Computational Chemical Dynamics Conference Abstracts




Computational Chemical Dynamics Conference

Submitted Abstracts

Kelly E. Anderson and J. Ilja Siepmann

Departments of Chemistry and of Chemical Engineering and Materials Science
University of Minnesota

Investigating the effects of methanol and n-octane entrainers on solubility and microscopic structure in supercritical carbon dioxide

Isobaric-isothermal Gibbs ensemble Monte Carlo simulations were carried out for systems of supercritical carbon dioxide and hexamethylbenzene to investigate the effects of entrainer presence on solubility and structure in the supercritical solvent. The entrainers methanol and n-octane were used to contrast polar versus non-polar cosolvent effects. Preliminary results using the transferable potentials for phase equilibria-united atom (TraPPE-UA) force field indicate that the presence of either cosolvent promotes solubility relative to the neat solvent system. These results are consistent with the experimental results of Dobbs, Wong, Lahiere, and Johnston. A detailed structural analysis is performed to relate changes in the microscopic structure to the solubility enhancement.

N. Balucani, D. Skouteris, G. Capozza, E. Segoloni, P. Casavecchia, M. H. Alexander, G. Capecchi and H.-J. Werner

The dynamics of prototype abstraction reactions: Crossed beam experiments and exact quantum scattering calculations on coupled ab initio potential energy surfaces for the reaction Cl(2P3/2,1/2) + H2

Abstract not recieved.

N. Balucani, G. Capozza, E. Segoloni, L. Cartechini, R. Bobbenkamp, P. Casavecchia, L. Bañares, F. J. Aoiz, P. Honvault, B. Bussery-Honvault, J.-M. Launay

The Dynamics of Prototype Insertion Reactions: Crossed Beam Experiments versus Quantum and Quasiclassical Trajectory Scattering Calculations on Ab Initio Potential Energy Surfaces for C(1D) + H2 and N(2D) + H2

Abstract not recieved.

Divesh Bhatt, J. Ilja Siepmann

Department of Chemistry
University of Minnesota

Computational Investigation of the Stability of n-Propylbenzene Polymorphs

Polymorphism in n-propylbenzene is investigated using a variety of computational techniques. Polymorph Predictor (PP), a commercial software, can be used to predict possible polymorphic structures in different space groups and ranks them according to their minnimized lattice energies. Two different molecular mechanics force fields (COMPASS and Dreiding) were used and result in a different stability ranking for the crystal structures and space groups. For example, the COMPASS force field predicts that a crystal structure with space group P21/c is most stable, whereas, the Dreiding force field yields a crystal in the space group C2c. In addition, the lattice energies of all were computed using the TraPPE (Transferable Potentials for Phase Equilibria) force field, and a surprisingly low correlation of the lattice energies and stability rankins observed.

The lattice energies obtained above only reflect the relative stability of the crystals structures at 0 K. Thus, constant stress-isothermal Monte Carlo simulations (1 bar, 170 K) of the crystal structures where performed starting from the 0 K lattice structures. The ranking of the enthalpies of the crystal structures was found to be substantially different than for the lattice energies, thereby suggesting that lattice energies are a poor indicator for relative polymorph stability. Additionally, Gibbs Ensemble Monte Carlo (GEMC) simulations were employed to investigate the solid-vapor phase equilibria and to rank the different polymorphs in order of their free energies that can be obtained from the saturated vapor pressures.

Edward A. Boudreaux and Eric Baxter

Department of Chemistry
University of New Orleans

A Study in the Dynamics of Formation and Properties of Cr2 and Mo2 From Their Respective Atoms via the SCMEG-MO Method

The SCMEH-MO method of molecular orbital calculations has enjoyed some 25 years of development and applications, with amazing success in a wide variety of molecular systems. This presentation addresses the dynamics of formation and bonding in two molecules which have proven to be great challanges to "state of the art" methods of computational quantum chemistry.

It is shown,for what appears to be the first time, that both valence and valence-core electrons are involved in the formation and equilibrium bonding stabilities of Cr2 and Mo2. Both the theoretical foundation and the unique conditions for the manifestation of these effects are the topics of substance for this presentation.

Piergiorgio Casavecchia

Dynamics of polyatomic radical-molecule and radical-radical reactions from crossed beam reactive scattering experiments using "soft" electron-impact ionization: A challenge for 21st century computational chemical dynamics

Abstract not recieved.

