diagram of alloys depicts a single plateau which confirms desorption to be a one-step process. Both alloy systems display a normal isotope effect as indicated by higher plateau pressure for deuterides as compared to hydrides. The enthalpy and entropy changes for the desorption reactions were calculated using Van't Hoff equation. The separation factor derived from PCT data indicates that Pd0.89Ag0.05Au0.06 alloy has a significantly high separation factor as compared to other Pd based alloys reported in the literature. To get more insight into this behavior, non-isothermal kinetic analysis of H2/D2 desorption based on Differential Scanning Calorimeter (DSC) data acquired at four different heating rates was conducted and activation energies of desorption were calculated by invoking Kissinger's equation. The activation energies of alloys were found to be lower than that of palladium which explains the high separation factor of alloys as compared to Pd. The desired combination of higher separation factor and lower activation energy makes Pd0.89Ag0.05Au0.06 alloy a promising material for practical application in the separation of hydrogen isotopes via self-displacement gas chromatography (SDGC).

Thermodynamics and kinetics hydrogen isotope effects of hydrogen/deuterium absorption in Pd-5wt.%Pt alloy

Palladium–Platinum (Pd–Pt) alloys have excellent hydrogen isotope separation properties near room temperature compared to pure palladium (Pd), and are expected to be used as filling materials in thermal cyclic absorption (TCAP) columns. However, most studies on the thermodynamic and kinetic hydrogen isotope effects of hydrogen absorption/deuterium in Pd–Pt alloys have been conducted at temperatures of 273 K or above, with few studies involving the performance of TCAP in the cold-cycle operating temperature range (253 K–303 K). In this work, the performance of Pd-5wt.%Pt alloy for the absorption of hydrogen/deuterium at low temperatures (263 K–323 K) was investigated. The results show that the enthalpy and entropy of dissolution of hydrogen and deuterium in the α-phase of Pd-5wt.%Pt alloy are −11.77 kJ/mol (H), 104.00 J mol−1 K−1(H); −5.50 kJ/mol (D), 87.88 J mol−1 K−1 (D), respectively, which are lower than the enthalpy and entropy change values of pure Pd materials. The apparent reaction activation energies of protium and deuterium absorption are −4.14 kJ/mol (H2) and −11.87 kJ/mol (D2), respectively, which are lower than those of pure palladium. The excellent hydrogen isotope separation performance of Pd–Pt alloys may originate from the fast hydrogen absorption and discharge isotope kinetic performance.

Experimental determinations of carbon and hydrogen isotope fractionations and methane clumped isotope compositions associated with ethane pyrolysis from 550 to 600 °C

Methane clumped isotope compositions signify the relative natural abundances of rare, doubly substituted isotopic species of methane (13CH3D and 12CH2D2) and have emerged as a new isotopic tool to trace the sources, sinks, and lifecycles of methane in the environment. Such measurements can identify equilibration (or re-equilibration) temperatures if found to be in isotopic equilibrium or non-equilibrium processes (e.g., kinetically controlled reactions or mixing) if not in isotopic equilibrium. Naturally occurring thermogenic methane—formed by the thermally activated breakdown of larger organic molecules—has been found to have clumped isotope compositions consistent with equilibrium at reasonable gas formation temperatures in some settings and non-equilibrium processes occurring during either formation, migration, storage, or extraction in others. To explore the potential controls on the isotopic composition of thermogenic methane, we conducted isothermal time-series ethane pyrolysis experiments at 550 and 600 °C to measure methane and ethane 13C/12C and D/H fractionations and methane clumped isotope compositions (resolved 13CH3D and 12CH2D2). We explore the effects of modifying the initial clumped isotope composition of ethane and the addition of water vapor to pyrolysis experiments. We observe that ethane and methane 13C/12C are controlled by kinetic isotope effects and Rayleigh distillation processes. In contrast, ethane and methane D/H and methane clumped isotope compositions appear to be controlled by a combination of these processes and hydrogen isotope exchange. The hydrogen isotope exchange processes lead to isotopic equilibrium as reaction completion is approached for both D/H (ethane/methane) and methane clumped isotope compositions. We develop a chemical model based on a mass balance approach that accounts for inheritance vs. hydrogen-abstraction formation pathways for singly and doubly substituted isotopologues of ethane and methane that is compared to the experimental data. The model allows the determination of carbon and hydrogen kinetic isotope effects associated with ethane cracking and hydrogen abstraction reactions that, where applicable, we compare to prior theoretical constraints. From the comparison of the model to the experimental data, we infer that the kinetically controlled ethane and methane bulk isotope compositions and methane clumped isotope compositions are controlled by kinetic isotope effects (both primary and secondary) associated with both CC bond and CH bond cleavage reactions. Specifically, the methane clumped isotope compositions likely result from a combination of clumped isotope effects associated with ethane breakdown and/or assembly of methane isotopologues (expressed in terms of γ-factor parameters ≠ 1) and combinatorial effects that arise probabilistically. We discuss our experimental results in the context of recent pyrolysis experiments and observations of naturally occurring thermogenic methane. We consider a proposal consistent with observations from nature that the hydrogen isotope exchange reactions that promote equilibration of methane isotopic molecules at or near formation temperature may be facilitated by free radicals generated by pyrolysis reactions. In this framework, isotope exchange effectively ceases when pyrolysis effectively ceases locking in compositions that can be consistent with peak formation temperatures.

