How do aldehyde side products by cytochrome P450? Theory reveals a state-specific multi-state scenario where the high-spin component leads to all side products

A theoretical study of alkene epoxidation, by the high-valent iron-oxo species (Compound I) of cytochrome P450, reveals a multi-state scenario in which the different products are generated in a state specific manner. All the low-spin doublet state processes are effectively concerted epoxide producing pathways. By contrast, all the high-spin quartet processes are stepwise and either lead to epoxide that does not conserve the isomeric identity of the alkene (cis or trans), or/and to by-products such as suicidal complexes and aldehydes. The product/state inventory is the following: (a) The epoxide with conserved alkene stereochemistry is generated from the low-spin doublet states of Compound I in a nonsynchronous but effectively concerted pathways that involve carbon radical (with Fe-III and Fe-IV) and cationic intermediates. (b) The epoxide with scrambled alkene stereochemistry is obtained from the quartet high-spin radical intermediate (with Fe-IV). (c) The suicidal complex, with a C-N bond between the alkene and the porphyrin, is obtained from the high-spin cationic state that possesses one electron in the sigma(xy)(*) orbital (the antibonding Fe-N orbital made from d(xy) and nitrogen sigma-hybrids). (d) The aldehyde by-product is obtained from the high-spin cationic state that possesses one electron in the sigma(z2)(*) orbital (the anti-bonding O-Fe-S orbital made from d(z2) and the oxo and sulfur sigma-hybrids). Factors controlling the competition between these processes are discussed. (C) 2004 Elsevier Inc. All rights reserved.