S. A. Corcelli, C. P. Lawrence, J. R. Schmidt, and J. L. Skinner

Theoretical Chemistry Institute and Department of Chemistry
University of Wisconsin

Computational Strategies for Ultrafast Vibrational Spectroscopy: Applications to Water and Aqueous Solutions

We present a new approach that combines electronic structure (ES) methods and molecular dynamics (MD) simulations for investigating the vibrational spectroscopy of condensed phase systems. This approach was first applied to the OH stretch band of dilute HOD in liquied D2) and the OD stretch band of dilute HOD in liquid h2O. OH and OD anharmonic frequencies and their corresponding infrared and raman transition intensities were calculated using density functional methods for 100 HOD(D2O)n and HOD(H2O)n(n = 4 - 9) clusters randomly selected from the liquid water simulations. It was found that the infrared transition dipole moments vary considerably with environment, while the raman intensities do not. The ES frequencies and transition dipole moments were both found to linearly correlate (r=0.9) with the component of the electric field from the solvent along the bond of interest. These linear relationships were used in MD simulations to compute the frequency time-correlation function (FTCF) and the infrared and raman absorption line shapes. FTCFs were calculated for three different water models: TIP4P, SPC/E, and SPC-FQ. The long time decay of the FTCF for the polarizable SPC-FQ model (1.45 ps) compared more favorably with recent vibrational echo experiments (1.4 ps) than the fixed charge models SPC/E (1.0 ps) and TIP4P (0.9 ps). The infrared and raman absorption line shapes for the SPC-FQ model were in qualitative agreement with the experiment over the tempurature range 10 °C - 90 °C. To demonstrate the transferability of the combined ES/MD approach, the vibrational spectroscopy of aqueous solutions of N-methylacetamide (NMA) and urea was investigated.

J. Espinosa-García, J.C. Corchado, M. Navarrete and C. Rangel

Universidad de Extremadura
Badajoz (Spain)

New hybrid method integrating quantum molecular or molecular mechanics methods with analytical potential energy surfaces. Kinetics and dynamics applications.

The theoretical study of large polyatomic systems represents a challenge for Theoretical Chemistry, because the application of ab initio high-level calculations is prohibitive. To circumvent this problem of large systems/high-level, DFT methods have been widely used in the literature. However, when breaking/forming bonds are involved, transition-state zone, DFTs fail to perform well, and generally underestimate the barrier height by several kcal/mol.

An interesting and economic alternative approach to this problem is represented by the integrated methods, which describe different parts of the large system with different theoretical approaches: the “model” system, which is the more active site where the breaking/forming bonds are involved, is treated at QM high-level; and the “rest” of the system is treated at a lower-level, QM or MM. These methods are denoted as QM:QM or QM:MM combinations, respectively.

In this communication a new integrated scheme is proposed and tested, where the QM high-level description of the “model” system is replaced by an analytical potential energy surface, PES. The new scheme is denoted PES:QM or PES:MM, depending on which low-level, QM or MM, is used. Although we shall test it on a reactive system based on the reaction-path construction, and hence obtain kinetics and dynamics information, it can be applied to calculate any property for any system or compound. The hydrogen abstraction reaction CH3CH3 + H  CH3CH2 + H2 was chosen as test, because there is a lot of theoretical and experimental information (heat of reaction, activation energy, tunneling effect, rate constants, etc.) available for comparison.

Benjamin F. Gherman, William B. Tolman, and Christopher J. Cramer

Department of Chemistry and Supercomputer Institute
University of Minnesota

Modeling Dioxygen Activation at Monocopper Enzyme Sites

The activation of molecular oxygen at monocopper centers plays an important role in biology, and in particular with regard to the biosynthesis of neurohormones by the Cu-containing enzymes dopamine β-monooxygenase (DβM) and the peptidylglycine α-hydroxylating monooxygena (PHM) component of the bifunctional peptidylglycine α-amidating monooxygenase (PAM). I order to gain an understanding of the first stage of the catalysis (i.e. dioxygen activation at the monocopper active sites), 1:1 Cu/O2 adducts coordinated to the biomimetic β-diketiminate and anilido-imine ligands of Tolman and co-workers[1,2] have been studied using a combination of DFT and CASPT2 methods, a protocol which stems from earlier work on closely related systems.[3,4] Calculations have focused on characterizing the formation of the adducts, interconversion between side-on and end-on adducts, and properties of the protonated forms of the adducts. These results[5] will form the foundation for subsequent work on O–O bond cleavage to potentially yield Cu(III)=O species as well as modeling of the hydroxylation reaction carried out by the Cu/O2 adducts (i.e. the second stage of the DβM and PHM reactions).