Kinetic hydrogen isotope effects and flow compensation strategy of the ZrCo-based chemical beds in full-scale SDS demo-system

ZrCo-based bed, commonly exploited to efficiently process large quantities of mixed hydrogen isotope gases, is one of crucial component in the storage and delivery system (SDS) of the tritium plant in fusion reactor. However, the hydrogen isotope effect on the desorption and adsorption processes of the ZrCo based assemblies has not yet clearly addressed, especially for a full-scale (100 g of Tritium) bed assembly. Herein, the kinetic hydrogen isotope effect of the full-scale ZrCo-based chemical beds, including ZrCo, Zr0.8Ti0.2Co, Zr0.8Hf0.2Co, with different hydrogen isotope ratios in SDS demo-system are investigated in detail. This work shows that the kinetic adsorption isotope effect of all ZrCo based beds are independent of pressure, temperature, and hydrogen isotope ratio. Conversely, the kinetic hydrogen isotope fluctuation is prominent during hydrogen desorption, where the proportion of protium is relatively high at the initial stage. After that, deuterium is easier to release. Finally, the protium gradually increases. The fluctuating degree is in the sequence of Zr0.8Ti0.2Co > ZrCo > Zr0.8Hf0.2Co for types of chemical beds, and 50%D + 50%H > 90%D + 10%H > 10%D + 90%H for isotope components. Based on experimental details, a flow compensation approach to ensure a stable ratio of hydrogen isotope gas is proposed. This work is of crucial significance for practical-scale SDS system operation, which is the prerequisite for the steady-state running of the fusion reactor.

Heterogeneous activation of persulfate by FeS – Surface influence on selectivity

Recently, several studies have been published on sulfate radicals as strong oxidants and methods of their generation. A shift from homogeneous to heterogeneous methods can be observed for persulfate activation as a means of generating sulfate radicals. However, to date, the influence of the surface on the radical chemistry has not been examined in detail.In the present study, homogeneous persulfate activation methods (elevated temperature and dissolved iron(II)) are compared with heterogeneous activation by FeS. The selectivity patterns for the oxidation of chlorinated ethenes, ethanes and benzenes differ notably depending on the persulfate activation method. The obtained kinetic data are in conformity with the hypothesis that sulfate radicals generated at the FeS surface have different selectivities than freely dissolved radicals. The assumption of different reactive species is further confirmed by the determination of hydrogen kinetic isotope effects (H-KIEs) for the oxidation of methanol isotopologues with sulfate radicals using the set of activation methods. The H-KIE obtained in the heterogeneous system (H-KIEFeS = 3.0 ± 0.3) is significantly higher than for the homogeneous system (H-KIEhom = 2.3 ± 0.1). This leads us to the conclusion that sulfate radicals generated on the FeS surface react as surface-associated radicals, which show a different selectivity pattern than freely dissolved radicals.Furthermore, the apparent second-order rate constants in homogenous solution at ambient temperature, ranging from 5 · 107 to 1.7 · 109 M−1 s−1, were determined for the reaction of sulfate radicals with various chlorinated hydrocarbons under study.

