(Graph Presented) The detection and kinetic characterization of a cytochrome P450 model compound I, [OFeIV-4-TMPyP]+ (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of Fe III-4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin π-radical cation. This intermediate was subsequently transformed into the well-characterized OFeIV-4-TMPyP. Global analysis afforded a second-order rate constant k1 = (1.59 ± 0.06) × 10 7 M-1 s-1 for the formation of 1 followed by a first-order decay with k2 = 8.8 ± 0.1 s-1. 1H and 13C NMR determined 9-xanthydrol to be the major product (∼90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% 18OH2 indicated 21% 18O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d2 revealed a modest kinetic isotope effect, kH/kD = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k3 = (3.6 ± 0.3) × 106 M-1 s-1. Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C-H hydroxylation that correlated well with the scissile C-H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C-H bond cleavage onto the Brønsted- Evans-Polanyi relationship for similar substrates determined the H-OFe IV-4-TMPyP bond dissociation energy to be ∼100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry