2.3 pMMO
41
Cu III
Cu III
O
O
Cu III
Cu II
O
O
Cu
Cu
O
O
CH 3
H
Cu III
Cu II
O
O
H
CH 3
Cu
Cu
O
O
H 3 C H
Cu II
Cu II
O
O
H
H 3 C
Cu II
Cu II
O
O
His
His
His
NH3
Fig. 2.12 Proposed reaction mechanism for the hydroxylation of methane by pMMO using a model
involving a di-nuclear copper catalytic site
Cu III
O
+ CH 4
Cu III
O
CH 3
H
Cu II
OH
CH 3
Cu III
OH
CH 3
Cu III
O CH 2
Cu I
+ CH 3 OH
Fig. 2.13 Proposed reaction mechanism for the hydroxylation of methane by pMMO based on a
model involving a mono-nuclear copper catalytic site
cooperative mechanism” [101]. The possibility of methanol production via a nonradical mechanism has also been reported based on a quantum chemical calculation
[102].
On the other hand, in the mono-nuclear copper site model, a Cu
III –oxo species
is assumed to be an active species [92], although this species has not been observed
experimentally. The corresponding reaction mechanism is shown schematically in
Fig. 2.13. The Cu
III –oxo species binds a methane molecule and abstracts a hydrogen
atom from the methane with a barrier of 22 kJ mol
−1 . A methyl radical and an OH
group are produced by the hydrogen abstraction, followed by the rebound of the OH
group to the methyl radical to produce methanol. The barrier for the rebound of the
methyl radical to the OH group was calculated to be 53 kJ mol
−1 . At the beginning
of the reaction, the triplet spin state is favorable, while the singlet spin state is more
favorable for the intermediate. Therefore, the spin state shifts from the triplet state
to the singlet state in this reaction mechanism; this so-called spin-crossover was also
found to make the reaction favorable in a previous study. Overall, the reaction of
methane with the Cu
III –oxo species is exothermic, and the activation barriers are
lower than those in the reaction mechanism based on a di-nuclear copper site (107 kJ
mol
−1 ).
2.3.4 Substrate-Binding Cavity
As no crystal structures of pMMO containing analogs of the substrate or product
have been obtained yet, the substrate-binding site of pMMO has not been identified
41
Cu III
Cu III
O
O
Cu III
Cu II
O
O
Cu
Cu
O
O
CH 3
H
Cu III
Cu II
O
O
H
CH 3
Cu
Cu
O
O
H 3 C H
Cu II
Cu II
O
O
H
H 3 C
Cu II
Cu II
O
O
His
His
His
NH3
Fig. 2.12 Proposed reaction mechanism for the hydroxylation of methane by pMMO using a model
involving a di-nuclear copper catalytic site
Cu III
O
+ CH 4
Cu III
O
CH 3
H
Cu II
OH
CH 3
Cu III
OH
CH 3
Cu III
O CH 2
Cu I
+ CH 3 OH
Fig. 2.13 Proposed reaction mechanism for the hydroxylation of methane by pMMO based on a
model involving a mono-nuclear copper catalytic site
cooperative mechanism” [101]. The possibility of methanol production via a nonradical mechanism has also been reported based on a quantum chemical calculation
[102].
On the other hand, in the mono-nuclear copper site model, a Cu
III –oxo species
is assumed to be an active species [92], although this species has not been observed
experimentally. The corresponding reaction mechanism is shown schematically in
Fig. 2.13. The Cu
III –oxo species binds a methane molecule and abstracts a hydrogen
atom from the methane with a barrier of 22 kJ mol
−1 . A methyl radical and an OH
group are produced by the hydrogen abstraction, followed by the rebound of the OH
group to the methyl radical to produce methanol. The barrier for the rebound of the
methyl radical to the OH group was calculated to be 53 kJ mol
−1 . At the beginning
of the reaction, the triplet spin state is favorable, while the singlet spin state is more
favorable for the intermediate. Therefore, the spin state shifts from the triplet state
to the singlet state in this reaction mechanism; this so-called spin-crossover was also
found to make the reaction favorable in a previous study. Overall, the reaction of
methane with the Cu
III –oxo species is exothermic, and the activation barriers are
lower than those in the reaction mechanism based on a di-nuclear copper site (107 kJ
mol
−1 ).
2.3.4 Substrate-Binding Cavity
As no crystal structures of pMMO containing analogs of the substrate or product
have been obtained yet, the substrate-binding site of pMMO has not been identified