74 research outputs found
Frontier Molecular Orbital Contributions to Chlorination versus Hydroxylation Selectivity in the Non-Heme Iron Halogenase SyrB2
The
ability of an Fe<sup>IV</sup>O intermediate in SyrB2
to perform chlorination versus hydroxylation was computationally evaluated
for different substrates that had been studied experimentally. The
π-trajectory for H atom abstraction (Fe<sup>IV</sup>O
oriented perpendicular to the C–H bond of substrate) was found
to lead to the <i>S</i> = 2 five-coordinate HO–Fe<sup>III</sup>–Cl complex with the C<sup>•</sup> of the
substrate, π-oriented relative to both the Cl<sup>–</sup> and the OH<sup>–</sup> ligands. From this ferric intermediate,
hydroxylation is thermodynamically favored, but chlorination is intrinsically
more reactive due to the energy splitting between two key redox-active
dπ* frontier molecular orbitals (FMOs). The splitting is determined
by the differential ligand field effect of Cl<sup>–</sup> versus
OH<sup>–</sup> on the Fe center. This makes chlorination effectively
competitive with hydroxylation. Chlorination versus hydroxylation
selectivity is then determined by the orientation of the substrate
with respect to the HO–Fe–Cl plane that controls either
the Cl<sup>–</sup> or the OH<sup>–</sup> to rebound
depending on the relative π-overlap with the substrate C radical.
The differential contribution of the two FMOs to chlorination versus
hydroxylation selectivity in SyrB2 is related to a reaction mechanism
that involves two asynchronous transfers: electron transfer from the
substrate radical to the iron center followed by late ligand (Cl<sup>–</sup> or OH<sup>–</sup>) transfer to the substrate
Anisotropic Covalency Contributions to Superexchange Pathways in Type One Copper Active Sites
Type
one (T1) Cu sites deliver electrons to catalytic Cu active
sites: the mononuclear type two (T2) Cu site in nitrite reductases
(NiRs) and the trinuclear Cu cluster in the multicopper oxidases (MCOs).
The T1 Cu and the remote catalytic sites are connected via a Cys-His
intramolecular electron-transfer (ET) bridge, which contains two potential
ET pathways: P1 through the protein backbone and P2 through the H-bond
between the Cys and the His. The high covalency of the T1 Cu–S(Cys)
bond is shown here to activate the T1 Cu site for hole superexchange
via occupied valence orbitals of the bridge. This covalency-activated
electronic coupling (<i>H</i><sub>DA</sub>) facilitates
long-range ET through both pathways. These pathways can be selectively
activated depending on the geometric and electronic structure of the
T1 Cu site and thus the anisotropic covalency of the T1 Cu–S(Cys)
bond. In NiRs, blue (π-type) T1 sites utilize P1 and green (σ-type)
T1 sites utilize P2, with P2 being more efficient. Comparing the MCOs
to NiRs, the second-sphere environment changes the conformation of
the Cys-His pathway, which selectively activates <i>H</i><sub>DA</sub> for superexchange by blue π sites for efficient
turnover in catalysis. These studies show that a given protein bridge,
here Cys-His, provides different superexchange pathways and electronic
couplings depending on the anisotropic covalencies of the donor and
acceptor metal sites
Molecular Origin of Rapid versus Slow Intramolecular Electron Transfer in the Catalytic Cycle of the Multicopper Oxidases
Kinetic
measurements on single-turnover processes in laccase established fast
type-1 Cu to trinuclear Cu cluster (TNC) intramolecular electron transfer
(IET) in the reduction of the native intermediate (NI), the fully
oxidized form of the enzyme formed immediately after O–O bond
cleavage in the mechanism of O<sub>2</sub> reduction. Alternatively,
slow IET kinetics was observed in the reduction of the resting enzyme,
which involves proton-coupled electron transfer on the basis of isotope
measurements. The >10<sup>3</sup> difference between the IET rates
for these two processes confirms that the NI, rather than the resting
enzyme that has been defined by crystallography, is the fully oxidized
form of the TNC in catalytic turnover. Computational modeling showed
that reduction of NI is fast because of the larger driving force
associated with a more favorable proton affinity of its μ<sub>3</sub>-oxo moiety generated by reductive cleavage of the O–O
bond. This defines a unifying mechanism in which reductive cleavage
of the O–O bond is coupled to rapid IET in the multicopper
oxidases
The Role of Chloride in the Mechanism of O<sub>2</sub> Activation at the Mononuclear Nonheme Fe(II) Center of the Halogenase HctB
Mononuclear nonheme
Fe(II) (MNH) and α-ketoglutarate (α-KG)
dependent halogenases activate O<sub>2</sub> to perform oxidative
halogenations of activated and nonactivated carbon centers. While
the mechanism of halide incorporation into a substrate has been investigated,
the mechanism by which halogenases prevent oxidations in the absence
of chloride is still obscure. Here, we characterize the impact of
chloride on the metal center coordination and reactivity of the fatty
acyl-halogenase HctB. Stopped-flow kinetic studies show that the oxidative
transformation of the Fe(II)-α-KG-enzyme complex is >200-fold
accelerated by saturating concentrations of chloride in both the absence
and presence of a covalently bound substrate. By contrast, the presence
of substrate, which generally brings about O<sub>2</sub> activation
at enzymatic MNH centers, only has an ∼10-fold effect in the
absence of chloride. Circular dichroism (CD) and magnetic CD (MCD)
studies demonstrate that chloride binding triggers changes in the
metal center ligation: chloride binding induces the proper binding
of the substrate as shown by variable-temperature, variable-field
(VTVH) MCD studies of non-α-KG-containing forms and the conversion
from six-coordinate (6C) to 5C/6C mixtures when α-KG is bound.
