6 research outputs found
Excited-State Tautomerization of 7āAzaindole in Nonpolar Solution: A Theoretical Study Based on Liquid-Phase Potential Surfaces of Mean Force
Excited state tautomerization of
a 7-azaindole (7AI) complex with
one methanol molecule in heptane was studied using variational transition
state theory including multidimensional tunneling (VTST/MT) with the
dielectric continuum model for the solvent effect. Electronic structures
and energies for reactants and transition state (TS) in solution were
computed at the complete active space self-consistent field (CASSCF)
level with second-order multireference perturbation theory (MRPT2)
to take into consideration of dynamic electron correlation. The polarizable
continuum model using the integral equation formalism (IEFPCM) and
the SMD model were used for the excited-state solvent effect. Excited-state
surfaces of potential of the mean force in solution were generated
for the first time at the MRPT2//SMD/CASSCFĀ(10,9)/6-31GĀ(d,p) level.
The position of TS on the reaction coordinate substantially depended
on the dynamic electron correlation. The two protons in the excited-state
tautomerization were transferred in a concerted but asynchronous process.
Calculated HH/DD kinetic isotope effect (KIE) and the ratio of Arrhenius
pre-exponential factors, <i>A</i>(HH)/<i>A</i>(DD), agreed very well with the corresponding experimental values.
The shape of the adiabatic energy surfaces in the excited-state strongly
depended on the position of isotopes due to the asynchronicity of
the reaction path, and the tunneling effect was essential for reproducing
experimental KIEs. The pyrrolic proton moved a twice longer distance
by tunneling than the hydroxyl proton in the most probable tunneling
path at 292 K. This study strongly suggests that the mechanism of
the excited-state double proton transfer in heptane is triggered by
proton transfer from the pyrrolic nitrogen of 7AI to alcohol (protolytic
pathway), rather than by proton transfer from alcohol to the pyridine
nitrogen of 7AI (solvolytic pathway)
Is It Fe(III)-Oxyl Radical That Abstracts Hydrogen in the CāH Activation of TauD? A Theoretical Study Based on the DFT Potential Energy Surfaces
Taurine:Ī±-ketoglutarate dioxygenase
(TauD) is one of the most important enzymes in the Ī±-ketoglutarate
dioxygenase family, which are involved in many important biochemical
processes. TauD converts taurine into amino acetaldehyde and sulfite
at its nonheme iron center, and a large H/D kinetic isotope effect
(KIE) has been found in the hydrogen atom transfer (HAT) of taurine
suggesting a large tunneling effect. Recently, highly electrophilic
FeĀ(III)-oxyl radicals have been proposed as a key species for HAT
in the catalytic mechanism of CāH activation, which might be
prepared prior to the actual HAT. In order to investigate this hypothesis
and large tunneling effect, DFT potential energy surfaces along the
intrinsic reaction path were generated. The predicted rate constants
and H/D KIEs using variational transition-state theory including multidimensional
tunneling, based on these potential surfaces, have excellent agreement
with experimental data. This study revealed that the reactive processes
of CāH activation consisted of two distinguishable parts: (1)
the substrate approaching the FeĀ(IV)-oxo center without CāH
bond cleavage, which triggers the catalytic process by inducing metal-to-ligand
charge transfer to form the FeĀ(III)-oxyl species, and (2) the actual
HAT from the substrate to the FeĀ(III)-oxyl species. Most of the activation
energy was used in the first part, and the actual HAT required only
a small amount of energy to overcome the TS with a very large tunneling
effect. The donorāacceptor interaction between Ļ<sub>CāH</sub> and Ļ<sub>FeāO</sub><sup>*</sup> orbitals reduced the activation energy
significantly to make CāH activation feasible
Kinetic Isotope Effects as a Probe for the Protonolysis Mechanism of Alkylmetal Complexes: VTST/MT Calculations Based on DFT Potential Energy Surfaces
Protonolysis
by platinum or palladium complexes has been extensively studied because
it is the microscopic reverse of the CāH bond activation reaction.
The protonolysis of (COD)ĀPt<sup>II</sup>Me<sub>2</sub>, which exhibits
abnormally large kinetic isotope effects (KIEs), is proposed to occur
via a concerted pathway (S<sub>E</sub>2 mechanism) with large tunneling.
However, further investigation of KIEs for the protonolysis of ZnMe<sub>2</sub> and others led to a conclusion that there is no noticeable
correlation between the mechanism and magnitude of KIE. In this study,
we demonstrated that variational transition state theory including
multidimensional tunneling (VTST/MT) could accurately predict KIEs
and Arrhenius parameters of the protonolysis of alkylmetal complexes
based on the potential energy surfaces generated by density functional
theory. The predicted KIEs, <i>E</i><sub>a</sub><sup><i>D</i></sup> ā <i>E</i><sub>a</sub><sup>H</sup> values, and <i>A</i><sub>H</sub>/<i>A</i><sub>D</sub> ratios for the protonolysis of (COD)ĀPt<sup>II</sup>Me<sub>2</sub> and Zn<sup>II</sup>Me<sub>2</sub> by TFA agreed very well
with experimental values. The protonolysis of ZnMe<sub>2</sub> with
the concerted pathway has a very flat potential energy surface, which
produces a very small tunneling effect and therefore a small KIE.
