Extending limits of chlorine kinetic isotope effects

Chlorine kinetic isotope effects exceeding semiclassical limits were observed in enzyme-catalyzed reactions, but their source has not been yet identified. Herein we show that unusually large chlorine kinetic isotope effects are associated with reactions in which chlorine is the central atom that is being passed between two heavy atoms. The origin of these large values is the ratio of imaginary frequencies for light-to-heavy species (the so-called temperature-independent factor).     

Analysis of kinetic isotope effects for nonadiabatic reactions

Factors influencing the rates of quantum mechanical particle transfer reactions in many-body systems are discussed. The investigations are carried out on a simple model for a proton transfer reaction that captures generic features seen in more realistic models of condensed phase systems. The model involves a bistable quantum oscillator coupled to a one-dimensional double-well reaction coordinate, which is in turn coupled to a bath of harmonic oscillators. Reactive-flux correlation functions that involve quantum-classical Liouville dynamics for chemical species operators and quantum equilibrium sampling are used to estimate the reaction rates. Approximate analytical expressions for the quantum equilibrium structure are derived. Reaction rates are shown to be influenced significantly by both the quantum equilibrium structure and nonadiabatic dynamics. Nonadiabatic dynamical effects are found to play the major role in determining the magnitude of the kinetic isotope effect for the model transfer reaction.     

Large concentration effects on the magnitude of secondary alpha-deuterium kinetic isotope effects

Because the secondary alpha-deuterium kinetic isotope effects in some S_N2 and E2 reactions are strongly concentration dependent, isotope effects measured at a single concentration could lead to erroneous conclusions about the mechanisms and transition state structures.     

Large kinetic isotope effects with unsymmetrical transition states

The results of a series of calculations for a wide, unrestricted variation in the force constants for the making and breaking bonds and their interaction constant are presented for the abstraction reactions of CH₂D₂ with Cl atoms. A wide range of asymmetrical force constants leads to a high kinetic isotope effect as has been pointed out by others for a more restricted range of consideration. These results pointedly contradict the assumed connection between a high kinetic isotope effect and a symmetrical transition state. It is found by examining the atomic displacements of the normal mode that the motion of the H or D atom in the real stretch of the transition state will often have little influence on the isotope effect because the mode can be dominated by end group motions. It is further found that a 3-center model multiplied by a constant factor to account for the contributions of the other vibrations is capable of very satisfactorily reproducing the more rigorous 6-center calculations.     

Chlorine kinetic isotope effects on the haloalkane dehalogenase reaction.

We have found chlorine kinetic isotope effects on the dehalogenation catalyzed by haloalkane dehalogenase from Xanthobacter autotrophicus GJ10 to be 1.0045 +/- 0.0004 for 1,2-dichloroethane and 1.0066 +/- 0.0004 for 1-chlorobutane. The latter isotope effect approaches the intrinsic chlorine kinetic isotope effect for the dehalogenation step. The intrinsic isotope effect has been modeled using semiempirical and DFT theory levels using the ONIOM QM/QM scheme. Our results indicate that the dehalogenation step is reversible; the overall irreversibility of the enzyme-catalyzed reaction is brought about by a step following the dehalogenation.     

Some analytical aspects of the measurement of heavy-atom kinetic isotope effects

Measurements of the kinetic isotope effects of heavy atoms are discussed from an analytical point of view. The discussion includes the choice of isotope, the procedure for measurement, and methods for increasing the kinetic isotope effect. The procedures most commonly used and newly developed techniques are presented and their advantages and disadvantages considered.     

Nitrogen kinetic isotope effects on the decarboxylation of 4-pyridylacetic acid

Nitrogen kinetic isotope effects on the decarboxylation of 4-pyridylacetic acid have been measured in solvents of different polarity and have been found to vary from the inverse value of 0.994 to the normal value of 1.002 upon increase of water content of the binary dioxane--water solvent from 25% to 75% (v/v), respectively. These changes were successfully modeled theoretically and shown to originate from the large inverse nitrogen isotope effect on the equilibrium between acidic and zwitterionic forms.     

Carbon-14 kinetic isotope effects in the debromination of dibromocinnamic acid

The kinetic isotope effect (KIE) method was applied to study the mechanism of elimination of bromine from erythro-a,b-dibromocinamic acid. The large ¹⁴C KIE for both a- and b-position of side chain of erythro-a,b-dibromocinamic acid proves that elimination of bromine leading to formation of (E)-cinnamic acid proceeds via E2 mechanism.     