Odd Gropen

Department of Chemistry
University of Tromsø

Relativistic Quantum Mechanical Methods in Molecular Calculations. Theory and Applications

During the last decade elements heavier than the second row transition metals have become readily accessible for theoretical studies due to the development of relativistic methods1. At the spin-free level, i.e. when spin-orbit interactions are neglected, calculations using relativistic ECPs on complexes involving the third row transition elements are common2. It is also possible, but less frequent, to make spin-free ab initio calculations using the no-pair approximation, one component equations obtained after transformation of the Dirac equation3. Concerning the spin-orbit effect, it is fairly well known how to calculate it; and this may be done at the two-component level by the no-pair form of the Hamiltonian or by solving the four-component Dirac equation.

Over the last fifteen years we have emphasized on the development of methods for systems containing heavy elements with the ultimate goal to study catalyses. We have developed methods for doing Relativistic Effective Core Potentials, one component Douglas-Kroll method, two component Douglas-Kroll Spin-Orbit method4 and relativistic four component methods5.

Theory and results from several applications are presented6.

1. J. Almlöf and O. Gropen, in: Reviews in Computational Chemistry, Vol. 8, (1996),K.B. Lipkowitz and D.B. Boyd, Eds., VCH Publishers, New York.

2. O. Gropen, in Methods in Computational Chemistry, Vol. 2, S. Wilson, Ed., Plenum, New York, 1988, pp.109-135.

3. B.A. Hess, Phys. Rev. A, 32, 756-763 (1985); B.A. Hess, Phys. Rev. A, 32, 3742-3748 (1986); R. Samzow, B.A. Hess, and G. Jansen, J. Chem. Phys. 96, 1227-1231 (1992).

4. M. Sjøvoll, J. Olsen and O. Gropen, (1997),97,301

5. T. Saue, K. Fagri, T. Helgaker and O. GropenMol. Phys, 1997,91,937

6. PdH (TCA,1997), Tl (TCA, 1997), NbS (J. Mol.Spec, 1997), Pth2+ (Theochem, 451(1998)22), AuH2 (Manuscript),ThO(Manuscript)

George A. Hagedorn

Department of Mathematics and Center for Statistical Mechanics and Mathematical Physics
Verginia Polytechnic Institute and State University

Non-Adiabatic Scattering Wave Functions in a Simple Born-Oppenheimer Model

We describe recent mathematical results, obtained in collaboration with Professor Alain Hoye, that concern non-adiabatic transitions in a simple molecular dynamics model. We study scattering theory for the time-dependent molecular Schrodinger equation

in the small e (Born-Oppenheimer) limit. We assume the electron Hamiltonian h(x) has finitely many levels and consider the propagation of coherent nuclear states with sufficiently high total energy. We further assume two of the electronic levels are isolated from the rest of the electron Hamiltonian's spectrum and have an avoided crossing with a small e-independent gap. We compute the leading order behavior for the nuclear wave function associated with the non-adiabatic transition that is generated as the nuclei move through the avoided crossing.

This component is of order exp(-C/e2). It propagates asymptotically as a free Gaussian in the nuclear variables, and its momentum is shifted. The total transition probability for this transition and the momentum shift are both larger than what one would expect from a naive approximation and energy conservation.

John L. Lewin and Christopher J. Cramer

Department of Chemistry and Supercomputer Institute
University of Minnesota Supercomputing Institute

Rapid Quantum Mechanical Models for the Computational Estimation of C-H Bond Dissociation Energies as a Measure of Metabolic Stability

Several relatively inexpensive levels of theory are surveyed together with alternative algorithmic methods for the estimation of C-H bond dissociation energies (BDEs), such energies being useful for the prediction of metabolic stability in drug-like molecules. In particular, bond stretching potentials of several C-H bonds are computed using the AM1, PM3, HF/MIDI!, and B3LYP/MIDI! levels of electronic structure theory, and selected points are fit to Morse and parabolic potentials. BDEs computed by an AM1 fit to the Morse function show the smallest mean unsigned error in prediction (+/-3-4kcal-1) over 32 diverse C-H bonds. An alternative method for correlating the AM1 parabolic force constant from a two-point unrelaxed potential provides only a slightly decreased accuracy and is computationally particularly inexpensive. Both methods should prove to be useful for the rapid in silico screening of drug-like molecules for the metabolic stability to C-H bond oxidizing enzymes.