Kinetic Hydrogen Isotope Effects and Flow Compensation Strategy of the Zrco-Based Chemical Beds in Full-Scale Sds Demo-System

Kinetic Hydrogen Isotope Effects and Flow Compensation Strategy of the Zrco-Based Chemical Beds in Full-Scale Sds Demo-System

Tunneling of Hydrogen and Deuterium Atoms on Interstellar Ices (Ih and ASW)

Model calculations are performed to investigate the kinetic isotope effect of hydrogen and deuterium atom diffusion on hexagonal ice and amorphous solid water. Comparisons with experimental results by Kuwahata et al. (Phys. Rev. Lett., Sep. 2015, 115 (13), 133201) at 10 K are made. The experimentally derived kinetic isotope effect on amorphous solid water is reproduced by transition state theory. The experimentally found kinetic isotope effect on hexagonal ice is much larger than on amorphous solid water and is not reproduced by transition state theory. Additional calculations using model potentials are made for the hexagonal ice, but the experimental kinetic isotope effect is not fully reproduced. A strong influence of temperature is observed in the calculations. The influence of tunnelling is discussed in detail and related to the experiments. The calculations fully support the claims by the Kuwahata et al. (Phys. Rev. Lett., Sep. 2015, 115 (13), 133201) that on amorphous solid water the diffusion is predominantly by thermal hopping while on the polycrystalline ice tunnelling diffusion contributes significantly.

Unravelling the process of petroleum hydrocarbon biodegradation in different filter materials of constructed wetlands by stable isotope fractionation and labelling studies

Maintaining and supporting complete biodegradation during remediation of petroleum hydrocarbon contaminated groundwater in constructed wetlands is vital for the final destruction and removal of contaminants. We aimed to compare and gain insight into biodegradation and explore possible limitations in different filter materials (sand, sand amended with biochar, expanded clay). These filters were collected from constructed wetlands after two years of operation and batch experiments were conducted using two stable isotope techniques; (i) carbon isotope labelling of hexadecane and (ii) hydrogen isotope fractionation of decane. Both hydrocarbon compounds hexadecane and decane were biodegraded. The mineralization rate of hexadecane was higher in the sandy filter material (3.6 µg CO2 g−1 day−1) than in the expanded clay (1.0 µg CO2 g−1 day−1). The microbial community of the constructed wetland microcosms was dominated by Gram negative bacteria and fungi and was specific for the different filter materials while hexadecane was primarily anabolized by bacteria. Adsorption / desorption of petroleum hydrocarbons in expanded clay was observed, which might not hinder but delay biodegradation. Very few cases of hydrogen isotope fractionation were recorded in expanded clay and sand & biochar filters during decane biodegradation. In sand filters, decane was biodegraded more slowly and hydrogen isotope fractionation was visible. Still, the range of observed apparent kinetic hydrogen isotope effects (AKIEH = 1.072–1.500) and apparent decane biodegradation rates (k = − 0.017 to − 0.067 day−1) of the sand filter were low. To conclude, low biodegradation rates, small hydrogen isotope fractionation, zero order mineralization kinetics and lack of microbial biomass growth indicated that mass transfer controlled biodegradation.

Can Alkaline Hydrolysis of γ-HCH Serve as a Model Reaction to Study Its Aerobic Enzymatic Dehydrochlorination by LinA?

Hexachlorocyclohexane (HCH) isomers constitute a group of persistent organic pollutants. Their mass production and treatment have led to a global environmental problem that continues to this day. The characterization of modes of degradation of HCH by isotope fractionation is a current challenge. Multi isotope fractionation analysis provides a concept to characterize the nature of enzymatic and chemical transformation reactions. The understanding of the kinetic isotope effects (KIE) on bond cleavage reaction contributes to analyses of the mechanism of chemical and enzymatic reactions. Herein, carbon, chlorine, and hydrogen kinetic isotope effects are measured and predicted for the dehydrochlorination reaction of γ-HCH promoted by the hydroxyl ion in aqueous solution. Quantum mechanical (QM) microsolvation with an implicit solvation model and path integral formalism in combination with free-energy perturbation and umbrella sampling (PI-FEP/UM) and quantum mechanical/molecular mechanical QM/MM potentials for including solvent effects as well as calculating isotope effects are used and analyzed with respect to their performance in reproducing measured values. Reaction characterization is discussed based on the magnitudes of obtained isotope effects. The comparative analysis between the chemical dehydrochlorination of γ-HCH in aqueous media and catalyzed reaction by dehydrochlorinase, LinA is presented and discussed. Based on the values of isotope effects, these two processes seem to occur via the same net mechanism.

