10 research outputs found
Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy
The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function
X-ray Absorption Spectroscopy as a Probe of Ligand Noninnocence in Metallocorroles: The Case of Copper Corroles
The question of ligand noninnocence in Cu corroles has long been a topic of discussion. Presented herein is a Cu K-edge X-ray absorption spectroscopy (XAS) study, which provides a direct probe of the metal oxidation state, of three Cu corroles, Cu[TPC], Cu[Br8TPC], and Cu[(CF3)8TPC] (TPC = meso-triphenylcorrole), and the analogous Cu(II) porphyrins, Cu[TPP], Cu[Br8TPP], and Cu[(CF3)8TPP] (TPP = meso-tetraphenylporphyrin). The Cu K rising-edges of the Cu corroles were found to be about 0ā1 eV upshifted relative to the analogous porphyrins, which is substantially lower than the 1ā2 eV shifts typically exhibited by authentic Cu(II)/Cu(III) model complex pairs. In an unusual twist, the Cu K pre-edge regions of both the Cu corroles and the Cu porphyrins exhibit two peaks split by 0.8ā 1.3 eV. Based on time-dependent density functional theory calculations, the lower- and higher-energy peaks were assigned to a Cu 1s ā 3dx2āy2 transition and a Cu 1s ā corrole/porphyrin Ļ* transition, respectively. From the Cu(II) porphyrins to the corresponding Cu corroles, the energy of the Cu 1s ā 3dx2āy2 transition peak was found to upshift by 0.6ā0.8 eV. This shift is approximately half that observed between Cu(II) to Cu(III) states for well-deļ¬ned complexes. The Cu K-edge XAS spectra thus show that although the metal sites in the Cu corroles are more oxidized relative to those in their Cu(II) porphyrin analogues, they are not oxidized to the Cu(III) level, consistent with the notion of a noninnocent corrole. The relative importance of Ļ-donation versus corrole Ļ-radical character is discussed
A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate ComplexīøThe Product of the Reaction of Nitrogen Monoxide (Ā·NO<sub>(g)</sub>) with a Ferric-Superoxide Species
Peroxynitrite
(<sup>ā</sup>OONī»O, PN) is a reactive
nitrogen species (RNS) which can effect deleterious nitrative or oxidative
(bio)Āchemistry. It may derive from reaction of superoxide anion (O<sub>2</sub><sup>ā¢ā</sup>) with nitric oxide (Ā·NO)
and has been suggested to form an as-yet unobserved bound heme-iron-PN
intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD)
enzymes, which facilitate a Ā·NO homeostatic process, i.e., its
oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin
porphyrinate-Fe<sup>III</sup> complex [(P<sup>Im</sup>)ĀFe<sup>III</sup>(<sup>ā</sup>OONī»O)] (<b>3</b>) (P<sup>Im</sup>; a porphyrin moiety with a covalently tethered imidazole axial ābaseā
donor ligand) has been identified and characterized by various spectroscopies
(UVāvis, NMR, EPR, XAS, resonance Raman) and DFT calculations,
following its formation at ā80 Ā°C by addition of Ā·NO<sub>(g)</sub> to the heme-superoxo species, [(P<sup>Im</sup>)ĀFe<sup>III</sup>(O<sub>2</sub><sup>ā¢ā</sup>)] (<b>2</b>). DFT
calculations confirm that <b>3</b> is a six-coordinate low-spin
species with the PN ligand coordinated to iron via its terminal peroxidic
anionic O atom with the overall geometry being in a <i>cis</i>-configuration. Complex <b>3</b> thermally transforms to its
isomeric low-spin nitrato form [(P<sup>Im</sup>)ĀFe<sup>III</sup>(NO<sub>3</sub><sup>ā</sup>)] (<b>4a</b>). While previous (bio)Āchemical
studies show that phenolic substrates undergo nitration in the presence
of PN or PN-metal complexes, in the present system, addition of 2,4-di-<i>tert</i>-butylphenol (<sup>2,4</sup>DTBP) to complex <b>3</b> does not lead to nitrated phenol; the nitrate complex <b>4a</b> still forms. DFT calculations reveal that the phenolic H atom approaches
the terminal PN O atom (farthest from the metal center and ring core),
effecting OāO cleavage, giving nitrogen dioxide (Ā·NO<sub>2</sub>) plus a ferryl compound [(P<sup>Im</sup>)ĀFe<sup>IV</sup>ī»O]
(<b>7</b>); this rebounds to give [(P<sup>Im</sup>)ĀFe<sup>III</sup>(NO<sub>3</sub><sup>ā</sup>)] (<b>4a</b>).The generation
and characterization of the long sought after ferriheme peroxynitrite
complex has been accomplished
Hydroxo-Bridged Dicopper(II,III) and -(III,III) Complexes: Models for Putative Intermediates in Oxidation Catalysis
A macrocyclic
ligand (L<sup>4ā</sup>) comprising two pyridineĀ(dicarboxamide)
donors was used to target reactive copper species relevant to proposed
intermediates in catalytic hydrocarbon oxidations by particulate methane
monooxygenase and heterogeneous zeolite systems. Treatment of LH<sub>4</sub> with base and CuĀ(OAc)<sub>2</sub>Ā·H<sub>2</sub>O yielded