In the presence of substrate, a site with square pyramidal five-coordinate
(5C) geometry is observed, which is required for O<sub>2</sub> activation
at enzymatic MNH centers. In the absence of substrate an unusual trigonal
bipyramidal site is formed, which accounts for the observed slow,
uncoupled reactivity. Molecular dynamics simulations suggest that
the binding of chloride to the metal center of HctB leads to a conformational
change in the enzyme that makes the active site more accessible to
the substrate and thus facilitates the formation of the catalytically
competent enzyme–substrate complex. Results are discussed in
relation to other MNH dependent halogenases
Comparison of High-Spin and Low-Spin Nonheme Fe<sup>III</sup>–OOH Complexes in O–O Bond Homolysis and H‑Atom Abstraction Reactivities
The geometric and electronic structures and reactivity
of an <i>S</i> = 5/2 (HS) mononuclear nonheme (TMC)Fe<sup>III</sup>–OOH
complex are studied by spectroscopies, calculations, and kinetics
and compared with the results of previous studies of <i>S</i> = 1/2 (LS) Fe<sup>III</sup>–OOH complexes to understand parallels
and differences in mechanisms of O–O bond homolysis and electrophilic
H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)Fe<sup>III</sup>–OOH]<sup>2+</sup> complex is found to involve axial
ligand coordination and a crossing to the LS surface for O–O
bond homolysis. Both HS and LS Fe<sup>III</sup>–OOH complexes
are found to perform direct H-atom abstraction reactions but with
very different reaction coordinates. For the LS Fe<sup>III</sup>–OOH,
the transition state is late in O–O and early in C–H
coordinates. However, for the HS Fe<sup>III</sup>–OOH, the
transition state is early in O–O and further along in the C–H
coordinate. In addition, there is a significant amount of electron
transfer from the substrate to the HS Fe<sup>III</sup>–OOH
at transition state, but that does not occur in the LS transition
state. Thus, in contrast to the behavior of LS Fe<sup>III</sup>–OOH,
the H-atom abstraction reactivity of HS Fe<sup>III</sup>–OOH
is found to be highly dependent on both the ionization potential and
the C–H bond strength of the substrate. LS Fe<sup>III</sup>–OOH is found to be more effective in H-atom abstraction for
strong C–H bonds, while the higher reduction potential of HS
Fe<sup>III</sup>–OOH allows it to be active in electrophilic
reactions without the requirement of O–O bond cleavage. This
is relevant to the Rieske dioxygenases, which are proposed to use
a HS Fe<sup>III</sup>–OOH to catalyze <i>cis</i>-dihydroxylation
of a wide range of aromatic compounds
Two-Electron Reduction versus One-Electron Oxidation of the Type 3 Pair in the Multicopper Oxidases
Multicopper oxidases (MCOs) utilize
an electron shuttling Type
1 Cu (T1) site in conjunction with a mononuclear Type 2 (T2) and a
binuclear Type 3 (T3) site, arranged in a trinuclear copper cluster
(TNC), to reduce O<sub>2</sub> to H<sub>2</sub>O. Reduction of O<sub>2</sub> occurs with limited overpotential indicating that all the
coppers in the active site can be reduced via high-potential electron
donors. Two forms of the resting enzyme have been observed in MCOs:
the alternative resting form (AR), where only one of the three TNC
Cu’s is oxidized, and the resting oxidized form (RO), where
all three TNC Cu’s are oxidized. In contrast to the AR form,
we show that in the RO form of a high-potential MCO, the binuclear
T3 Cu(II) site can be reduced via the 700 mV T1 Cu. Systematic spectroscopic
evaluation reveals that this proceeds by a two-electron process, where
delivery of the first electron, forming a high energy, metastable
half reduced T3 state, is followed by the rapid delivery of a second
energetically favorable electron to fully reduce the T3 site. Alternatively,
when this fully reduced binuclear T3 site is oxidized via the T1 Cu,
a different thermodynamically favored half oxidized T3 form, i.e.,
the AR site, is generated. This behavior is evaluated by DFT calculations,
