Elucidation of the mechanism of N-demethylation catalyzed by cytochrome P450 monooxygenase is facilitated by exploiting nitrogen-15 heavy isotope effects

15N heavy isotope effects are especially useful when detail is sought pertaining to the reaction mechanism for the cleavage of a C–N bond. Their potential in assisting to describe the mechanism of N-demethylation of tertiary amines by the action of cytochrome P450 monooxygenase has been investigated. As a working model for the first step, oxidation of the N-methyl group to N-methoxyl, tropine and a cytochrome P450 monooxygenase reaction centre composed of a truncated heme with sulfhydryl as the axial ligand were used. It is apparent that this first step of the reaction proceeds via a hydrogen atom transfer mechanism. Transition states for this step are described for both the high spin (4TSH) and low spin (2TSH) pathways in both gas and solvation states. Hence, overall normal secondary 15N KIE could be calculated for the reaction path modeled in the low spin state, and inverse for the reaction modeled in the high spin state. This partial reaction has been identified as the probable rate limiting step. The model for the second step, fission of the C–N bond, consisted of N-methoxylnortropine and two molecules of water. A transition state described for this step, TSCN, gives a strongly inverse overall theoretical 15N KIE.