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Rudolf K Thauer - One of the best experts on this subject based on the ideXlab platform.
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Binding of Coenzyme B induces a major conformational change in the active site of methyl-Coenzyme M reductase.
Journal of the American Chemical Society, 2010Co-Authors: Sieglinde Ebner, Rudolf K Thauer, Bernhard Jaun, Meike Goenrich, Jeffrey HarmerAbstract:Methyl-Coenzyme M reductase (MCR) is the key enzyme in methane formation by methanogenic Archaea. It converts the thioether methyl-Coenzyme M and the thiol Coenzyme B into methane and the heterodisulfide of Coenzyme M and Coenzyme B. The catalytic mechanism of MCR and the role of its prosthetic group, the nickel hydrocorphin Coenzyme F430, is still disputed, and no intermediates have been observed so far by fast spectroscopic techniques when the enzyme was incubated with the natural substrates. In the presence of the competitive inhibitor Coenzyme M instead of methyl-Coenzyme M, addition of Coenzyme B to the active Ni(I) state MCRred1 induces two new species called MCRred2a and MCRred2r which have been characterized by pulse EPR spectroscopy. Here we show that the two MCRred2 signals can also be induced by the S-methyl- and the S-trifluoromethyl analogs of Coenzyme B. 19F-ENDOR data for MCRred2a and MCRred2r induced by S-CF3-Coenzyme B show that, upon binding of the Coenzyme B analog, the end of the 7-thi...
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Nickel-alkyl bond formation in the active site of methyl-Coenzyme m reductase.
Metal ions in life sciences, 2009Co-Authors: Bernhard Jaun, Rudolf K ThauerAbstract:Methyl-Coenzyme M reductase (MCR) catalyzes the methane-forming step in methanogenic archaea and most probably also the methane-oxidizing step in methanotrophic archaea. The enzyme contains Coenzyme F430 as prosthetic group. F430 is a nickel porphinoid that has to be in the reduced Ni(I) state for the enzyme to be active. The presently discussed catalytic mechanisms of MCR can in principle be divided into two basic models. In one model the key intermediate features a methyl-Ni(III) species being either formed in a nucleophilic substitution reaction or in an oxidative addition reaction. In the other model first the thioether sulfur of methyl-Coenzyme M binds to the Ni(I), which subsequently results in the release of the methyl group as methyl radical leaving behind a Ni(II)-sulfur bond. The experimental evidence for and against a methyl-nickel intermediate is reviewed.
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Coordination and binding geometry of methyl-Coenzyme M in the red1m state of methyl-Coenzyme M reductase
Journal of Biological Inorganic Chemistry, 2008Co-Authors: Dariush Hinderberger, Rudolf K Thauer, Bernhard Jaun, Stefan Mayr, Sieglinde Ebner, Meike Goenrich, Markus Reiher, Jeffrey HarmerAbstract:Methane formation in methanogenic Archaea is catalyzed by methyl-Coenzyme M reductase (MCR) and takes place via the reduction of methyl-Coenzyme M (CH3-S-CoM) with Coenzyme B (HS-CoB) to methane and the heterodisulfide CoM-S-S-CoB. MCR harbors the nickel porphyrinoid Coenzyme F430 as a prosthetic group, which has to be in the Ni(I) oxidation state for the enzyme to be active. To date no intermediates in the catalytic cycle of MCRred1 (red for reduced Ni) have been identified. Here, we report a detailed characterization of MCRred1m (''m'' for methyl-Coenzyme M), which is the complex of MCRred1a (''a'' for absence of substrate) with CH3-S-CoM. Using continuous-wave and pulse electron paramagnetic reso- nance spectroscopy in combination with selective isotope labeling ( 13 C and 2 H) of CH3-S-CoM, it is shown that CH3-S-CoM binds in the active site of MCR such that its thioether sulfur is weakly coordinated to the Ni(I) of F430. The complex is stable until the addition of the second sub- strate, HS-CoB. Results from EPR spectroscopy, along with quantum mechanical calculations, are used to characterize the electronic and geometric structure of this complex, which can be regarded as the first intermediate in the catalytic mechanism.