Theoretical Investigation into Rate-Determining Factors in Electrophilic Aromatic Halogenation.

The halogenation of monosubstituted benzenes in aqueous solvent was studied using density functional theory at the PCM-M06-2 X/6-311G(d,p) level. The reaction with Cl2 begins with the formation of C atom coordinated π-complex and is followed by the formation of the σ-complex, which is rate-determining. The final part proceeds via the abstraction of the proton by a water molecule or a weak base. We evaluated the use of the σ-complex as a model for the rate-determining transition state (TS) and found that this model is more accurate the later the TS comes along the reaction coordinate. This explains the higher accuracy of the model for halogenations (late TS) compared to nitrations (early TS); that is, the more deactivated the substrate the later the TS. The halogenation with Br2 proceeds with a similar mechanism as the corresponding chlorination, but the bromination has a very late rate-determining TS that is similar to the σ-complex in energy. The iodination with ICl follows a different mechanism than chlorination and bromination. After the formation of the π-complex, the reaction proceeds in a concerted manner without a σ-complex. This reaction has a large primary hydrogen kinetic isotope effect in agreement with experimental observations.

The mechanism of a ligand-promoted C(sp3)-H activation and arylation reaction via palladium catalysis: theoretical demonstration of a Pd(II)/Pd(IV) redox manifold.

Density functional theory (DFT) computations (BP86 and M06-L) have been utilized to elucidate the detailed mechanism of a palladium-catalyzed reaction involving pyridine-type nitrogen-donor ligands that significantly expands the scope of C(sp(3))-H activation and arylation. The reaction begins with precatalyst initiation, followed by substrate binding to the Pd(II) center through an amidate auxiliary, which directs the ensuing bicarbonate-assisted C(sp(3))-H bond activation producing five-membered-ring cyclopalladate(II) intermediates. These Pd(II) complexes further undergo oxidative addition with iodobenzene to form Pd(IV) complexes, which proceed by reductive C-C elimination/coupling to give final products of arylation. The base-assisted C(sp(3))-H bond cleavage is found to be the rate-determining step, which involves hydrogen bond interactions. The mechanism unravels the intimate involvement of the added 2-picoline ligand in every phase of the reaction, explains the isolation of the cyclopalladate intermediates, agrees with the observed kinetic hydrogen isotope effect, and demonstrates the Pd(II)/Pd(IV) redox manifold.

Predicting CH4 Dissociation Kinetics on Metals: Trends, Sticking Coefficients, H Tunneling, and Kinetic Isotope Effect

The rate constant (W) of catalytic dissociation of a methane molecule to adsorbed CH3 and H and its temperature dependence are studied in the framework of a tunneling model which considers the reaction as a result of quantum transitions between discrete vibrational energy levels of an initial C–H bond in methane and energy continuum of the dissociated final state while the system reaches the transition state along other classical degrees of freedom. This step is assumed to be the rate determining one in many reactions of commercial significance like steam and dry reforming of methane. This approach has previously been successfully applied to other catalytic reactions of H atom transfer. Numerical calculations of the kinetic characteristics for the reaction proceeding over surfaces of Ni(111), Pt(111), Cu(111), Pd(111), Ru(001), and Ir(111) are interpreted in terms of the Brønsted–Evans–Polanyi (BEP) relationship. The good correlation and the theory show that the main discriminating parameter which determines the trend within a set of metals is the total adsorption energy of a methyl fragment and an H atom. The BEP relationship is used for prediction of the rate constants of the dissociation on rhodium, silver, and gold (111) surfaces and for predicting the activation energy on bimetallic surfaces. Sticking coefficients are estimated using the W values and adsorption energy of the methane molecule; the latter is approximately assumed to be the same for all considered surfaces. These estimations are compared with the experimental sticking coefficients for several metals showing quantitative agreement. Hydrogen kinetic isotope effects (KIEs) and their temperature dependencies are calculated for the dissociation on nickel and ruthenium surfaces, and the KIE estimates at fixed temperatures of iridium and rhodium are compared with the corresponding experimental data. Results of this comparison justify the application of the model for the description of the catalytic dissociation of the C–H bond.

