12 research outputs found
Setting an Upper Limit on the Myoglobin Iron(IV)Hydroxide p<i>K</i><sub>a</sub>: Insight into Axial Ligand Tuning in Heme Protein Catalysis
To provide insight into the ironÂ(IV)Âhydroxide
p<i>K</i><sub>a</sub> of histidine ligated heme proteins,
we have probed the
active site of myoglobin compound II over the pH range of 3.9â9.5,
using EXAFS, MoÌssbauer, and resonance Raman spectroscopies.
We find no indication of ferryl protonation over this pH range, allowing
us to set an upper limit of 2.7 on the ironÂ(IV)Âhydroxide p<i>K</i><sub>a</sub> in myoglobin. Together with the recent determination
of an ironÂ(IV)Âhydroxide p<i>K</i><sub>a</sub> ⌠12
in the thiolate-ligated heme enzyme cytochrome P450, this result provides
insight into Natureâs ability to tune catalytic function through
its choice of axial ligand
Direct Observation of Oxygen Rebound with an Iron-Hydroxide Complex
The
rebound mechanism for alkane hydroxylation was invoked over
40 years ago to help explain reactivity patterns in cytochrome P450,
and subsequently has been used to provide insight into a range of
biological and synthetic systems. Efforts to model the rebound reaction
in a synthetic system have been unsuccessful, in part because of the
challenge in preparing a suitable metal-hydroxide complex at the correct
oxidation level. Herein we report the synthesis of such a complex.
The reaction of this species with a series of substituted radicals
allows for the direct interrogation of the rebound process, providing
insight into this uniformly invoked, but previously unobserved process
Direct Observation of Oxygen Rebound with an Iron-Hydroxide Complex
The
rebound mechanism for alkane hydroxylation was invoked over
40 years ago to help explain reactivity patterns in cytochrome P450,
and subsequently has been used to provide insight into a range of
biological and synthetic systems. Efforts to model the rebound reaction
in a synthetic system have been unsuccessful, in part because of the
challenge in preparing a suitable metal-hydroxide complex at the correct
oxidation level. Herein we report the synthesis of such a complex.
The reaction of this species with a series of substituted radicals
allows for the direct interrogation of the rebound process, providing
insight into this uniformly invoked, but previously unobserved process
Spectroscopic Investigations of Catalase Compound II: Characterization of an Iron(IV) Hydroxide Intermediate in a Non-thiolate-Ligated Heme Enzyme
We
report on the protonation state of <i>Helicobacter pylori</i> catalase compound II. UV/visible, MoÌssbauer, and X-ray absorption
spectroscopies have been used to examine the intermediate from pH
5 to 14. We have determined that HPC-II exists in an ironÂ(IV) hydroxide
state up to pH 11. Above this pH, the ironÂ(IV) hydroxide complex transitions
to a new species (p<i>K</i><sub>a</sub> = 13.1) with MoÌssbauer
parameters that are indicative of an ironÂ(IV)-oxo intermediate. Recently,
we discussed a role for an elevated compound II p<i>K</i><sub>a</sub> in diminishing the compound I reduction potential. This
has the effect of shifting the thermodynamic landscape toward the
two-electron chemistry that is critical for catalase function. In
catalase, a diminished potential would increase the selectivity for
peroxide disproportionation over off-pathway one-electron chemistry,
reducing the buildup of the inactive compound II state and reducing
the need for energetically expensive electron donor molecules
Oxygen-Atom Transfer Reactivity of Axially Ligated Mn(V)âOxo Complexes: Evidence for Enhanced Electrophilic and Nucleophilic Pathways
Addition
of anionic donors to the manganeseÂ(V)âoxo corrolazine
complex Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz) has a dramatic influence
on oxygen-atom transfer (OAT) reactivity with thioether substrates.
The six-coordinate anionic [Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz)Â(X)]<sup>â</sup> complexes (X = F<sup>â</sup>, N<sub>3</sub><sup>â</sup>, OCN<sup>â</sup>) exhibit a âŒ5
cm<sup>â1</sup> downshift of the MnâO vibrational mode
relative to the parent Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz) complex
as seen by resonance Raman spectroscopy. Product analysis shows that
the oxidation of thioether substrates gives sulfoxide product, consistent
with single OAT. A wide range of OAT reactivity is seen for the different
axial ligands, with the following trend determined from a comparison
of their second-order rate constants for sulfoxidation: five-coordinate
â thiocyanate â nitrate < cyanate < azide <
fluoride âȘ cyanide. This trend correlates with DFT calculations
on the binding of the axial donors to the parent Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz) complex. A Hammett study was performed with <i>p</i>-X-C<sub>6</sub>H<sub>4</sub>SCH<sub>3</sub> derivatives and [Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz)Â(X)]<sup>â</sup> (X = CN<sup>â</sup> or F<sup>â</sup>) as the oxidant, and unusual
âV-shapedâ Hammett plots were obtained. These results
are rationalized based upon a change in mechanism that hinges on the
ability of the [Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz)Â(X)]<sup>â</sup> complexes to function as either an electrophilic or weak nucleophilic
oxidant depending upon the nature of the <i>para</i>-X substituents.