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Structure of an F430 Variant from Archaea Associated with Anaerobic Oxidation of Methane
Journal of the American Chemical Society, 2008Co-Authors: Stefan Mayr, Rudolf K Thauer, Seigo Shima, Martin Kruger, Christopher Latkoczy, Detlef Günther, Friedrich Widdel, Bernhard JaunAbstract:Microbial mats collected at cold methane seeps in the Black Sea carry out anaerobic oxidation of methane (AOM) to carbon dioxide using sulfate as the electron acceptor. These mats, which predominantly consist of sulfate-reducing bacteria and archaea of the ANME-1 and ANME-2 type, contain large amounts of proteins very similar to methyl-Coenzyme M reductase from methanogenic archaea. Mass spectrometry of mat samples revealed the presence of two nickel-containing cofactors in comparable amounts, one with the same mass as Coenzyme F430 from methanogens (m/z = 905) and one with a mass that is 46 Da higher (m/z = 951). The two cofactors were isolated and purified, and their constitution and absolute configuration were determined. The cofactor with m/z = 905 was proven to be identical to Coenzyme F430 from methanogens. For the m/z = 951 species, high resolution ICP-MS pointed to F430 + CH2S as the molecular formula, and LA-ICP-SF MS finally confirmed the presence of one sulfur atom per nickel. Esterification ga...
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biosynthesis of Coenzyme F430 a nickel porphinoid involved in methanogenesis
Ciba Foundation Symposium 180 - The Biosynthesis of the Tetrapyrrole Pigments, 2007Co-Authors: Rudolf K Thauer, Lutz G BonackerAbstract:: Coenzyme F430 is the prosthetic group of methyl-Coenzyme-M reductase, which catalyses the final step of methane formation in methanogenic bacteria. The Coenzyme is a nickel-containing macrocyclic tetrapyrrole of unique structure. We describe the biosynthesis of this nickel porphinoid from L-glutamate via 5-aminolaevulinic acid, uroporphyrinogen III and dihydrosirohydrochlorin, the binding of the Coenzyme to methyl-Coenzyme-M reductase and the regulation of Coenzyme F430 biosynthesis. We end with some evolutionary considerations on the biosynthesis of macrocyclic tetrapyrroles and remarks on the degradation of these compounds under anaerobic conditions.
Stephen W. Ragsdale - One of the best experts on this subject based on the ideXlab platform.
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Spectroscopic and Kinetic Studies of the Reaction of Bromopropanesulfonate with Methyl-Coenzyme
2020Co-Authors: Ryan C Kunz, Yihchern Horng, Stephen W. RagsdaleAbstract:Methyl-Coenzyme M reductase (MCR) catalyzes the final step of methanogenesis in which Coenzyme B and methyl-Coenzyme M are converted to methane and the heterodisulfide, CoMSSCoB. MCR also appears to initiate anaerobic methane oxidation (reverse methanogenesis). At the active site of MCR is Coenzyme F430, a nickel tetrapyrrole. This paper describes the reaction of the active MCRred1 state with the potent inhibitor, 3-bromopropanesulfonate (BPS; I50 50 nM) by UV-visible and EPR spectroscopy and by steady-state and rapid kinetics. BPS was shown to be an alternative substrate of MCR in an ionic reaction that is Coenzyme B-independent and leads to debromination of BPS and formation of a distinct state (“MCRPS”) with an EPR signal that was assigned to a Ni(III)-propylsulfonate species (Hinderberger, D., Piskorski, R. P., Goenrich, M., Thauer, R. K., Schweiger, A., Harmer, J., and Jaun, B. (2006) Angew. Chem. Int. Ed. Engl. 45, 3602–3607). A similar EPR signal was generated by reacting MCRred1 with several halogenated sulfonate and carboxylate substrates. In rapid chemical quench experiments, the propylsulfonate ligand was identified by NMR spectroscopy and high performance liquid chromatography as propanesulfonic acid after protonolysis of the MCRPS complex. Propanesulfonate formation was also observed in steady-state reactions in the presence of Ti(III) citrate. Reaction of the alkylnickel intermediate with thiols regenerates the active MCRred1 state and eliminates the propylsulfonate group, presumably as the thioether. MCRPS is catalytically competent in both the generation of propanesulfonate and reformation of MCRred1. These results provide evidence for the intermediacy of an alkylnickel species in the final step in anaerobic methane oxidation and in the initial step of methanogenesis.
