MECHANISM OF C-H ACTIVATION BY DIIRON METHANE MONOOXYGENASES - QUANTUM-CHEMICAL STUDIES

A variety of plausible mechanisms for methane hydroxylation in methane monooxygenases (MMO) have been tested by high-level quantum chemical methods on model systems with simple ligands chosen on the basis of th e MMO crystal structure and the available biophysical data. One pathwa y survives the present level of tests in having intermediates with pla usible energies and structures and a low-energy transition state for C -H abstraction. In this proposed pathway, the Fe-2(II,II) dinuclear ir on site of the reduced form of MMO reacts with O-2 to give two differe nt Fe-2(III,III) peroxo species and, after O-O bond cleavage, an Fe-2( IV,IV) bis-mu-oxo species probably directly analogous to ''compound Q' ' of MMO. As a result of the large Jahn-Teller distortions in the d(4) bis-mu-oxo species, the Fe-O-Fe bridges are highly asymmetric, allowi ng the system to open up easily to a key Fe-III-O-Fe-V=O intermediate that is shown to be capable of reacting with methane via a low-energy transition state. This intermediate is shown to be better regarded as having the structure Fe-III-O-Fe-IV-O-., with radical character at the terminal oxo group. After H atom abstraction from methane, the methyl radical recombines very rapidly with the Fe center via a weak Fe-CH3 bond. With the loss of CH3OH, an Fe-III-O-Fe-III dimer is formed that requires reduction to form the Fe-2(II,II) starting species. In additi on to the work on the dinuclear species, results on a number of releva nt mononuclear Fe(III) and Fe(IV) species are also reported.