Shuhua Ma, Jiali Gao

Department of Chemistry
University of Minnesota

Molecular Dynamics Simulation of the N-terminal Auto-cleavage of SARS-CoV Main Proteinase

Severe acute respiratory syndrome coronavirus(SARS-CoV) has been identified as the causative agent of SARS. Like other 3C-like human corano virus main proteases, the SARS-CoV main proteinase plays an important role in medicating viral replication and transcription functions through extensive proteolytic processing of two overlapping replicase polyproteins, pp1a and pp1ab. Therefor, understanding the proteolytic process by SARS-CoV main proteinase will help successful inhibition of this enzyme and treatment of SARS.

SARS-CoV main proteinase is a member of cysteine proteases with a Cys-His active site dyad. Starting from the X-ray crystal structure of SARS-CoV main proteinase determined at pH 7.6 (which is an optimal pH for SARS-CoV main proteinase activity), we carried out molecular dynamics simulations using a combined QM/MM method on its N-terminal auto-clevage catalysis (substrate: H2N-Thr-Ser-Ala-Val-Leu-GlnSer-Gly-Phe-Arg-COOH, the cleavage site is GlnSer). The 2-dimensional potential of mean force along the whole reaction pathway has been computed, and it shows that the acylation step is rate-limiting with a combination of concerted (reactant and product state regions) and step-wise (transition state region) mechanism, whereas the deacylation step is a well concerted process. In this presentation, the catalytic mechanism of SARS-CoV main proteinase will be discussed in detail.

Thomas Miller

Physical & Theoretical Chemistry Laboratory
Oxford University

Torsional Path Integral Method for Predicting Biomolecular Conformation

The computational simulation of chemical and biological processes rely heavily on the ability of theoretical methods to accurately predict the preferred conformation of flexible mollecules. Unfortunately, because of the large number of possible conformations and the subtle factors that determine their relative stability, this prediction is extremely challenging.

Standard theoretical methods for predicting molecular conformation generally make one of two assumptions. The first is the neglect of vibrational anharmonicity in the potential energy surface which can lead to failures in the description of conformational entropy and the effects of temperature on molecular conformation. Alternatively, when standard techniques do account for anharmonicity, they often neglect the important role of quantum mechanics in describing the light degrees of freedom, such as those involved in hydrogen bonding and solvation.

By applying the path integral Monte Carlo (PIMC) technique to the internal torsional motions of flezible molecules, we have developed the torional PIMC techniqe for predictiong molecular conformation. This new approach is efficient enough to apply to large molecules and accureate enough to quantitatively reporoduce experimental results. Completed applications to a neurotransmitter molecule and to the glycine amino acid highlight the advantage of the method for cases in which anharmonicity and quantum effects are critical.

Most recently, the torional PIMC technique has been extended to simulate the effects of explicit solvation on molecular conformation, and quantum dynamical calculations have been performed to explain a missing conformer of glycine.

  • Miller TF and Clary DC. J. Chem. Phys. 116 (2002) 8262.
  • Miller TF and Clary DC. J. Chem. Phys. 119 (2003) 68.
  • Miller TF and Clary DC. J. Phys. Chem. B 108 (2004) 2484.
  • Miller TF and Clary DC. Phys. Chem. Chem. Phys. 6 (2004) 2563.

John C. Polanyi

Department of Chemistry
University of Toronto

Molecular Dynamics of Surface Reaction, Followed a Molecule-At-A-Time by STM

Experiments will be described in which light, low-energy electrons or heat have been used to induce reactions between sub-monolayers of adsorbed halides and silicon substrates. The molecular reaction dynamics were inferred in some detail from the STM images. Reagents were halobenzenes and both short and long-chain halides. The molecules were observed to self-assemble and to undergo localised reaction with the substrate. These findings have been modeled by quantum mechanics; the results will be described.

The experiments were performed by Duncan Rogers, Harikumar Rajamma, Sergey Dobrin, Xuekun Lu, Jody Yang, Rhys Jones, Zafar Waqar and Iain McNab, and the theory by Fedor Y. Naumkin, Cherif Matta, Ioannis Petsalakis and Giannoula Theodorakopoulos, all at the University of Toronto.