Insights into the elimination mechanisms employed for the degradation of different hexachlorocyclohexane isomers using kinetic isotope effects and docking studies

Modern DFT functional as well as the continuum solvation model have been applied in order to theoretically predict carbon, chlorine and hydrogen kinetic isotope effects (KIEs) during aerobic degradation of four hexachlorocyclohexane isomers (α, β, δ and γ). A small model of the hexachlorocyclohexane dehydrochlorinase (LinA) active site comprising its catalytic dyad has been constructed based on the receptor–ligand complexes suggested by docking studies and compared to the respective reaction modeled in aqueous solution. In‐depth analysis of chlorine and hydrogen KIEs patterns clearly indicates different transition states in aqueous solution and in the model mimicking the protein active site. Although all isomers seem to undergo a concerted E2 mechanism, the contribution of proton transfer and carbon–chlorine leaving group bond stretch in the transition states differs for the different isomers giving rise to totally different magnitudes of predicted isotope effects. Copyright © 2013 John Wiley & Sons, Ltd.

A Tunnel Model for Activated Hydrogen Dissociation on Metal Surfaces

Activated dissociative adsorption of a hydrogen molecule on metal surfaces is studied in the framework of the precursor mediated dissociation (PMD) mechanism according to which the dissociation proceeds through a molecular-adsorbed state (precursor) which then dissociates to H atoms. The PMD activation energies and rate constants are calculated by considering quantum transitions from various vibration levels of initial state into a continuous manifold of energies of the final dissociated state. The geometric parameters required are taken from the literature for a nickel surface while on other metals the same geometry is assumed to hold but the energies differ. This approach is applied to gain an insight into specific details of the reaction mechanism. The Bronsted–Evans–Polanyi relationship coefficients are obtained and used for prediction of the catalytic reactivity of a wide range of (111) metals and alloys. Hydrogen kinetic isotope effect (KIE) of the reaction on the palladium surface and its temperatu...

Parallel and competitive pathways for substrate desaturation, hydroxylation, and radical rearrangement by the non-heme diiron hydroxylase AlkB.

A purified and highly active form of the non-heme diiron hydroxylase AlkB was investigated using the diagnostic probe substrate norcarane. The reaction afforded C2 (26%) and C3 (43%) hydroxylation and desaturation products (31%). Initial C-H cleavage at C2 led to 7% C2 hydroxylation and 19% 3-hydroxymethylcyclohexene, a rearrangement product characteristic of a radical rearrangement pathway. A deuterated substrate analogue, 3,3,4,4-norcarane-d(4), afforded drastically reduced amounts of C3 alcohol (8%) and desaturation products (5%), while the radical rearranged alcohol was now the major product (65%). This change in product ratios indicates a large kinetic hydrogen isotope effect of ∼20 for both the C-H hydroxylation at C3 and the desaturation pathway, with all of the desaturation originating via hydrogen abstraction at C3 and not C2. The data indicate that AlkB reacts with norcarane via initial C-H hydrogen abstraction from C2 or C3 and that the three pathways, C3 hydroxylation, C3 desaturation, and C2 hydroxylation/radical rearrangement, are parallel and competitive. Thus, the incipient radical at C3 either reacts with the iron-oxo center to form an alcohol or proceeds along the desaturation pathway via a second H-abstraction to afford both 2-norcarene and 3-norcarene. Subsequent reactions of these norcarenes lead to detectable amounts of hydroxylation products and toluene. By contrast, the 2-norcaranyl radical intermediate leads to C2 hydroxylation and the diagnostic radical rearrangement, but this radical apparently does not afford desaturation products. The results indicate that C-H hydroxylation and desaturation follow analogous stepwise reaction channels via carbon radicals that diverge at the product-forming step.

