Theoretical Study of N-Demethylation of Substituted N,N-Dimethylanilines by Cytochrome P450: The Mechanistic Significance of Kinetic Isotope Effect Profiles
journal contributionposted on 2007-07-05, 00:00 authored by Yong 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.
energy levelKIEPnonenzymatic carbinolamine decompositionKIEs increasesDMAisotope effect profilesnonenzymatic environmentCpdreaction pathwaysseries exhibitsKinetic Isotope Effect ProfilesThe mechanismHOMOHAT energy barriersTheoretical StudyCNproton shiftMechanistic SignificancereactivityTS structuresDFT calculationsbioinorganic chemistryhydrogen atom transferhydroxylation