Yuan-Ping Pang

Mayo Clinic College of Medicine

3D Model of a Substrate-Bound SARS Chymotrypsin-Like Cysteine Proteinase Predicted by Multiple Molecular Dynamics Simulations: Catalytic Efficiency Regulated by Substrate Binding

Severe acute respiratory syndrome (SARS) is a contagious and deadly disease caused by a new coronavirus. The protein sequence of the chymotrypsin-like cysteine proteinase (CCP) responsible for SARS viral replication has been identified as a target for developing anti-SARS drugs. Here, I report the ATVRLQp1Ap1'-bound CCP 3D model predicted by 420 different molecular dynamics simulations (2.0 ns for each simulation with a 1.0-fs time step). This theoretical model was released at the Protein Data Bank (PDB; code: 1P76) before the release of the first X-ray structure of CCP (PDB code: 1Q2W). In contrast to the catalytic dyad observed in X-ray structures of CCP and other coronavirus cysteine proteinases, a catalytic triead comprising Asp187, His41, and Cys145 is found in the theoretical model of the substrate-bound CCP. The simulations of the CCP complex suggest that the substrate binding leads to the displacement of a water molecule entrapped by Asp187 and His41, thus converting the dyad to a more efficient catalytic triad. The CCP complex structure has an expanded active-site pocket that is useful for anti-SARS drug design. In addition, this work demonstrates that the SWISS- MODEL-based homology modeling followed by a refinement with multiple molecular dynamics simulations is able to generate the 3D structure of the substrate-bound CCP from its amino acid sequence with a root-mean-square deviation of 1.89 angstroms for all heavy atoms of the protein relative to the corresponding X-ray structure. This work was supported by DARPA, ARO, USAMRAA, NIH/NIAID, HPCMO, SDSC, MSI and compaq.

Laura L. Perissinotti and Dario A. Estrin

Departamento de Quimica Inorganica, Analitica y Quimica Fisica / INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires

Solvent Effects in the Transnitrosation Reaction

S-nitrosothiols (RSNO) are involved in many bioregulation functions, they may play an important role in NO storage, transport and delivery. RSNOs can react with thiols, so the NO transport from one thiol to another can be effected by the transnitrosation reaction. One possible reaction of this type is protein modification by cysteine S-nitrosation, an important mechanism for the regulation of the protein function.

We have studied the model reaction between methanethiol (1) and nitroso methanethiol (2), by using continuum and QM-MM techniques at the DFT level. QM-MM free energy profiles have been obtained using the steered molecular dynamics approach proposed by Jarzynski.

Neeraj Rai and J. Ilja Siepmann

Department of Chemistry and Department of Chemicall Engineering and Materials Science
University of Minnesota

Extending the transferable potentials for phace equilibria force field to aromatic heterocycles and investigating polymorphism in ROY

Polymorphism plays an important role in many industrial applications such as pharmaceutics and fat based food products. A good force field is very crucial for predicting polymorphism in molecular crystals. To this end we have extended the transferable potential for phase equilibria (TraPPE) force field to nitrogen containgin aromatic heterocycles, e.g. pyridine and pyrimidine. Monte Carlo simulations in the canonical Gibbs ensemble were carried out to determine the parameters that reproduce the critical temperature and saturated liquid densities. For pyridine the TraPPE-UA model gives more accurate results than the OPLS-AA model despite less computational cost for the former model.

In order to see the efficacy of molecular simulations in predicting polymorphism we have carried out constant-stress simulations for 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY). We have used two different sampling approaches either treating ROY as totally rigid or allowing torsional flexibility to see how conformational freedom affects the stability of the different polymorphs of ROY. The simulations predict different stability order than experiment which indicates the challenges in polymorph prediction.

Sachchida N. Rai (Computer Centre, Bijni Complex, Laitumkhrah, North-Easter Hill University), Heinz-Peter Liebermann (Theoretische Chemie, Bergische-Universitaet-Gesamthochschule), Robert J. Buenker (Theoretische Chemie, Bergische-Universitaet-Gesamthochschule), Mineo Kimura (Graduate School of Science and Engineering, Yamaguchi University), H. Suno (Faculty of Engineering, Yamaguchi University), and R. K. Janev (National Institute of Fusion Science, Oroshi-cho, Toki, Gifu, Japan and Macedonian Academy of Sciences and Arts)

A brief review of our recent theoretical studies on electron capture and direct elastic scattering in collisions of protons with key hydrocarbons are presented. A molecular representation is adopted within a fully quantum-mechanical approach. Different approaches of the proton toward the target molecule are considered. The results of ab initio CI calculations of the potential curves and nonadiabatic coupling matrix elements for H+ + C2H4 collisions and the differential cross sections for elastic scattering and electron capture during H+ + CH2 collisions at 0.5 KeV and 1.5 KeV are described and interpreted.