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The Reaction Mechanism of Methyl-Coenzyme M Reductase HOW AN ENZYME ENFORCES STRICT BINDING ORDER
Journal of Biological Chemistry, 2015Co-Authors: Thanyaporn Wongnate, Stephen W. RagsdaleAbstract:Methyl-Coenzyme M reductase (MCR) is a nickel tetrahydrocorphinoid (Coenzyme F430) containing enzyme involved in the biological synthesis and anaerobic oxidation of methane. MCR catalyzes the conversion of methyl-2-mercaptoethanesulfonate (methyl-SCoM) and N-7-mercaptoheptanoylthreonine phosphate (CoB7SH) to CH4 and the mixed disulfide CoBS-SCoM. In this study, the reaction of MCR from Methanothermobacter marburgensis, with its native substrates was investigated using static binding, chemical quench, and stopped-flow techniques. Rate constants were measured for each step in this strictly ordered ternary complex catalytic mechanism. Surprisingly, in the absence of the other substrate, MCR can bind either substrate; however, only one binary complex (MCR·methyl-SCoM) is productive whereas the other (MCR·CoB7SH) is inhibitory. Moreover, the kinetic data demonstrate that binding of methyl-SCoM to the inhibitory MCR·CoB7SH complex is highly disfavored (Kd = 56 mm). However, binding of CoB7SH to the productive MCR·methyl-SCoM complex to form the active ternary complex (CoB7SH·MCR(NiI)·CH3SCoM) is highly favored (Kd = 79 μm). Only then can the chemical reaction occur (kobs = 20 s−1 at 25 °C), leading to rapid formation and dissociation of CH4 leaving the binary product complex (MCR(NiII)·CoB7S−·SCoM), which undergoes electron transfer to regenerate Ni(I) and the final product CoBS-SCoM. This first rapid kinetics study of MCR with its natural substrates describes how an enzyme can enforce a strictly ordered ternary complex mechanism and serves as a template for identification of the reaction intermediates.
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Biochemistry of methyl-Coenzyme M reductase: the nickel metalloenzyme that catalyzes the final step in synthesis and the first step in anaerobic oxidation of the greenhouse gas methane
Metal ions in life sciences, 2014Co-Authors: Stephen W. RagsdaleAbstract:Methane, the major component of natural gas, has been in use in human civilization since ancient times as a source of fuel and light. Methanogens are responsible for synthesis of most of the methane found on Earth. The enzyme responsible for catalyzing the chemical step of methanogenesis is methyl-Coenzyme M reductase (MCR), a nickel enzyme that contains a tetrapyrrole cofactor called Coenzyme F430, which can traverse the Ni(I), (II), and (III) oxidation states. MCR and methanogens are also involved in anaerobic methane oxidation. This review describes structural, kinetic, and computational studies aimed at elucidating the mechanism of MCR. Such studies are expected to impact the many ramifications of methane in our society and environment, including energy production and greenhouse gas warming.
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In vivo activation of methyl-Coenzyme M reductase by carbon monoxide.
Frontiers in Microbiology, 2013Co-Authors: Yuzhen Zhou, Alexandria E. Dorchak, Stephen W. RagsdaleAbstract:Methyl-Coenzyme M reductase (MCR) from methanogenic archaea catalyzes the rate-limiting and final step in methane biosynthesis. Using Coenzyme B (CoBSH) as the two-electron donor, MCR reduces methyl-Coenzyme M (CH3-SCoM) to methane and the mixed disulfide, CoBS-SCoM. MCR contains an essential redox-active nickel tetrahydro¬corphinoid cofactor, Coenzyme F430, at its active site. The active form of the enzyme (MCRred1) contains Ni(I)-F430. Rapid and efficient conversion of MCR to MCRred1 is important for elucidating the enzymatic mechanism, yet this reduction is difficult because the Ni(I) state is subject to oxidative inactivation. Furthermore, no in vitro methods have yet been described to convert Ni(II) forms into MCRred1. Since 1991, it has been known that MCRred1 from Methanothermobacter marburgensis can be generated in vivo when cells are purged with 100% H2. Here we show that purging cells or cell extracts with CO can also activate MCR. The rate of in vivo activation by CO is about 15 times faster than by H2 (130 min-1 and 8 min-1, respectively) and CO leads to two-fold higher MCRred1 than H2. Unlike H2-dependent activation, which exhibits a 10-h lag time, there is no lag for CO-dependent activation. Based on cyanide inhibition experiments, CODH is required for the CO-dependent activation. Formate, which also is a strong reductant, cannot activate MCR in M. marburgensis in vivo.
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Structural Analysis of a Ni-Methyl Species in Methyl-Coenzyme M Reductase from Methanothermobacter marburgensis
Journal of the American Chemical Society, 2011Co-Authors: Peder E. Cedervall, Stephen W. Ragsdale, Xianghui Li, Ritimukta Sarangi, Britt Hedman, Carrie M. WilmotAbstract:We present the 1.2 A resolution X-ray crystal structure of a Ni-methyl species that is a proposed catalytic intermediate in methyl-Coenzyme M reductase (MCR), the enzyme that catalyzes the biological formation of methane. The methyl group is situated 2.1 A proximal of the Ni atom of the MCR Coenzyme F430. A rearrangement of the substrate channel has been posited to bring together substrate species, but Ni(III)-methyl formation alone does not lead to any observable structural changes in the channel.
Bernhard Jaun - One of the best experts on this subject based on the ideXlab platform.
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Binding of Coenzyme B induces a major conformational change in the active site of methyl-Coenzyme M reductase.
Journal of the American Chemical Society, 2010Co-Authors: Sieglinde Ebner, Rudolf K Thauer, Bernhard Jaun, Meike Goenrich, Jeffrey HarmerAbstract:Methyl-Coenzyme M reductase (MCR) is the key enzyme in methane formation by methanogenic Archaea. It converts the thioether methyl-Coenzyme M and the thiol Coenzyme B into methane and the heterodisulfide of Coenzyme M and Coenzyme B. The catalytic mechanism of MCR and the role of its prosthetic group, the nickel hydrocorphin Coenzyme F430, is still disputed, and no intermediates have been observed so far by fast spectroscopic techniques when the enzyme was incubated with the natural substrates. In the presence of the competitive inhibitor Coenzyme M instead of methyl-Coenzyme M, addition of Coenzyme B to the active Ni(I) state MCRred1 induces two new species called MCRred2a and MCRred2r which have been characterized by pulse EPR spectroscopy. Here we show that the two MCRred2 signals can also be induced by the S-methyl- and the S-trifluoromethyl analogs of Coenzyme B. 19F-ENDOR data for MCRred2a and MCRred2r induced by S-CF3-Coenzyme B show that, upon binding of the Coenzyme B analog, the end of the 7-thi...
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Nickel-alkyl bond formation in the active site of methyl-Coenzyme m reductase.
