Theoretical Study of N-Demethylation of Substituted N,N-Dimethylanilines by Cytochrome
P450: The Mechanistic Significance of Kinetic Isotope Effect Profiles
posted on 2007-07-05, 00:00authored byYong Wang, Devesh Kumar, Chuanlu Yang, Keli Han, Sason Shaik
The mechanism of N-demethylation of N,N-dimethylanilines (DMAs) by cytochrome P450, a highly debated
topic in mechanistic bioinorganic chemistry (Karki, S. B.; Dinnocenczo, J. P.; Jones, J. P.; Korzekwa, K. R.
J. Am. Chem. Soc.1995, 117, 3657), is studied here using DFT calculations of the reactions of the active
species of the enzyme, Compound I (Cpd I), with four para-(H, Cl, CN, NO2) substituted DMAs. The
calculations resolve mechanistic controversies, offer a consistent mechanistic view, and reveal the following
features: (a) the reaction pathways involve C−H hydroxylation by Cpd I followed by a nonenzymatic
carbinolamine decomposition. (b) C−H hydroxylation is initiated by a hydrogen atom transfer (HAT) step
that possesses a “polar” character. As such, the HAT energy barriers correlate with the energy level of the
HOMO of the DMAs. (c) The series exhibits a switch from spin-selective reactivity for DMA and p-Cl-DMA to two-state reactivity, with low- and high-spin states, for p-CN-DMA and p-NO2-DMA. (d) The
computed kinetic isotope effect profiles (KIEPs) for these scenarios match the experimentally determined
KIEPs. Theory further shows that the KIEs and TS structures vary in a manner predicted by the Melander−Westheimer postulate: as the substituent becomes more electron withdrawing, the TS is shifted to a later
position along the H-transfer coordinate and the corresponding KIEs increases. (e) The generated carbinolaniline
can readily dissociate from the heme and decomposes in a nonenzymatic environment, which involves water
assisted proton shift.