For comparison, the one-electron-oxidized cationic Mn<sup>V</sup>(O)Â(TBP<sub>8</sub>Cz<sup>âą+</sup>) complex yielded a linear Hammett relationship
for all substrates (Ï = â1.40), consistent with a straightforward
electrophilic mechanism. This study provides new, fundamental insights
regarding the influence of axial donors on high-valent Mn<sup>V</sup>(O) porphyrinoid complexes
O<sub>2</sub>-Evolving Chlorite Dismutase as a Tool for Studying O<sub>2</sub>-Utilizing Enzymes
The direct interrogation of fleeting intermediates by
rapid-mixing
kinetic methods has significantly advanced our understanding of enzymes
that utilize dioxygen. The gasâs modest aqueous solubility
(<2 mM at 1 atm) presents a technical challenge to this approach,
because it limits the rate of formation and extent of accumulation
of intermediates. This challenge can be overcome by use of the heme
enzyme chlorite dismutase (Cld) for the rapid, <i>in situ</i> generation of O<sub>2</sub> at concentrations far exceeding 2 mM.
This method was used to define the O<sub>2</sub> concentration dependence
of the reaction of the class Ic ribonucleotide reductase (RNR) from <i>Chlamydia trachomatis</i>, in which the enzymeâs Mn<sup>IV</sup>/Fe<sup>III</sup> cofactor forms from a Mn<sup>II</sup>/Fe<sup>II</sup> complex and O<sub>2</sub> via a Mn<sup>IV</sup>/Fe<sup>IV</sup> intermediate, at effective O<sub>2</sub> concentrations as high
as âŒ10 mM. With a more soluble receptor, myoglobin, an O<sub>2</sub> adduct accumulated to a concentration of >6 mM in <15
ms. Finally, the CâH-bond-cleaving Fe<sup>IV</sup>âoxo
complex, <b>J</b>, in taurine:α-ketoglutarate dioxygenase
and superoxoâFe<sub>2</sub><sup>III/III</sup> complex, <b>G</b>, in <i>myo</i>-inositol oxygenase, and the tyrosyl-radical-generating
Fe<sub>2</sub><sup>III/IV</sup> intermediate, <b>X</b>, in <i>Escherichia coli</i> RNR, were all accumulated to yields more
than twice those previously attained. This means of <i>in situ</i> O<sub>2</sub> evolution permits a >5 mM âpulseâ
of
O<sub>2</sub> to be generated in <1 ms at the easily accessible
Cld concentration of 50 ÎŒM. It should therefore significantly
extend the range of kinetic and spectroscopic experiments that can
routinely be undertaken in the study of these enzymes and could also
facilitate resolution of mechanistic pathways in cases of either sluggish
or thermodynamically unfavorable O<sub>2</sub> addition steps
Oxygen-atom transfer reactivity of axially ligated Mn(V)âoxo complexes: Evidence for enhanced electrophilic and nucleophilic pathways
Addition of anionic donors to the manganese(V)âoxo corrolazine complex MnV(O)(TBP8Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The sixcoordinate anionic [MnV(O)(TBP8Cz)(X)]â complexes (X = Fâ, N3â, OCNâ) exhibit a âŒ5 cmâ1 downshift of the MnâO vibrational mode relative to the parent MnV(O)(TBP8Cz) complex as seen by resonance Raman spectroscopy. Product analysis shows that the oxidation of thioether substrates gives sulfoxide product, consistent with single OAT. A wide range of OAT reactivity is seen for the different axial ligands, with the following trend determined from a comparison of their second-order rate constants for sulfoxidation: five-coordinate â thiocyanate â nitrate < cyanate < azide < fluoride âȘ cyanide. This trend correlates with DFT calculations on the binding of the axial donors to the parent MnV(O)(TBP8Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 derivatives and [MnV(O)(TBP8Cz)(X)]â(X = CNâ or Fâ) as the oxidant, and unusual âV-shapedâ Hammett plots were obtained. These results are rationalized based upon a change in mechanism that hinges on the ability of the [MnV(O)(TBP8Cz)(X)]â complexes to function as either an electrophilic or weak nucleophilic oxidant depending upon the nature of the para-X substituents. For comparison, the oneelectron-oxidized cationic MnV(O)(TBP8Czâą+) complex yielded a linear Hammett relationship for all substrates (Ï = â1.40), consistent with a straightforward electrophilic mechanism. This study provides new, fundamental insights regarding the influence of axial donors on high-valent MnV(O) porphyrinoid complexes