Energy of hydroxylation reaction

It was shown (Likhtenshtein, 1988a), that the redox potential of a center which can abstract hydrogen in compound RH with the dissociation energy Drh, can be estimated by the following relationship  

According to Eq. 3.1, for the reaction to occur by the one-electron mechanism of hydrogen abstraction from an alkane in a free state (for instance, from a terminal methyl 

According to a widely accepted consensus, the complexation of 02 with heme followed by the two-step electron transition leads to the formation of the state [Fe2+02 ], in which the dioxygen adopts an active form capable of hydroxylating the substrates, including saturated hydrocarbons (Coon et. al., 1981 Guengerich and Macdonald 1984). The complexation and the first electron transfer proceed rapidly and take place at atmospheric pressure of dioxygen froml0 2to 10 3s. The second electron transfer is a relatively slow reaction (for the enzyme from P. Putida, k = 4 s 1), which commonly limits the entire process. 

The activation of oxygen in the heme iron coordination sphere and reactions of the activated species with substrates are very rapid processes and can, in principle, occur by the following mechanisms. 

The oxenoid mechanism implicates the insertion of an oxygen atom (oxenoid) across the C-H bond of the hydrocarbon (Hamilton et al., 1973). This mechanism is evidently energetically preferable, since it is accompanied by the formation of three bonds, two of which, O-H and C-O, are extremely strong and compensate the rupture of the relatively weak C-H and 0-0 bonds. The transition state, however, involves the formation of a three-membered ring with oxygen, whose formation is accompanied by a strain with an energy of about 30 kcal/mole. More over, the insertion of O to C-H or H-H bonds is a symmetrically forbidden process. 

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