Theoretical study of N-dealkylation of N-cyclopropyl-N-methylaniline catalyzed by cytochrome P450: insight into the origin of the regioselectivity

Abstract

The mechanism of N-dealkylation of N-cyclopropyl-N-methylaniline () catalyzed by cytochrome P450 (P450) was investigated using density functional theory. This reaction involves two steps. The first one is a Calpha-H hydroxylation on the N-substituent to form a carbinolaniline complex, and the second is a decomposition of the carbinolaniline to yield cyclopropanone (or formaldehyde) and N-methylaniline (or N-cyclopropylaniline). Our calculations demonstrate that the first step proceeds in a spin-selective mechanism (SSM), mostly on the low-spin (LS) doublet state. The rate-limiting Calpha-H activation is an isotope-sensitive hydrogen atom transfer (HAT) step. The environmental effect switches the regioselectivity of this reaction from a competition between N-decyclopropylation and N-demethylation to a clear preference for N-demethylation. This preference is consistent with former experimental studies. However, it is not in accord with the normal DeltaE-BDE correlation since the BDE of Calpha-H on the methyl group is higher than that on the cyclopropyl group. Insight into the origin of the preference for N-demethylation reveals that tertiary amine is different from normal hydrocarbons, possessing a unique piPh-piC-N conjugated system. The electron delocalization effect of the piPh-piC-N conjugated system in makes the transition state pose a polar character, and the bulk polarity and hydrogen bonding capability of the protein pocket can exert a remarkable effect on the regioselectivity of N-dealkylation of . Decomposition of carbinolaniline is a water-assisted proton-transfer process in the nonenzymatic environment. The ring-intact cyclopropanone formed in the reaction sheds some light on the inability of to inactivate P450 during its N-decyclopropylation.

Recent density functional theory model calculations of drug metabolism by cytochrome P450

“…The calculated energy barriers for nicotine 5 -hydroxylation are higher than the barriers for amine C ␣ -H hydroxylation [102,103]. This is because the amines used in the previous studies are deprotonated, whereas the nicotine used in this study is a protonated one, with a proton on the pyrrolidine nitrogen.…”

“…A subsequent DFT study on N-cyclopropyl-N-methylaniline (CMA) [103] shows similar electron delocalization effect of the Ph -N C conjugated system. The electron delocalization effect of the Ph -N C conjugated system makes the transition state possess a polar character, and therefore, the bulk polarity and hydrogen bonding capability of the protein pocket exert a remarkable effect on the regioselectivity of the CMA N-dealkylation reaction and cause a preference for N-demethylation over N-decyclopropylation.…”

“…This is because the amines used in the previous studies are deprotonated, whereas the nicotine used in this study is a protonated one, with a proton on the pyrrolidine nitrogen. In the deprotonated amines, the Ph -N C conjugative interaction delocalizes the electron distribution of the N C H reaction center and, thus, stabilizes the transition state [102,103]. However, the protonated nicotine species has a proton on the N atom of the pyrrolidine ring such that no lone pair is available for the conjugative interaction.…”

Quantum-Mechanical Studies of Reactions Performed by Cytochrome P450 Enzymes

“…with hydrogen-abstraction and rebound steps, of which the first is rate limiting. Because of the importance of understanding these reaction types in drug degradation, a number of theoretical studies have been performed, in several cases including the non-enzymatic reaction, 62,71,72,[74][75][76][77] as are summarized in Figure 5.…”

Theoretical study on the atmospheric transformation mechanism of pirimiphos-methyl initiated by O3

“…Previous studies have reported that the activation energy for decomposition of carbinolaniline is greatly reduced by the participation of a water molecule in the reaction (Wang et al, 2007;Li et al, 2009). This may imply that the N-dealkylation process is favorable in the presence of water.…”