Formation of the active species of cytochrome P450 by using iodosylbenzene: A case for spin-selective reactivity

The generation of the active species for the enzyme cytochrome P450 by using the highly versatile oxygen surrogate iodosylbenzene (PhIO) often produces different results compared with the native route, in which the active species is generated through O-2 uptake and reduction by NADPH. One of these differences that is addressed here is the deuterium kinetic isotope effect (KIE) jump observed during N-dealkylation of N,N-dimethylaniline (DMA) by P450, when the reaction conditions change from the native to the PhIO route. The paper presents a theoretical analysis targeted to elucidate the mechanism of the reaction of PhIO with heme, to form the high-valent iron-oxo species Compound I (CpdI), and define the origins of the KIE jump in the reaction of CpdI with DMA. It is concluded that the likely origin of the KIE jump is the spin-selective chemistry of the enzyme cytochrome P450 under different preparation procedures. In the native route, the reaction proceeds via the doublet spin state of CpdI and leads to a low KIE value. PhIO, however, diverts the reaction to the quartet spin state of CpdI, which leads to the observed high KIE values. The KIE jump is reproduced here experimentally for the dealkylation of N,N-dimethyl-4-(methylthio)aniline, by using intramolecular KIE measurements that avoid kinetic complexities. The effect of PhIO is compared with N,N-dimethylaniline-N-oxide (DMAO), which acts both as the oxygen donor and the substrate and leads to the same KIE values as the native route.