which reveal that the protein backbone plays a significant role in
controlling the environment of the active site coppers. This allows
for the formation of the metastable, half reduced state and thus the
complete reductive activation of the enzyme for catalysis
Correlation of the Electronic and Geometric Structures in Mononuclear Copper(II) Superoxide Complexes
The geometry of mononuclear copper(II)
superoxide complexes has been shown to determine their ground state
where side-on bonding leads to a singlet ground state and end-on complexes
have triplet ground states. In an apparent contrast to this trend,
the recently synthesized (HIPT<sub>3</sub>tren)Cu<sup>II</sup>O<sub>2</sub><sup>•–</sup> (<b>1</b>) was proposed
to have an end-on geometry and a singlet ground state. However, reexamination
of <b>1</b> with resonance Raman, magnetic circular dichroism,
and <sup>2</sup>H NMR spectroscopies indicate that <b>1</b> is,
in fact, an end-on superoxide species with a triplet ground state
that results from the single Cu<sup>II</sup>O<sub>2</sub><sup>•–</sup> bonding interaction being weaker than the spin-pairing energy
Substrate and Metal Control of Barrier Heights for Oxo Transfer to Mo and W Bis-dithiolene Sites
Reaction coordinates for oxo transfer from the substrates
Me<sub>3</sub>NO, Me<sub>2</sub>SO, and Me<sub>3</sub>PO to the biologically
relevant Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)<sub>2</sub>]<sup>−</sup> where mdt = 1,2-dimethyl-ethene-1,2-dithiolate(2-),
and from Me<sub>2</sub>SO to the analogous W(IV) complex, have been
calculated using density functional theory. In each case, the reaction
first proceeds through a transition state (TS1) to an intermediate
with substrate weakly bound, followed by a second transition state
(TS2) around which breaking of the substrate X–O bond begins.
By analyzing the energetic contributions to each barrier, it is shown
that the nature of the substrate and metal determines which transition
state controls the rate-determining step of the reaction
Substrate and Metal Control of Barrier Heights for Oxo Transfer to Mo and W Bis-dithiolene Sites
Reaction coordinates for oxo transfer from the substrates
Me<sub>3</sub>NO, Me<sub>2</sub>SO, and Me<sub>3</sub>PO to the biologically
relevant Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)<sub>2</sub>]<sup>−</sup> where mdt = 1,2-dimethyl-ethene-1,2-dithiolate(2-),
and from Me<sub>2</sub>SO to the analogous W(IV) complex, have been
calculated using density functional theory. In each case, the reaction
first proceeds through a transition state (TS1) to an intermediate
with substrate weakly bound, followed by a second transition state
(TS2) around which breaking of the substrate X–O bond begins.
By analyzing the energetic contributions to each barrier, it is shown
that the nature of the substrate and metal determines which transition
state controls the rate-determining step of the reaction
Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites
Two
[Cu<sub>2</sub>O]<sup>2+</sup> cores have been identified as
the active sites of low temperature methane hydroxylation in the zeolite
Cu-MOR. These cores have similar geometric and electronic structures,
yet different reactivity with CH<sub>4</sub>: one reacts with a much
lower activation enthalpy. In the present study, we couple experimental
reactivity and spectroscopy studies to DFT calculations to arrive
at structural models of the Cu-MOR active sites. We find that the
more reactive core is located in a constricted region of the zeolite
lattice. This leads to close van der Waals contact between the substrate
and the zeolite lattice in the vicinity of the active site. The resulting
enthalpy of substrate adsorption drives the subsequent H atom abstraction
stepa manifestation of the “nest” effect seen
in hydrocarbon cracking on acid zeolites. This defines a mechanism
to tune the reactivity of metal active sites in microporous materials
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