NDLI: A comprehensive test set of epoxidation rate constants for iron(iv)

Content Provider	Royal Society of Chemistry (RSC)
Author	Kumar, Suresh Kumar, Devesh Fornarini, Simonetta Crestoni, Maria Elisa Visser, Sam P. de Sainna, Mala A.
Copyright Year	2015
Abstract	Cytochrome P450 enzymes are heme based monoxygenases that catalyse a range of oxygen atom transfer reactions with various substrates, including aliphatic and aromatic hydroxylation as well as epoxidation reactions. The active species is short-lived and difficult to trap and characterize experimentally, moreover, it reacts in a regioselective manner with substrates leading to aliphatic hydroxylation and epoxidation products, but the origin of this regioselectivity is poorly understood. We have synthesized a model complex and studied it with low-pressure Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry (MS). A novel approach was devised using the reaction of [FeIII(TPFPP)]+ (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion) with iodosylbenzene as a terminal oxidant which leads to the production of ions corresponding to [FeIV(O)(TPFPP+˙)]+. This species was isolated in the gas-phase and studied in its reactivity with a variety of olefins. Product patterns and rate constants under Ideal Gas conditions were determined by FT-ICR MS. All substrates react with [FeIV(O)(TPFPP+˙)]+ by a more or less efficient oxygen atom transfer process. In addition, substrates with low ionization energies react by a charge-transfer channel, which enabled us to determine the electron affinity of [FeIV(O)(TPFPP+˙)]+ for the first time. Interestingly, no hydrogen atom abstraction pathways are observed for the reaction of [FeIV(O)(TPFPP+˙)]+ with prototypical olefins such as propene, cyclohexene and cyclohexadiene and also no kinetic isotope effect in the reaction rate is found, which suggests that the competition between epoxidation and hydroxylation – in the gas-phase – is in favour of substrate epoxidation. This notion further implies that P450 enzymes will need to adapt their substrate binding pocket, in order to enable favourable aliphatic hydroxylation over double bond epoxidation pathways. The MS studies yield a large test-set of experimental reaction rates of iron(IV)–oxo porphyrin cation radical complexes, so far unprecedented in the gas-phase, providing a benchmark for calibration studies using computational techniques. Preliminary computational results presented here confirm the observed trends excellently and rationalize the reactivities within the framework of thermochemical considerations and valence bond schemes.
Starting Page	1516
Ending Page	1529
Page Count	14
File Format	HTM / HTML PDF
ISSN	20416520
Volume Number	6
Issue Number	2
Journal	Chemical Science
DOI	10.1039/c4sc02717e
Language	English
Publisher	Royal Society of Chemistry
Access Restriction	Subscribed
Subject Keyword	Isotope Hydroxylation Regioselectivity Aliphatic compound Cytochrome P450 Electron affinity Fourier-transform ion cyclotron resonance Cyclohexadiene Ion Cyclohexene Reaction rate Propene Cyclotron Alkene Ionization Porphyrin Epoxide Covalent bond Hydrogen atom abstraction Nuclear magnetic resonance Mass spectrometry Nucleophile
Content Type	Text
Resource Type	Article
Subject	Chemistry

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