Metal ions in life sciences, 2009Co-Authors: Bernhard Jaun, Rudolf K ThauerAbstract:Methyl-Coenzyme M reductase (MCR) catalyzes the methane-forming step in methanogenic archaea and most probably also the methane-oxidizing step in methanotrophic archaea. The enzyme contains Coenzyme F430 as prosthetic group. F430 is a nickel porphinoid that has to be in the reduced Ni(I) state for the enzyme to be active. The presently discussed catalytic mechanisms of MCR can in principle be divided into two basic models. In one model the key intermediate features a methyl-Ni(III) species being either formed in a nucleophilic substitution reaction or in an oxidative addition reaction. In the other model first the thioether sulfur of methyl-Coenzyme M binds to the Ni(I), which subsequently results in the release of the methyl group as methyl radical leaving behind a Ni(II)-sulfur bond. The experimental evidence for and against a methyl-nickel intermediate is reviewed.
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Coordination and binding geometry of methyl-Coenzyme M in the red1m state of methyl-Coenzyme M reductase
Journal of Biological Inorganic Chemistry, 2008Co-Authors: Dariush Hinderberger, Rudolf K Thauer, Bernhard Jaun, Stefan Mayr, Sieglinde Ebner, Meike Goenrich, Markus Reiher, Jeffrey HarmerAbstract:Methane formation in methanogenic Archaea is catalyzed by methyl-Coenzyme M reductase (MCR) and takes place via the reduction of methyl-Coenzyme M (CH3-S-CoM) with Coenzyme B (HS-CoB) to methane and the heterodisulfide CoM-S-S-CoB. MCR harbors the nickel porphyrinoid Coenzyme F430 as a prosthetic group, which has to be in the Ni(I) oxidation state for the enzyme to be active. To date no intermediates in the catalytic cycle of MCRred1 (red for reduced Ni) have been identified. Here, we report a detailed characterization of MCRred1m (''m'' for methyl-Coenzyme M), which is the complex of MCRred1a (''a'' for absence of substrate) with CH3-S-CoM. Using continuous-wave and pulse electron paramagnetic reso- nance spectroscopy in combination with selective isotope labeling ( 13 C and 2 H) of CH3-S-CoM, it is shown that CH3-S-CoM binds in the active site of MCR such that its thioether sulfur is weakly coordinated to the Ni(I) of F430. The complex is stable until the addition of the second sub- strate, HS-CoB. Results from EPR spectroscopy, along with quantum mechanical calculations, are used to characterize the electronic and geometric structure of this complex, which can be regarded as the first intermediate in the catalytic mechanism.
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Structure of an F430 Variant from Archaea Associated with Anaerobic Oxidation of Methane
Journal of the American Chemical Society, 2008Co-Authors: Stefan Mayr, Rudolf K Thauer, Seigo Shima, Martin Kruger, Christopher Latkoczy, Detlef Günther, Friedrich Widdel, Bernhard JaunAbstract:Microbial mats collected at cold methane seeps in the Black Sea carry out anaerobic oxidation of methane (AOM) to carbon dioxide using sulfate as the electron acceptor. These mats, which predominantly consist of sulfate-reducing bacteria and archaea of the ANME-1 and ANME-2 type, contain large amounts of proteins very similar to methyl-Coenzyme M reductase from methanogenic archaea. Mass spectrometry of mat samples revealed the presence of two nickel-containing cofactors in comparable amounts, one with the same mass as Coenzyme F430 from methanogens (m/z = 905) and one with a mass that is 46 Da higher (m/z = 951). The two cofactors were isolated and purified, and their constitution and absolute configuration were determined. The cofactor with m/z = 905 was proven to be identical to Coenzyme F430 from methanogens. For the m/z = 951 species, high resolution ICP-MS pointed to F430 + CH2S as the molecular formula, and LA-ICP-SF MS finally confirmed the presence of one sulfur atom per nickel. Esterification ga...