(Me<sub>4</sub>N)<sub>2</sub>[L<sub>2</sub>Cu<sub>4</sub>(Ī¼<sub>4</sub>-O)] (<b>1</b>) or (Me<sub>4</sub>N)Ā[LCu<sub>2</sub>(Ī¼-OH)] (<b>2</b>), depending on conditions. Complex <b>2</b> was found to undergo two reversible 1-electron oxidations
via cyclic voltammetry and low-temperature chemical reactions. On
the basis of spectroscopy and theory, the oxidation products were
identified as novel hydroxo-bridged mixed-valent CuĀ(II)ĀCuĀ(III) and
symmetric CuĀ(III)<sub>2</sub> species, respectively, that provide
the first precedence for such moieties as oxidation catalysis intermediates
Formylglycine-generating enzyme binds substrate directly at a mononuclear Cu(I) center to initiate O 2
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Formylglycine-generating enzyme binds substrate directly at a mononuclear Cu(I) center to initiate O 2 activation
The formylglycine-generating enzyme (FGE) is required for the posttranslational activation of type I sulfatases by oxidation of an active-site cysteine to C
-formylglycine. FGE has emerged as an enabling biotechnology tool due to the robust utility of the aldehyde product as a bioconjugation handle in recombinant proteins. Here, we show that Cu(I)-FGE is functional in O
activation and reveal a high-resolution X-ray crystal structure of FGE in complex with its catalytic copper cofactor. We establish that the copper atom is coordinated by two active-site cysteine residues in a nearly linear geometry, supporting and extending prior biochemical and structural data. The active cuprous FGE complex was interrogated directly by X-ray absorption spectroscopy. These data unambiguously establish the configuration of the resting enzyme metal center and, importantly, reveal the formation of a three-coordinate tris(thiolate) trigonal planar complex upon substrate binding as furthermore supported by density functional theory (DFT) calculations. Critically, inner-sphere substrate coordination turns on O
activation at the copper center. These collective results provide a detailed mechanistic framework for understanding why nature chose this structurally unique monocopper active site to catalyze oxidase chemistry for sulfatase activation
A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate ComplexīøThe Product of the Reaction of Nitrogen Monoxide (Ā·NO<sub>(g)</sub>) with a Ferric-Superoxide Species
Peroxynitrite
(<sup>ā</sup>OONī»O, PN) is a reactive
nitrogen species (RNS) which can effect deleterious nitrative or oxidative
(bio)Āchemistry. It may derive from reaction of superoxide anion (O<sub>2</sub><sup>ā¢ā</sup>) with nitric oxide (Ā·NO)
and has been suggested to form an as-yet unobserved bound heme-iron-PN
intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD)
enzymes, which facilitate a Ā·NO homeostatic process, i.e., its
oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin
porphyrinate-Fe<sup>III</sup> complex [(P<sup>Im</sup>)ĀFe<sup>III</sup>(<sup>ā</sup>OONī»O)] (<b>3</b>) (P<sup>Im</sup>; a porphyrin moiety with a covalently tethered imidazole axial ābaseā
donor ligand) has been identified and characterized by various spectroscopies
(UVāvis, NMR, EPR, XAS, resonance Raman) and DFT calculations,
following its formation at ā80 Ā°C by addition of Ā·NO<sub>(g)</sub> to the heme-superoxo species, [(P<sup>Im</sup>)ĀFe<sup>III</sup>(O<sub>2</sub><sup>ā¢ā</sup>)] (<b>2</b>). DFT
calculations confirm that <b>3</b> is a six-coordinate low-spin
species with the PN ligand coordinated to iron via its terminal peroxidic
anionic O atom with the overall geometry being in a <i>cis</i>-configuration. Complex <b>3</b> thermally transforms to its
isomeric low-spin nitrato form [(P<sup>Im</sup>)ĀFe<sup>III</sup>(NO<sub>3</sub><sup>ā</sup>)] (<b>4a</b>). While previous (bio)Āchemical
studies show that phenolic substrates undergo nitration in the presence
of PN or PN-metal complexes, in the present system, addition of 2,4-di-<i>tert</i>-butylphenol (<sup>2,4</sup>DTBP) to complex <b>3</b> does not lead to nitrated phenol; the nitrate complex <b>4a</b> still forms. DFT calculations reveal that the phenolic H atom approaches
the terminal PN O atom (farthest from the metal center and ring core),
effecting OāO cleavage, giving nitrogen dioxide (Ā·NO<sub>2</sub>) plus a ferryl compound [(P<sup>Im</sup>)ĀFe<sup>IV</sup>ī»O]
(<b>7</b>); this rebounds to give [(P<sup>Im</sup>)ĀFe<sup>III</sup>(NO<sub>3</sub><sup>ā</sup>)] (<b>4a</b>).The generation
and characterization of the long sought after ferriheme peroxynitrite
complex has been accomplished