ion phase that leads to an alkyl radical coordinated to the iron-hydroxo complex by a weak OH---C hydrogen bond, labeled as CI; (ii) an alkyl (or OH) rotation phase whereby the alkyl group achieves a favorable orientation for rebound; and (iii) a rebound phase that leads to C-O bond making and the ferric-alcohol complexes, 4,2P. The two profiles remain close in energy throughout the first two phases and then bifurcate. Whereas the HS state exhibits a significant barrier and a genuine TS for rebound, in the LS state, once the right orientation of the alkyl group is achieved, the LS rebound proceeds in a virtually barrier-free fashion to the alcohol. As such, alkane hydroxylation proceeds by TSR,70-72,120 in which the HS mechanism is truly stepwise with a finite lifetime for the radical intermediate, whereas the LS mechanism is effectively concerted with an ultrashort lifetime for the radical intermediate. Subsequent studies of ethane and camphor hydroxylation by the Yoshizawa group117,181-183 arrived at basically the same conclusion, that the mechanism is typified by TSR. The differences between the results of Shaik et al.130,173,177-180 and Yoshizawa et al.117,181-183 were rationalized recently71,72 and shown to arise owing to technical problems and the choice of the mercaptide ligand,117,181-183 which is a powerful electron donor and is too far from the representation of cysteine in the protein environment. The most recent study of camphor hydroxylation, which was done at a higher quality,117 converged to the picture reported by Shaik et al.130,173,177-180 and shows a stepwise HS process with a barrier of more than 3 kcal/mol for C-O bond formation by rebound of the camphoryl radical vis-à-vis an effectively concerted LS process for which this barrier is 0.7 kcal mol-1 and is the rotational barrier for reaching the rebound position. By referring to Figure 21, it is possible to rationalize the clock data of Newcomb in a simple manner. The apparent lifetimes are based on the assumption that there is a single state that leads to the reaction, such that the radical lifetime can be quantitated from the rate constant of free radical rearrangement and the ratio of rearranged to unrearranged alcohol product. However, in TSR, the rearranged (R) product is formed only/mainly on the HS surface, while the unrearranged (U) product is formed mainly on Figure 20. Formal descriptions of iron(III)-peroxo, iron(III)-hydroperoxo, and iron(V)-oxo species with indication of the negative charges. The roles “electrophile” or “nucleophile” are assigned according to the charge type. Reprinted with permission from ref 7. Copyright 2000 Springer-Verlag Heidelberg. 3964 Chemical Reviews, 2004, Vol. 104, No. 9 Meunier et al.

Mechanism of oxidation reactions catalyzed by cytochrome p450 enzymes.

Theoretical perspective on the structure and mechanism of cytochrome P450 enzymes.

ion from camphor is affected by the choices made during setup.131 In this context, it should be noted that early QM/MM work on different P450cam intermediates and on different steps of the catalytic cycle employed separate setups that had been generated independently, which raises the question whether it is possible to handle the whole cycle with the same QM/MM setup. This is indeed the case: following a standard protocol that starts from the X-ray structure of the ferrous dioxygen complex and that involves careful assignment of protonation states and hydration, one can generate a common setup that gives QM/MM results for P450cam which are generally consistent with those obtained previously with individual setups.54 This confirms that setup issues can be resolved in a consistent manner if proper care is exercised. P450 Enzymes Modeled by QM/MM Calculations Chemical Reviews, 2010, Vol. 110, No. 2 955

P450 Enzymes: Their Structure, Reactivity, and Selectivity—Modeled by QM/MM Calculations

This review describes examples of remarkable acceleration of metal-catalyzed oxidation reactions by certain additives. In some cases, reactions proceed 2 or 10 times more rapidly in comparison with the process in the additive’s absence, in other cases, reactions become possible only in the presence of the additive. Varying ligands at the metal center or additives, one can not only dramatically improve yields of oxygenates but also control the selectivity of the reaction. Understanding mechanisms of the additive’s action is very important for search of new efficient catalysts and catalytic systems. Additives considered in the review can play roles of the ligands at metal ion or proton or electron transfer reagents and they mimic certain enzymes (the active center or its environment). Often the mechanism of the effect of additives on the reaction rate and the product yield is unknown, and the main aim of the review is to attract investigator’s attention in creating new efficient catalytic systems, which contain not only a metal ion but also a necessary “additive”.

http://shulpin.narod.ru/JMolCatal2002.pdf

Metal-catalyzed hydrocarbon oxygenations in solutions: the dramatic role of additives: a review

Spin state energies of iron complexes are important for biochemical applications such as the catalytic cycle of cytochrome P450. Due to the size of these systems and the presence of iron, accurate computational results can be obtained only with density functional theory (DFT). Validation of exchange-correlation (xc) DFT functionals for predicting the correct spin ground state of iron complexes is a rather unexplored area. In this contribution we report a systematic study on the performance of several xc functionals for seven iron complexes that are experimentally found to have either a low, intermediate, or high spin ground state. Standard xc functionals like LDA, BLYP, and PBE are found to disfavor high spin states, whereas hybrid and some meta-GGA functionals do provide the correct spin ground state for all molecules. Recently improved pure DFT functionals such as Handy's optimized exchange (OPTX) also perform well. The origin for the apparent performance of the DFT functionals has been addressed and seems to be related to the inclusion of fourth-order terms (s 4 ) of the dimensionless (or reduced) density gradient s in the exchange functional.

and Samuël P. de Visser

Papers

Medium Polarization and Hydrogen Bonding Effects on Compound I of Cytochrome P450: What Kind of a Radical Is It Really?