Orlando Roberto-Netoa, Fernando R. Ornellasb, and Francisco B. C. Machadoc

a. Divisao Fotonica, Instituto de Estudos Avancados, Centro Tecnico Aeroespacial
b. Instituto de Quimica, Universidade de Sao Paulo
c. Depertamento de Quimica, Instituto Tecnologico de Aeronautica, Centro Tecnico Aeroespacial

Dual-level direct dynamics calculations of the kinetic isotope effects for the CH4 + F -> CH3 + HF abstraction reaction

Kinetic isotope effects (KIEs) for hydrogen abstraction from the isotopomers CH4, 13CH4, and CD4 by fluorine atoms have been studied by the variational transition state theory with multidimensional tunneling contributions (VTST/MT). Low-level calculations of the potential surface were carried out within the AM1-SRP approach. High-level structural and energitic properties of the reactants, transition state, and products were used to interpolate corrections to the low-level calculations. The values of the classical barrier height (V) and the energy of reaction (E), which are employed as high-level energetic parameters, are set equal to 1.8 kcal/mol and -28.5 kcal/mol, respectively. The chemical dynamics results show that large-curvature tunneling (LCT) paths provide the dominant contribution, with significant participation of excited vibrational states. At the CVT/uOMT level, the calculated value of H/D KIE is equal to 1.74 at 298 K, which is in very close agreement with experimental (1.4 - 1.7). The 12C/13C KIE is calculated to have a very small variation in the tempurature range of 200 to 2000 K. CVT/uOMT activation energies at the temperature ranges of 200 - 300 K and 200 - 400 K are equal to 0.59 and 0.73 kcal/mol, respectively, which are comparable to the experimental values of 0.43 and 0.53 kcal/mol.

Alessandro Troisi, Abraham Nitzan, Michael Galperin and Mark A. Ratner

Department of Chemistry
Northwestern University and Center for Nanotechnology

Molecular Transport Junctions: Inelastic and Switching Effects

Electrical conduction through molecular wires, both in single-molecule junctions and in adlayers, can include broad mechanistic variation. In this presentation, we will focus on the effects of vibrational coupling, inelasticity, hysteresis, intramolecular isomerization and stochastic switching.

The calculations will be based on a non-equilibrium Green's function approach, combined with appropriate modeling for the electronic Hamiltonian. In particular, to get an appropriately correct answer it is required to deal with full self-consistency in the electron and vibrational Green's functions -- such consistency can be straightforwardly obtained within the Keldysh approach.

Lin Wang, Nitish Agrawal, Baoyu Hong, and Amnon Kohen

Department of Chemistry
University of Iowa

H-tunneling, Coupled Motion, and Environmentally Coupled H-Transfer in Enzymatic Reactions

Theoretical studies commonly address only events that are close to the H-transfer transition state, while for most experimental data that step is only one component in a complex kinetic expression. Our studies are directly aimed at exposing that step and comparing the experimental and theoretical results. Dihydrofolate reductase (DHFR) and thymidylate synthase (TS) from E. coli were examined using isotopically labeled cofactors (H, D, and T). 1š and 2š Kinetic isotope effects (KIEs) were measured and the intrinsic KIEs and their temperature dependency were examined. The experimental findings with DHFR at 25 šC are in excellent agreement with QM/MM calculations by Truhlar and by Hammes-Schiffer. All Arrhenius preexponential factors were above the semiclassical range, which indicates a contribution of quantum mechanical H-tunneling to reactions1 rate. Initial velocity experiments over the same temperature range enabled calculations of activation parameters. Energies of activation for both enzymes were small, but significant, which indicated probable contribution of3 environmentally coupled tunneling2 to reactions1 rate. The ability to study the nature of the chemical transformation in these kinetically complex enzymatic cascades result in data that are directly comparable to simulation and shaded a new light on the use of the Swain-Schaad relationship and 2š KIEs in determining the location and nature of the transition tate.