DFT calculations for methyl cation complexed within a constrained cage of water molecules permit the controlled manipulation of the “axial” donor/acceptor distance and the “equatorial” distance to hydrogen-bond acceptors. The kinetic isotope effect k(CH₃)/k(CT₃) for methyl transfer within a cage with a short axial distance becomes less inverse for shorter equatorial C⋅⋅⋅O distances: a decrease of 0.5 Å results in a 3 % increase at 298 K. Kinetic isotope effects in AdoMet-dependent methyltransferases may be m∧odulated by CH⋅⋅⋅O hydrogen bonding, and factors other than axial compression may contribute, at least partially, to recently reported isotope-effect variations for catechol-O-methyltransferase and its mutant structures.

DFT calculations for methyl cation complexed within a constrained cage of water molecules permit the controlled manipulation of the “axial” donor/acceptor distance and the “equatorial” distance to hydrogen-bond acceptors. The kinetic isotope effect k(CH₃)/k(CT₃) for methyl transfer within a cage with a short axial distance becomes less inverse for shorter equatorial C⋅⋅⋅O distances: a decrease of 0.5 Å results in a 3 % increase at 298 K. Kinetic isotope effects in AdoMet-dependent methyltransferases may be m∧odulated by CH⋅⋅⋅O hydrogen bonding, and factors other than axial compression may contribute, at least partially, to recently reported isotope-effect variations for catechol-O-methyltransferase and its mutant structures.

Computational simulations for chloromethane hydrolysis have been performed using hybrid quantum-mechanical/molecular-mechanical methods with explicit solvation by large numbers of water molecules. In the first part of the paper, we present results for 2° ²H₃, 1° ¹⁴C, and 1° ³⁷Cl kinetic isotope effects (KIEs) at 298 K with both the AM1/TIP3P and B3LYP/6-31G* QM methods for the nucleophile H₂O and electrophile CH₃Cl surrounded by 496 solvating TIP3P water molecules. An initial Hessian computed for a subset of this system including up to 104 MM water molecules was reduced in size by successive deletion of rows and columns, and KIEs were evaluated for each. We suggest that accurate calculations of KIEs in solvated systems should involve a subset Hessian including the substrate together with any solvent atoms making specific interactions with any isotopically substituted atom. In the second part of the paper, the ensemble-averaged 2° α-²H₃ KIE calculated with the B3LYP/6-31+G(d,p)/TIP3P method is shown to be in good agreement with experiment. This comparison is meaningful because it includes consideration of uncertainties owing to sampling of a range of representative thermally accessible solvent configurations. We also present ensemble-averaged ¹⁴C and ³⁷Cl KIEs which have not as yet been determined experimentally. Copyright © 2013 John Wiley & Sons, Ltd.

The existence of solvent fluctuations leads to populations of reactant-state (RS) and transition-state (TS) configurations and implies that property calculations must include appropriate averaging over distributions of values for individual configurations. Average kinetic isotope effects 〈KIE〉 for NC⁻+EtClNCEt+Cl⁻ in DMSO solution at 30 °C are best obtained as the ratio 〈f_RS〉/〈f_TS〉 of isotopic partition function ratios separately averaged over all RS and TS configurations. In this way the hybrid AM1/OPLS-AA potential yields 〈KIE〉 values for all six isotopic substitutions (2° α-²H₂, 2° β-²H₃, α-¹¹C/¹⁴C, leaving group ³⁷Cl, and nucleophile ¹³C and ¹⁵N) for this reaction in the correct direction as measured experimentally. These thermally-averaged calculated KIEs may be compared meaningfully with experiment, and only one of them differs in magnitude from the experimental value by more than one standard deviation from the mean. This success contrasts with previous KIE calculations based upon traditional methods without averaging. The isotopic partition function ratios are best evaluated using all (internal) vibrational and (external) librational frequencies obtained from Hessians determined for subsets of atoms, relaxed to local minima or saddle points, within frozen solvent environments of structures sampled along molecular dynamics trajectories for RS and TS. The current method may perfectly well be implemented with other QM or QM/MM methods, and thus provides a useful tool for investigating KIEs in relation to studies of chemical reaction mechanisms in solution or catalyzed by enzymes.

Appreciation for the contribution of nuclear quantum effects (NQEs) to chemical reactivity predates transition-state theory (TST). Quantum corrections to rate constants for the reactions catalysed by lactate dehydrogenase (LDH) and formate dehydrogenase (FDH) and the same reactions in water are estimated by Bell's one-dimensional approximate method and give tunnelling contributions to catalysis of 1.6 and 0.95, respectively. Published results for NQEs, including both tunnelling and zero-point energies, estimated by the quantum classical path method for LDH, carbonic anhydrase, glyoxylase I and lipoxygenase, together with the corresponding reactions in water, are reviewed: the respective contributions to catalysis are 0.66, 5, 1 and 1. In the absence of better evidence that an enzymic rate enhancement is due to a significantly larger quantum correction for the enzyme-catalysed reaction than for an appropriate uncatalysed reference reaction, it is suggested that the term ‘quantum catalysis’ should be used with caution and restraint. Copyright © 2010 John Wiley & Sons, Ltd.