Dry Deposition of Biogenic Terpenes via Cationic Oligomerization on Environmental Aqueous Surfaces.

Unraveling the complex interactions between the atmosphere and the biosphere is critical for predicting climate changes. Although it is well-recognized that the large amounts of biogenic volatile organic compounds (BVOCs) emitted by plants must play important roles in this regard, current atmospheric models fail to account for their fate due to missing chemical sinks. Here, we applied online electrospray mass spectrometry to monitor aqueous microjets exposed to gaseous monoterpenes (α-pinene, β-pinene, and d-limonene) and found that these BVOCs are readily protonated (to C10H17(+)) and undergo oligomerization (to C20H33(+) and C30H49(+)) upon colliding with the surface of pH < 4 microjets. By considering that the yields of all products show inflection points at pH ≈ 3.5 and display solvent kinetic hydrogen isotope effects larger than 2, we conclude that the oligomerization process is initiated by weakly hydrated hydronium ions, H3O(+), present at the gas-water interface. Present results provide a universal mechanism for the dry deposition of unsaturated BVOCs and may account for recent observations on the uptake of terpenes in forest canopies and over grassland.

Kinetics of Catalytic OH Dissociation on Metal Surfaces

A tunnel mechanism of OH dissociation reaction on metal surfaces is suggested and analyzed. Expressions for the rate constant and the activation energy are obtained by calculating the transition probability on semiempirical potential energy surfaces in the framework of the nonadiabatic perturbation theory. The analysis of the transition probability leads to a linear Brønsted–Evans–Polanyi (BEP) relationship between the activation energies and the surface reaction energies (ΔI), correlated by Ea = 0.73ΔI + 18 (kcal/mol). This relationship is compared with published literature data. The effect of the electron coupling on the pre-exponential factor is discussed, and the kinetic isotope effect of hydrogen for the reaction is predicted.

Isotopic fractionation during soil uptake of atmospheric hydrogen

Abstract. Soil uptake of atmospheric hydrogen (H2) and the associated hydrogen isotope effect were studied using soil chambers in a Western Washington second-growth coniferous forest. Chamber studies were conducted during both winter and summer seasons to account for large natural variability in soil moisture content (4–50%) and temperature (6–22 °C). H2 deposition velocities were found to range from 0.01–0.06 cm s−1 with an average of 0.033 ± 0.008 cm s−1 (95% confidence interval). Consistent with prior studies, deposition velocities were correlated with soil moisture below 20% soil moisture content during the summer season. During winter, there was considerable variability observed in deposition velocity that was not closely related to soil moisture. The hydrogen kinetic isotope effect with H2 uptake was found to range from −24‰ to −109‰. Aggregate analysis of experimental data results in an average KIE of −57 ± 5‰ (95% CI). Some of the variability in KIE can be explained by larger isotope effects at lower (&lt;10%) and higher (&gt;30%) soil moisture contents. The measured KIE was also found to be correlated with deposition velocity, with smaller isotope effects occurring at higher deposition velocities. If correct, these findings will have an impact on the interpretation of atmospheric measurements and modeling of δD of H2.

Kinetic isotope effects of L-Dopa decarboxylase.

A mixed centroid path integral and free energy perturbation method (PI-FEP/UM) has been used to investigate the primary carbon and secondary hydrogen kinetic isotope effects (KIEs) in the amino acid decarboxylation of L-Dopa catalyzed by the enzyme L-Dopa decarboxylase (DDC) along with the corresponding uncatalyzed reaction in water. DDC is a pyridoxal 5'-phosphate (PLP) dependent enzyme. The cofactor undergoes an internal proton transfer between the zwitterionic protonated Schiff base configuration and the neutral hydroxyimine tautomer. It was found that the cofactor PLP makes significant contributions to lowering the decarboxylation barrier, while the enzyme active site provides further stabilization of the transition state. Interestingly, the O-protonated configuration is preferred both in the Michaelis complex and at the decarboxylation transition state. The computed kinetic isotope effects (KIE) on the carboxylate C-13 are consistent with that observed on decarboxylation reactions of other PLP-dependent enzymes, whereas the KIEs on the α carbon and secondary proton, which can easily be validated experimentally, may be used as a possible identification for the active form of the PLP tautomer in the active site of DDC.