John T. Groves

Aliphatic hydroxylation by highly purified liver microsomal cytochrome P-450. Evidence for a carbon radical intermediate

was oxidized. Analysis of the mass spectra of the products from the deuterated hydrocarbon showed that 25% of the exo-norborneol contained four deuterium atoms whereas 9% of the endo-norborneol contained three deuterium atoms. These results, wm indicate a very large isotope effect (kH/kD = 11.521) and a significant amount of epimerization for the hydroxylation of norbornane by cytochrome P-450, suggest an initial hydrogen abstraction to give a carbon radical intermediate. The heme-containing mixed function oxidase of liver micro- somes, cytochrome P-450, has been the subject of much investiga- tion because of its ability to catalyze epoxidation or hydroxyla- tion of a wide variety of organic compounds (cf. l-3). Several - lines of evidence suggest that the reactive oxygen intermediate is a higher valent iron-oxo species equivalent to (Fe013+

Metalloporphyrin Models for Cytochrome P-450

The elucidation of the molecular mechanisms of biological oxygen activation has been the focus of sustained attention for over a decade.1 In this time, cytochrome P-450 has become a Rosetta stone among the hemecontaining monooxygenases.2,3 The wide variety of oxygenations mediated by P-450 and the significance of these processes in steroid metabolism, drug detoxification, and the carcinogenic activation of polycyclic aromatic hydrocarbons have stimulated an effort to understand these processes. The selective hydroxylation of unactivated alkanes, in particular, lacks a classical paradigm in organic chemistry. Accordingly, there has been an effort to develop synthetic models of P-450 which might be used for the oxyfunctionalization of hydrocarbons. An understanding of the mechanism of these simple cases has begun to provide a conceptual base for the understanding of the enzymatic pathway.

Hydroxylation and expoxidation reactions catalyzed by synthetic metalloporphyrinates. Models related to the active oxygen species of cytochrome P-450

Abstract Iodosylbenzene has been found to react with catalytic amounts of synthetic metalloporphyrinates to give epoxides and alcohols from olefins and alkanes. Tetraphenylporphinatoiron(III) chloride (TPPFeCl) catalyzed the epoxidation of cis -stilbene to cis -stilbene oxide whereas the trans isomer was unreactive. Dioctyl protoporphyrin IX reacted with iodosylbenzene to give significant oxidation of the side chain alkyl groups. The distribution of products is interpreted to be the result of an intramolecular oxygen transfer reaction. Treatment of tetra-ortho-tolylporphinatoiron(III) chloride (TTPFeCl) with iodosylbenzene afforded a reactive intermediate which degraded the porphyrin in the absence of a oxidizable substrate. Similar oxidation of tetraphenylporphinatochromium(III) chloride with iodosylbenzene gave a stable intermediate which has been characterized as an oxochromium(V) complex.

Mössbauer effect study of tight spin coupling in oxidized chloro‐5,10,15,20‐tetra(mesityl)porphyrinatoiron(III)

Mossbauer spectra of a polycrystalline form of oxidized chloro‐5,10,15,20‐tetra(mesityl)porphyrin‐ atoiron(III) [Fe(TMP)Cl], compound A, were recorded over a range of temperatures (4.2–195 K) and magnetic fields (0–6 T). These spectra of compound A exhibit magnetic features which are markedly different from those of the analogous protein complexes, horse radish peroxidase compound I (HRP‐I) and compound ES of cytochrome c peroxidase, even though chemical evidence and optical spectroscopy indicate that compound A is similar to the others in comprising a Fe(IV) complex within a porphyrin cation radical. We interpret the data by employing a spin Hamiltonian model in which the central Fe(IV) complex, with S=1, is tightly coupled to a S=1/2 system of the oxidized porphyrin to yield a net S=3/2 system as suggested by the susceptibility measurements. The theoretical treatment yields information on the d‐electron energies which is similar to that more directly available in the peroxidase spectra. The strength of ...

A stereochemical probe of the fate of carbon radicals oxidized by metals

observation of radicals 2b such asjin the esr and the isolation of carbonyl products. There has appeared as yet no direct evidence for an alkoxy radical and in particular no report of a most typical alkoxy radical reaction, yhydrogen abstraction. Photolysis of an acetonitrile solution of 2-cyclohexylethanol &) in the presence of anhydrous ferric perchlorate with a Hanovia 450-watt lamp through a Pyrex filter led to complete reduction of iron(II1) to iron(I1) after 4 hr (quantum yield ~0.1). Concentration of the reaction mixture and analysis of the products after distillation afforded cyclohexyl acetaldehyde (&) and c& and trans-perhydrobenzofuran (J, anda (Table I, Scheme II). The first-order interpretation of these results is that an alkoxy radical (2, initially formed upon photodissociation of an iron(III)-2-cyclohexyl ethanol complex [J_ + Fe(III)J, undergoes intramolecular y-hydrogen abstraction in a manner analogous to the family of Barton-type reactions. 5