The predicted KIE for the stepwise protonolysis (S<sub>E</sub>(ox)
mechanism) of (COD)ĀPt<sup>II</sup>Me<sub>2</sub> was much smaller
than that of the concerted pathway, but greater than the KIE of the
concerted protonolysis of ZnMe<sub>2</sub>. A large KIE, which entails
a significant tunneling effect, could be used as an experimental probe
of the concerted pathway. However, a normal or small KIE should not
be used as an indicator of the stepwise mechanism, and the interplay
between experiments and reliable theory including tunneling would
be essential to uncover the mechanism correctly
Large Tunneling Effect on the Hydrogen Transfer in Bis(Ī¼-oxo)dicopper Enzyme: A Theoretical Study
Type-III copper-containing enzymes have dicopper centers
in their
active sites and exhibit a novel capacity for activating aliphatic
CāH bonds in various substrates by taking molecular oxygen.
Dicopper enzyme models developed by Tolman and co-workers reveal exceptionally
large kinetic isotope effects (KIEs) for the hydrogen transfer process,
indicating a significant tunneling effect. In this work, we demonstrate
that variational transition state theory allows accurate prediction
of the KIEs and Arrhenius parameters for such model systems. This
includes multidimensional tunneling based on state-of-the-art quantum-mechanical
calculations of the minimum-energy path (MEP). The computational model
of bisĀ(Ī¼-oxo)Ādicopper enzyme consists of 70 atoms, resulting
in a 204-dimensional potential energy surface. The calculated values
of <i>E</i><sub>a</sub><sup>H</sup> ā <i>E</i><sub>a</sub><sup>D</sup>, <i>A</i><sub>H</sub>/<i>A</i><sub>D</sub>, and the KIE at 233 K are ā1.86 kcal/mol,
0.51, and 28.1, respectively, for the isopropyl ligand system. These
values agree very well with experimental values within the limits
of experimental error. For the representative tunneling path (RTP)
at 233 K, the pre- and post-tunneling configurations are 3.3 kcal/mol
below the adiabatic energy maximum, where the hydrogen travels 0.54
Ć
by tunneling. We found that tunneling is very efficient for
hydrogen transfer and that the RTP is very different from the MEP.
It is mainly heavy atoms that move as the reaction proceeds from the
reactant complex to the pretunneling configuration, and the hydrogen
atom suddenly hops at that point
Proton Transfer Dependence on Hydrogen-Bonding of Solvent to the Water Wire: A Theoretical Study
The mechanism and dynamics of double proton transfer
dependence
on hydrogen-bonding of solvent molecules to the bridging water in
a water wire were studied by a direct ab initio dynamics approach
with variational transition-state theory including multidimensional
tunneling. Long-range proton transfers in solution and within enzymes
may have very different mechanisms depending on the p<i>K</i><sub>a</sub> values of participating groups and their electrostatic
interactions with their environment. For end groups that have acidic
or basic p<i>K</i><sub>a</sub> values, proton transfers
by the classical Grotthuss and āproton-holeā transfer
mechanisms, respectively, are energetically favorable. This study
shows that these processes are facilitated by hydrogen-bond accepting
and donating solvent molecule interactions with the water wire in
the transition state (TS), respectively. Tunneling also depends very
much on the hydrogen bonding to the water wire. All molecules hydrogen
bonded to the water wire, even if they raised and narrowed energy
barriers, reduced the tunneling coefficients of double proton transfer,
which was attributed to the increased effective mass of transferring
protons near the TS. The theoretical HH/DD KIE, including tunneling,
was in good agreement with experimental KIE values. These results
suggest that the classical Grotthuss and proton-hole transfer mechanisms
require quite different solvent (or protein) environments near the
TS for the most efficient processes
Determination of Spin Inversion Probability, HāTunneling Correction, and Regioselectivity in the Two-State Reactivity of Nonheme Iron(IV)-Oxo Complexes
We show by experiments that nonheme
Fe<sup>IV</sup>O species react
with cyclohexene to yield selective hydrogen atom transfer (HAT) reactions
with virtually no Cī»C epoxidation. Straightforward DFT calculations
reveal, however, that Cī»C epoxidation on the <i>S</i> = 2 state possesses a low-energy barrier and should contribute substantially
to the oxidation of cyclohexene by the nonheme Fe<sup>IV</sup>O species.
By modeling the selectivity of this two-site reactivity, we show that
an interplay of tunneling and spin inversion probability (SIP) reverses
the apparent barriers and prefers exclusive <i>S</i> = 1
HAT over mixed HAT and Cī»C epoxidation on <i>S</i> = 2. The model enables us to derive a SIP value by combining experimental
and theoretical results