An iron-oxygen intermediate formed during the catalytic cycle of cysteine dioxygenase.

Cysteine dioxygenase is a key enzyme in the breakdown of cysteine, but its mechanism remains controversial. A combination of spectroscopic and computational studies provides the first evidence of a short-lived intermediate in the catalytic cycle. The intermediate decays within 20 ms and has absorption maxima at 500 and 640 nm.

Multi-state epoxidation of ethene by cytochrome P450: a quantum chemical study.

The epoxidation of ethene by a model for Compound I of cytochrome P450, studied by the use of density functional B3LYP calculations, involves two-state reactivity (TSR) with multiple electromer species, hence "multi-state epoxidation". The reaction is found to proceed in stepwise and effectively concerted manners. Several reactive states are involved; the reactant is an (oxo)iron(IV) porphyrin cation radical complex with two closely lying spin states (quartet and doublet), both of which react with ethene to form intermediate complexes with a covalent C-O bond and a carbon-centered radical (radical intermediates). The radical intermediates exist in two electromers that differ in the oxidation state of iron; Por(+)(*)Fe(III)OCH(2)CH(2)(*) and PorFe(IV)OCH(2)CH(2)(*) (Por = porphyrin). These radical intermediates exist in both the doublet- and quartet spin states. The quartet spin intermediates have substantial barriers for transformation to the quartet spin PorFe(III)-epoxide complex (2.3 kcal mol(-)(1) for PorFe(IV)OCH(2)CH(2)(*) and 7.2 kcal mol(-)(1) for Por(+)(*)Fe(III)OCH(2)CH(2)(*)). In contrast, the doublet spin radicals collapse to the corresponding PorFe(III)-epoxide complex with virtually no barriers. Consequently, the lifetimes of the radical intermediates are much longer on the quartet- than on the doublet spin surface. The loss of isomeric identity in the epoxide and rearrangements to other products arise therefore mostly, if not only, from the quartet process, while the doublet state epoxidation is effectively concerted (Scheme 7). Experimental trends are discussed in the light of the computed mechanistic scheme, and a comparison is made with closely related mechanistic schemes deduced from experiment.

Stereospecific oxidation by compound I of cytochrome P450 does not proceed in a concerted synchronous manner.

Calculations show that the transition structure for the synchronous oxygen transfer by Compound I is a second order saddle point. The process is unlikely.

What is the difference between the manganese porphyrin and corrole analogues of cytochrome P450's compound I?

Density functional calculations on oxo-manganese complexes of corrole (1) and porphyrin (2 and 3) show a fundamental difference. The ground state of I is the singlet manganese(V) state, 1A(MnV), in which corrole is a closed shell. In contrast, 2 and 3 have high-spin manganese(IV) states, 1A1u and 3A2u respectively. This difference and the state ordering for each system are rationalized based on the competition between the intrinsic tendency of manganese to prefer high-spin electronic configurations, vis-à-vis the general tendency to prefer double occupancy in the low-lying orbitals. The outcome of this competition is determined primarily by the identity of the macrocycle, corrole versus porphyrin. Corrole with a small cavity holds the MnO moiety with a high off-plane displacement, and thereby prefers the low-spin state. On the other hand, porphyrin with the wider cavity holds the MnO moiety closer to the plane, and thereby prefers high-spin states.