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derivatives of Coenzyme F430 with a covalently attached α axial ligand part i
Helvetica Chimica Acta, 2003Co-Authors: Carsten Bauer, Bernhard JaunAbstract:X-Ray structures of the enzyme methyl-Coenzyme M reductase show that the Ni-center in the prosthetic group Coenzyme F430 is penta- or hexacoordinated with the carboxamide group of a glutamine residue occupying the axial coordination site on the α-side of the macrocycle. To obtain diastereoselectively coordinated complexes for mechanistic and spectroscopic studies of the free Coenzyme in solution, we aimed to prepare partial-synthetic derivatives of Coenzyme F430 that have a coordinating group attached via a linker to one of the propanoic acid side chains. By using molecular-mechanics calculations and two different conformational search methods, a set of 50 structures containing imidazole or pyridine units as potential ligands were computationally tested according to geometric criteria defining coordinating conformations. The best candidates proved to be proline-containing tri- and tetrapeptides with a methyl-histidine as the C-terminal residue. These linkers were synthesized, and their conformation was determined by NMR. Refinement of the molecular modeling by using the experimentally determined geometric restraints allowed us to decide that the tripeptide Pro-Pro-His(π-Me)-OMe (10) was the most promising of all tested structures for attachment to the side chain at C(3) or C(13) of F430.
Marcel Goubeaud - One of the best experts on this subject based on the ideXlab platform.
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Purified Methyl‐Coenzyme‐M Reductase is Activated when the Enzyme‐Bound Coenzyme F430 is Reduced to the Nickel(I) Oxidation State by Titanium(III) Citrate
FEBS Journal, 2004Co-Authors: Marcel Goubeaud, Guido Schreiner, Rudolf K ThauerAbstract:The nickel porphinoid, Coenzyme F430, is the prosthetic group of methyl-Coenzyme M reductase. The active form of the enzyme exhibits Ni-EPR signals designated as MCR-red1 and MCR-red2. The inactive form of the enzyme is either EPR silent or it exhibits a distinct Ni-EPR signal designated MCR-0x1. Evidence is presented here that the MCR-ox1 form of the enzyme can be converted in vitro to the MCR-red1 form by reduction with titanium(III) citrate at pH 9. During conversion, the specific activity increases with increasing MCR-red1 spin concentration from 2 U/mg to approximately 100 U/mg at spin concentrations higher than 80%. The reduced methyl-Coenzyme-M reductase shows an ultraviolet/visible spectrum characteristic for Coenzyme F430 in the Ni(I) oxidation state, with maxima at 386 nm and at 750 nm. The results indicate that methyl-Coenzyme-M reductase is activated when the enzyme-bound Coenzyme F430 is reduced to the Ni(I) oxidation state. The experiments were performed with purified methyl-Coenzyme-M reductase isoenzyme I of Methanobacterium thermoautotrophicum (strain Marburg).
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The biosynthesis of methylated amino acids in the active site region of methyl-Coenzyme M reductase.
Journal of Biological Chemistry, 2000Co-Authors: Thorsten Selmer, Marcel Goubeaud, Seigo Shima, Ulrich Ermler, Jorg Kahnt, Wolfgang Grabarse, Rudolf K ThauerAbstract:Abstract The global production of the greenhouse gas methane by methanogenic archaea reaches 1 billion tons per annum. The final reaction releasing methane is catalyzed by the enzyme methyl-Coenzyme M reductase. The crystal structure of methyl-Coenzyme M reductase from Methanobacterium thermoautotrophicumrevealed the presence of five modified amino acids within the α-subunit and near the active site region. Four of these modifications were C-, N-, and S-methylations, two of which, 2-(S)-methylglutamine and 5-(S)-methylarginine, have never been encountered before. We have now confirmed these modifications by mass spectrometry of chymotryptic peptides. With methyl-Coenzyme M reductase purified from cells grown in the presence of l-[methyl-D3]methionine, it was shown that the methyl groups of the modified amino acids are derived from the methyl group of methionine rather than from methyl-Coenzyme M, an intermediate in methane formation. The D3 labeling pattern was found to be qualitatively and quantitatively the same as in the two methyl groups of the methanogenic Coenzyme F430, which are known to be introduced viaS-adenosylmethionine. From the results, it is concluded that the methyl groups of the modified amino acids in methyl-Coenzyme M reductase are biosynthetically introduced by anS-adenosylmethionine-dependent post-translational modification. A mechanism for the methylation of glutamine at C-2 and of arginine at C-5 is discussed.
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purified methyl Coenzyme m reductase is activated when the enzyme bound Coenzyme F430 is reduced to the nickel i oxidation state by titanium iii citrate
FEBS Journal, 1997Co-Authors: Marcel Goubeaud, Guido Schreiner, Rudolf K ThauerAbstract:The nickel porphinoid, Coenzyme F430, is the prosthetic group of methyl-Coenzyme M reductase. The active form of the enzyme exhibits Ni-EPR signals designated as MCR-red1 and MCR-red2. The inactive form of the enzyme is either EPR silent or it exhibits a distinct Ni-EPR signal designated MCR-0x1. Evidence is presented here that the MCR-ox1 form of the enzyme can be converted in vitro to the MCR-red1 form by reduction with titanium(III) citrate at pH 9. During conversion, the specific activity increases with increasing MCR-red1 spin concentration from 2 U/mg to approximately 100 U/mg at spin concentrations higher than 80%. The reduced methyl-Coenzyme-M reductase shows an ultraviolet/visible spectrum characteristic for Coenzyme F430 in the Ni(I) oxidation state, with maxima at 386 nm and at 750 nm. The results indicate that methyl-Coenzyme-M reductase is activated when the enzyme-bound Coenzyme F430 is reduced to the Ni(I) oxidation state. The experiments were performed with purified methyl-Coenzyme-M reductase isoenzyme I of Methanobacterium thermoautotrophicum (strain Marburg).
Abhik Ghosh - One of the best experts on this subject based on the ideXlab platform.
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High-valent transition metal centers and noninnocent ligands in metalloporphyrins and related molecules: a broad overview based on quantum chemical calculations
Journal of Biological Inorganic Chemistry, 2001Co-Authors: Abhik Ghosh, Erik SteeneAbstract:Using density functional theory, we have carried out a quantum chemical survey of high-valent transition metal porphyrins and related compounds. Discussed herein are recent developments on metalloporphyrin π-cation radicals, high-valent manganese and iron porphyrins and heme protein intermediates, nickel(III) porphyrinoids, Coenzyme F430, and high-valent transition metal corroles. In particular, we focus on whether the molecules of interest feature "true" high-valent metal centers, whether the ligands are oxidized instead, i.e. are noninnocent, or whether the electronic structures fall somewhere along the continuum between these scenarios.
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a first principles quantum chemical study of Coenzyme F430 interplay of skeletal stereoisomerism and conformation in the stabilization of nickel i
Journal of the American Chemical Society, 2000Co-Authors: Abhik GhoshAbstract:Nonlocal density functional theory calculations, with full geometry optimization, are reported for the Ni(I) and low-spin Ni(II) forms of high-fidelity models of Coenzyme F430, the nickel tetracorphinoid cofactor of methylCoenzyme M reductase (MCR), and its 12,13-diepimer. The diepimer appears to exhibit the conformational characteristics of a typical hydroporphyrin in terms of a strong tendency to adopt highly ruffled conformations and short Ni(II)−N bond distances. In contrast, for native F430, the steric effects of peripheral substituents impose a potent planarizing influence on the ring system. The relative inability to ruffle implies that the N4 core of F430 cannot contract sufficiently to optimally coordinate a small low-spin Ni(II) ion. This appears to be the key factor that results in the stabilization of the larger Ni(I) and high-spin Ni(II) ions by the F430 ligand environment. The optimized Ni−N bond distances for the Ni(I)−F430 model compound are 198, 200, 203, and 214 pm and span an extremely ...