Carbon-14 kinetic isotope effects in the debromination of p-methyldibromocinnamic acid

The kinetic isotope effect (KIE) was applied in the study of the mechanism of bromine elimination from p-methyl-erythro-a,b-dibromocinnamic acid, successively labeled at the a and b carbons. The large ¹⁴C KIE for the b-position and small KIE for the a one of side chain of p-methyl-erythro-a,b-dibromocinamic acid proves that elimination of bromine leading to the formation of (E)-p-methylcinnamic acid proceeds via the E1 mechanism.     

Calculations of substituent and solvent effects on the kinetic isotope effects of Menshutkin reactions

Nitrogen, deuterium, halogen, and carbon kinetic isotope effects have been modeled for the Menshutkin reaction between methyl halides and substituted N,N-dimethylaniline at the HF/6-31G(d) level of theory augmented by the C-PCM continuum solvent model for several solvents. Systematic changes in geometries of the transition states and Gibbs free energies of activation have been found with phenyl ring substituents, solvent, and the leaving group. Kinetic isotope effects also change systematically; however, these changes are predicted to be small, inside the usual precision of the experimental measurements. On the contrary, no correlation has been found between the kinetic isotope effects and the Hammett constants for para substituents. Thus opposite to previous assumptions, our results indicate that kinetic isotope effects on the Menshutkin reaction cannot be used to predict the position of the transition state on the reaction coordinate.     

Competitive 15N kinetic isotope effects of nitrogenase-catalyzed dinitrogen reduction

Biological N2 fixation is achieved under ambient conditions by enzymatic catalysis. The enzyme nitrogenase has been studied extensively, but the N2 chemical reduction step is, by far, not rate limiting and hard to examine. A new method was developed that allows studying the reduction transition state within the enzyme's complex kinetic cascade by means of the 15N kinetic isotope effect on the reaction's second-order rate constant, V/K. A value of 1.7% +/- 0.2% was measured.     

Efficient estimators for quantum instanton evaluation of the kinetic isotope effects: application to the intramolecular hydrogen transfer in pentadiene

The quantum instanton approximation is used to compute kinetic isotope effects for intramolecular hydrogen transfer in cis-1,3-pentadiene. Due to the importance of skeleton motions, this system with 13 atoms is a simple prototype for hydrogen transfer in enzymatic reactions. The calculation is carried out using thermodynamic integration with respect to the mass of the isotopes and a path integral Monte Carlo evaluation of relevant thermodynamic quantities. Efficient "virial" estimators are derived for the logarithmic derivatives of the partition function and the delta-delta correlation functions. These estimators require significantly fewer Monte Carlo samples since their statistical error does not increase with the number of discrete time slices in the path integral. The calculation treats all 39 degrees of freedom quantum mechanically and uses an empirical valence bond potential based on a molecular mechanics force field.     

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions

This work describes the first experimental studies of deuterium kinetic isotope effects (KIEs) for the gas-phase E2 reactions of microsolvated systems. The reactions of F(-)(H(2)O)(n) and OH(-)(H(2)O)(n), where n = 0, 1, with (CH(3))(3)CX (X = Cl, Br), as well as the deuterated analogs of the ionic and neutral reactants, were studied utilizing the flowing afterglow-selected ion flow tube technique. The E2 reactivity is found to decrease with solvation. Small, normal kinetic isotope effects are observed for the deuteration of the alkyl halide, while moderately inverse kinetic isotope effects are observed for the deuteration of the solvent. Minimal clustering of the product ions is observed, but there are intriguing differences in the nature and extent of the clustering process. Electronic structure calculations of the transition states provide qualitative insight into these microsolvated E2 reactions.     

Primary and secondary kinetic isotope effects in E2 elimination reactions

Primary and secondary deuterium kinetic isotope effects have been measured for elimination of LCl (L = H or D) from some substituted 1,2-diaryl-1-chloroethanes. Although changes in these effects are in agreement with theoretical predictions, the high values of (E_H-E_D)_β and the significantly low A_H/A_D may suggest that either proton tunnelling or an internal return mechanism is complicating this E2 elimination.