Mechanism of the methane → methanol conversion reaction catalyzed by methane monooxygenase

A density functional study

Harold Basch, Koichi Mogi, Djamaladdin G. Musaev, Keiji Morokuma

Research output: Contribution to journalArticle

135 Citations (Scopus)

Abstract

The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH2)(H2O)Fe(m-O)22-HCOO)2Fe(NH2)(H2O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe(IV) atoms, L4F(IV)(μ-O)2Fe(IV)L4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L4Fe(IV)(μ-O)(μ-OH(· · · CH3))Fe(III)L4, which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe(III)-Fe(IV) mixed valence state. This short- lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L4Fe(III)(OHCH3)(μ- O)Fe(III)L4, which has an Fe(III)-Fe(III) core. The barrier of the rate- determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH4 oxidation reaction to form the methanol complex, I + CH4 → VI, was found to be exothermic by 39 kcal/mol.

Original languageEnglish
Pages (from-to)7249-7256
Number of pages8
JournalJournal of the American Chemical Society
Volume121
Issue number31
DOIs
Publication statusPublished - Aug 11 1999
Externally publishedYes

Fingerprint

methane monooxygenase
Conversion Disorder
Methane
Methanol
Hydroxyl Radical
Trichosanthin
Ligands
Hydroxylation
Diamond
Electronic structure
Atoms
Oxidation
Enzymes
Diamonds

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Mechanism of the methane → methanol conversion reaction catalyzed by methane monooxygenase : A density functional study. / Basch, Harold; Mogi, Koichi; Musaev, Djamaladdin G.; Morokuma, Keiji.

In: Journal of the American Chemical Society, Vol. 121, No. 31, 11.08.1999, p. 7249-7256.

Research output: Contribution to journalArticle

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abstract = "The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH2)(H2O)Fe(m-O)2(η2-HCOO)2Fe(NH2)(H2O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe(IV) atoms, L4F(IV)(μ-O)2Fe(IV)L4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L4Fe(IV)(μ-O)(μ-OH(· · · CH3))Fe(III)L4, which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe(III)-Fe(IV) mixed valence state. This short- lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L4Fe(III)(OHCH3)(μ- O)Fe(III)L4, which has an Fe(III)-Fe(III) core. The barrier of the rate- determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH4 oxidation reaction to form the methanol complex, I + CH4 → VI, was found to be exothermic by 39 kcal/mol.",
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N2 - The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH2)(H2O)Fe(m-O)2(η2-HCOO)2Fe(NH2)(H2O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe(IV) atoms, L4F(IV)(μ-O)2Fe(IV)L4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L4Fe(IV)(μ-O)(μ-OH(· · · CH3))Fe(III)L4, which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe(III)-Fe(IV) mixed valence state. This short- lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L4Fe(III)(OHCH3)(μ- O)Fe(III)L4, which has an Fe(III)-Fe(III) core. The barrier of the rate- determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH4 oxidation reaction to form the methanol complex, I + CH4 → VI, was found to be exothermic by 39 kcal/mol.

AB - The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH2)(H2O)Fe(m-O)2(η2-HCOO)2Fe(NH2)(H2O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe(IV) atoms, L4F(IV)(μ-O)2Fe(IV)L4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L4Fe(IV)(μ-O)(μ-OH(· · · CH3))Fe(III)L4, which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe(III)-Fe(IV) mixed valence state. This short- lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L4Fe(III)(OHCH3)(μ- O)Fe(III)L4, which has an Fe(III)-Fe(III) core. The barrier of the rate- determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH4 oxidation reaction to form the methanol complex, I + CH4 → VI, was found to be exothermic by 39 kcal/mol.

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