Cytochrome manganese porphyrin complexes

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

This scheme contains shortcuts. Only these shortcuts could be mimicked catalyti-cally and in an efficient way by many model compounds having a closely related structure. Indeed, Oj activation is difficult and complex, as one can notice on the mechanistic scheme. In particular, iron and manganese porphyrin complexes have been synthesized in order to mimic cytochrome P450 using single oxygen-atom donors such as H2O2, etc. (XO on the scheme). 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

D KIE associated with alkane hydroxylation by cytochromes P-450 and intermolec-ular D KIE in the alkane hydroxylations catalysed by manganese and iron porphyrin complexes... 

Figure 9 Bimetallic model system for cytochrome P-450 composed of a manganese porphyrin and a rhodium(III) bipyridine complex anchored to a vesicle bilayer.

Numerous model systems for cytochrome P-450 have focussed upon the use of porphyrin complexes of chromium, manganese and iron in the presence of O2, peracids, hydroperoxides, AT-oxides or iodosylarenes as terminal oxidant [5, 6]. It is generally agreed that these systems generate a high valent metal-oxo species which participates in a rebound mechanism to shuttle an oxygen atom to a hydrocarbon... 

It was also demonstrated that the Rh reduction was highly dependent on the nature of the charge of the surfactant and the reduced rhodium complex catalyzes the reduction of methylviologen, riboflavin, and various manganese(lll) porphyrins. A variety of substrates (pinene, stilbene, styrene, limonene, ethylbenzene) were epoxidized with this P450 cytochrome mimics and with TON in the same order... 

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