Enzymatic Methane Hydroxylation: sMMO and pMMO
69
reaction mechanisms for the same reaction catalyzed by copper-containing zeolites
[129–134].
7 Conclusions, Emerging Issues, and Challenges
This chapter reviewed mechanisms for the direct conversion of methane to methanol
on methane monooxygenase (soluble and particulate forms, (sMMO and pMMO))
obtained from density functional theory (DFT) calculations by various research
groups. The coordination number of iron cations in the active site of sMMO exhibiting
a direct reactivity toward substrate methane (MMOH Q ) is uncertain. Accordingly,
various groups used different models of the active site in various spin states. As
the active site of MMOH Q , coordinatively saturated and unsaturated (μ-O) 2 Fe(IV) 2
models where iron cations are ferromagnetically or antiferromagnetically coupled
were considered. Reflecting from different coordination environment of an iron
cation in the active site models, various mechanisms for the methane hydroxylation by MMOH Q have been proposed. In coordinatively saturated diiron models, a
bridging oxygen atom can activate a methane C–H bond in a homolytic manner to
form a methyl radical, together with the hydroxyl group bound to the diiron cations.
After the homolytic C–H bond activation, two reaction pathways open. One is a
radical rebound mechanism where a methyl radical, depart from the active site, is
rebound to the formed hydroxyl group. The other is nonsynchronous concerted mechanism where methyl radical that does not detach from the hydoxo group, and then
binding of a methyl group into the hydroxyl group proceeds in a barrierless fashion.
In contrast, coordinatively unsaturated diiron models have a vacant coordination site
on an iron cation, and it can be utilized to directly interact with methane through
the Fe–C bond formation. The direct Fe–C interactions facilitate the activation of a
methane C–H bond via a four-centered transition sate to form a HO–Fe–CH 3 intermediate. After the formation of the intermediate, the methyl group binds into the
hydroxyl group to form a methanol complex. Potential energy surfaces along these
mechanisms are completely different. Especially, different activation barriers for the
C–H bond dissociation were obtained.
Similar mechanistic analyses have be done for the methane hydroxylation on
pMMO whose active sites were assumed to be Cu(III)–O, (μ–O) 2 Cu(II)Cu(III),
and (μ-O)(μ-OH)Cu(II)Cu(III) structures. According to the DFT calculations, the
methane hydroxylation on pMMO proceeds via a nonradical intermediate, independent of the three types of active site. A further improvement in studying the
reactivity of MMO toward methane is to incorporate its entire protein by means of
mixed quantum mechanics/molecular mechanics (QM/MM) calculations. QM/MM
calculations found that protein environment has an influential impact on the potential energy surface of this reaction, because the total energies of transition states and
intermediates are affected by van der Waals and electrostatic interactions with the
protein. The above findings from the sophisticated computational approaches will
help to understand the experimental findings on the hydroxylation reactions.
69
reaction mechanisms for the same reaction catalyzed by copper-containing zeolites
[129–134].
7 Conclusions, Emerging Issues, and Challenges
This chapter reviewed mechanisms for the direct conversion of methane to methanol
on methane monooxygenase (soluble and particulate forms, (sMMO and pMMO))
obtained from density functional theory (DFT) calculations by various research
groups. The coordination number of iron cations in the active site of sMMO exhibiting
a direct reactivity toward substrate methane (MMOH Q ) is uncertain. Accordingly,
various groups used different models of the active site in various spin states. As
the active site of MMOH Q , coordinatively saturated and unsaturated (μ-O) 2 Fe(IV) 2
models where iron cations are ferromagnetically or antiferromagnetically coupled
were considered. Reflecting from different coordination environment of an iron
cation in the active site models, various mechanisms for the methane hydroxylation by MMOH Q have been proposed. In coordinatively saturated diiron models, a
bridging oxygen atom can activate a methane C–H bond in a homolytic manner to
form a methyl radical, together with the hydroxyl group bound to the diiron cations.
After the homolytic C–H bond activation, two reaction pathways open. One is a
radical rebound mechanism where a methyl radical, depart from the active site, is
rebound to the formed hydroxyl group. The other is nonsynchronous concerted mechanism where methyl radical that does not detach from the hydoxo group, and then
binding of a methyl group into the hydroxyl group proceeds in a barrierless fashion.
In contrast, coordinatively unsaturated diiron models have a vacant coordination site
on an iron cation, and it can be utilized to directly interact with methane through
the Fe–C bond formation. The direct Fe–C interactions facilitate the activation of a
methane C–H bond via a four-centered transition sate to form a HO–Fe–CH 3 intermediate. After the formation of the intermediate, the methyl group binds into the
hydroxyl group to form a methanol complex. Potential energy surfaces along these
mechanisms are completely different. Especially, different activation barriers for the
C–H bond dissociation were obtained.
Similar mechanistic analyses have be done for the methane hydroxylation on
pMMO whose active sites were assumed to be Cu(III)–O, (μ–O) 2 Cu(II)Cu(III),
and (μ-O)(μ-OH)Cu(II)Cu(III) structures. According to the DFT calculations, the
methane hydroxylation on pMMO proceeds via a nonradical intermediate, independent of the three types of active site. A further improvement in studying the
reactivity of MMO toward methane is to incorporate its entire protein by means of
mixed quantum mechanics/molecular mechanics (QM/MM) calculations. QM/MM
calculations found that protein environment has an influential impact on the potential energy surface of this reaction, because the total energies of transition states and
intermediates are affected by van der Waals and electrostatic interactions with the
protein. The above findings from the sophisticated computational approaches will
help to understand the experimental findings on the hydroxylation reactions.