A model "rebound" mechanism of hydroxylation by cytochrome P450: Stepwise and effectively concerted pathways, and their reactivity patterns

A two-state rebound mechanism of alkane hydroxylation by a model active spe cies of the enzyme cytochrome P450 is studied using density functional theo retic calculations. Theory corroborates Groves's rebound mechanism (Groves, J. T. J. Chem. Educ. 1985, 62, 928), with a key difference,namely that in the two-state rebound the reactivity and product distribution result from t he interplay of two reactive states of the active ferryl-oxene (Por(+.)FeO) species of the enzyme: one state is low-spin (doublet) and the other high- spin (quartet). Transition-state structures, intermediates, and product com plexes are identified for the two states. The bond activation in either one of the two states involves a hydrogen abstraction-like transition structur e. However, while in the high-spin state there forms a radical that has a s ignificant barrier for rebound, in the low-spin state the rebound is virtua lly barrierless. Even though one cannot ignore incursion of a small amount of radicals in the low-spin state, it is clear that the radical has a signi ficant lifetime mainly on the high-spin surface. The results are used to ga in insight into puzzling experimental data which emerge from studies of ult rafast radical clocks (e.g., Toy, P. H.; Newcomb, M.; Hollenberg, P. F., J. Am. Chem. Sec. 1998, 120, 7719), vis a vis the nature the transition state , deduced from kinetic isotope effect measurements (Manchester, J. I.; Dinn ocenzo, J. P.; Higgins, L. A.; Jones, J. P. J. Am. Chem. Sec. 1997, 119, 50 69) and stereochemical scrambling patterns (Groves, J. T.; McClusky, G. A.; White, R. E.; Goon, M. J. Biochem. Biophys. Res. Commun. 1978, 81, 154). U nderstanding the electronic structure of the various species leads to a pre dictive structure-reactivity picture, based on the two-state reactivity sce nario (Shaik, S.; Filatov, M.; Schroder, D.; Schwarz, H. Chem. fur. J. 1998 , 4, 193). The model makes it possible to predict the dependence of the rel ative rates of the two states, and of the corresponding steps as a function of the nature of the alkane, the resulting alkyl radical, and the binding capability of the thiolate proximal ligand of the active species.