ON THE MECHANISM OF OXYGEN INSERTION CATALYZED BY CYTOCHROME P-450. HYDROXYLATION WITH PARTIAL ALLYLIC REARRANGEMENT

The cumene hydroperoxide dependent oxidation of cyclohexene catalyzed cytochrome P-450 produced cyclohexene oxide and cyclohexenol. Catalytic Systems containing chlorotetraphenyl-porphinatoiron(III)-iodosylbenzene and chlorotetraphenylpor-phinatochromium (III)-iodosylbenzene gave the same products. The oxidation of 1,2-dideuterocyclohexene by these systems gave cyclohexenol with various degrees of allylic rearrangement. With the chromium catalyst nearly complete, allylic scrambling was observed whereas the iron catalyst and cytochrome P-450 gave only partial equilibration. A mechanism involving caged radical intermediates is presented to account for these results.

On the reorganization of homobullvalenyl carbonium ions: A convenient synthesis of homobullvalenone

There has been continued intense interest in the reorganization of (CH), hydrocarbons and ions particularly with regard to expectations based on theoretical grounds. 1 Toward this end we recently described the generation of enone i upon silver assisted solvolysis of dibromide 1. 2a We report here the ready conversion of dibromide 1 to the skeletally related a-bromotrifluoroacetate, 2. 2b Treatment of 2 with silver trifluoroacetate affords preparative quantities of homobullvalenone (z)3a confirming our prior suggestion of its intermediacy on the path to 5 More importantly, the isolation of two additional isomeric ketones appears to gilie some additional insight into the predominant modes of rearrangement in this manifold.

Models of Metalloenzymes: Carboxypeptidase A

Mechanistic and structural studies on bovine pancreatic carboxypeptidase A are reviewed. Pertinent model studies of metal catalyzed acyl transfer reactions are also reviewed. Results for the hydrolysis of a metal-complexing lactam (1), in which the metal is held perpendicular to the plane of the amide are described. Rate enhancements for Cu2+-1 are found to be greater than 106 with respect to the hydrolysis of the same ligand with no metal present. Similarly, a rate enhancement of 104–105 has been observed for Zn2+-1. A mechanism involving nucleophilic attack of a metal-bound hydroxide is proposed for this process.

Models of Metalloenzymes: Peroxidase and Cytochrome P-450

Evidence has emerged over the last decade to support an intermediate heme-ironoxo complex as the active oxygen species in the catalytic cycle of cytochrome P-450. Studies with deuterated substrates have shown that a large isotope effect and significant rearrangement accompany the aliphatic hydroxylation mediated by this enzyme. Model studies with chromium, manganese and iron porphyrins have shown that the latter stages of oxygen transfer can be simulated outside of the protein. An oxochromium (V) porphyrin complex has been characterized by its e. s. r. spectrum. An X-ray crystal structure of our oxochromium(IV) complex has been determined. An oxomanganese(V) porphyrin complex has been shown to hydroxylate alkanes under mild conditions. An iron(IV) porphyrin cation radical species which has spectroscopic features similar to those reported for peroxidase Compound I has been shown to transfer oxygen to hydrocarbon substrates.

Synthetic models of metalloenzymes

Abstract Metal ions play an important role in the enzymic catalysis of many metalloproteins. The molecular details of the catalytic cycle are often obscured by the complexity of the biological system. It has been the goal of our research for the past few years to elucidate the mechanisms of metalloenzymes through the synthesis of simple metal complexes that mimic the structure of the active sites. The reactivity of such metal complexes have provided insights into the enzyme mechanism. Further, successful enzyme models have provided a rational basis for the construction of synthetic, biomimetic catalysts. In this lecture, recent advances in the study of active site models of carboxypeptidase A, CPA, a zinc-containing protease, and cytochrome P-450, a heme-containing monooxygenase will be described. The role of zinc in the peptidase activity of CPA has been ascribed to coordination of the substrate amide carbonyl, coordination of a nucleophilic hydroxide or even to a less specific structural role. To choose among these possibilities we have synthesized a family of metal-complexing amides which does not allow a metal-carbonyl interaction. Large zinc- and copper-mediated rate enhancements (10 4 10 7 ) for amide hydrolysis are observed with these compounds. Kinetic and titrimetric measurements indicate that the deprotonation of a metal-bound water is a component of this catalysis. A mechanism for amide hydrolysis involving nucleophilic attack of a metal hydroxide is consistent with the observed results. The catalytic cycle of cytochrome P-450 has been suggested to involve a reactive oxo-iron intermediate which is responsible for oxygen transfer to the substrate. We have prepared the first synthetic example of an iron(IV)-porphyrin cation radical complex ( 1 ). This species has been shown to be extraordinarily reactive toward hydrocarbons. The physico-chemical characterization of 1 and the elucidation of the mechanism of olefin epoxidation and alkane hydroxylation will be described.