Mechanism of Oxygen Activation by

Also Other mechanisms of oxygen activation by flavohydroquinone have been postulated, among others the formation of oxaziridines, dioxetanes and carbonyl oxides but the resulting chromophores would probably absorb at wavelengths shorter than 370 nm. However, Bruice and his co-workers gathered... 

The mechanism of oxygen activation by P-450 has been extensively studied (7c, 5a, b) however, cmcial intermediates in the catalytic cycle are so unstable that it is not possible to examine their structures and reactivities. Accordingly, model systems for P-450 with synthetic metalloporphyrins have illuminated the enzymic reaction mechanism. For instance, the so-called picket fence porphyrin, /ncjo-tetra(a,a,a,a-pivalamidophenyl)porphyrin (TpivPP) Fe(II), prepared by Collman and co-workers, forms a stable complex with O2 6a, b). An X-ray... 

T.A. Dix and S.J. Benkovic, Mechanism of oxygen activation by pteridine-dependent... 

Such reactions are interesting as models for oxygen atom transfer in mechanisms of dioxygen activation by cytochrome P450 enzyme systems (49, 50, 51). 

Studies of the Mechanism of Oxygen Activation and Transfer Catalyzed by Cytochrome P 450... 

It is apparent that more than one oxygen activation pathway exists. The excited triplet state of tin porphyrins is known to be quenched in the presence of 2, suggesting a possible direct mechanism of O2 activation by the photosensitizer. We have examined reactions of both singlet O2 and superoxide anion under our reaction conditions. Chemically-produced superoxide (K02/18-crown-6) is not reactive under our experimental conditions. On the other hand, singlet oxygen, produced by irradiation of free base protoporphyrin (H2ProtoP), is reactive in the presence of tertiary amines and gives about the same hexanol to hexanone ratio (2.7, see Table 1) as is observed in the presence of the SnP photosensitizer. 

Answers to these questions are predicated on the ingenious design of new supra-molecular systems. First and foremost, well-defined model systems that allow the independent timing of proton and electron are needed some systems—notably those described in Section 4.4.10—are poised to attack this issue because they display unique spectroscopic signatures for the transfer of the electron and the proton. In addition to directly probing the sequence and rates of the two events, kinetic isotope effects for the proton and the electron transfers may be ascertained. Isotope effects have been used to infer PCET mechanisms in oxygen activation by model... 

In this review, we will focus on the mechanistic aspects of the oxygen activation and reactions of the P-450 systems involving metalloporphyrins. For this purpose, we will briefly outline some of the chemistry of P-450 and examine in detail the mechanisms of oxygen transfer by comparing the enzymic system to the model studies. At this time, applications of these model systems in synthetic chemistry have not been extensively pursued. 

In this review we have discussed the mechanistic features of P-450 reactions by comparing enzymic reactions with those of model porphyrin systems. Model porphyrin systems have allowed the complete dissection of the proposed mechanism of oxygen activation and transfer by P-450. Putative intermediates have been characterized, and each primitive transformation has now been observed. The similarities and differences in the oxygen activation between HRP and... 

Figure 30 (a) Reaction catalyzed by Sporobolomyces sa/mon/co/or carbonyl reductase (SSCR). (b) Three-dimensional view of NADPH binding in SSCR (PDR ID 1Y1P) with the formation of hydrophobic mobile loop (colored in violet), (c) Proposed mechanism of oxygen activation and hydride transfer in SSCR with CODE (119). 

Feigelson, P., Brady, F.O., in Molecular Mechanisms of Oxygen Activation, Ed. by Hayaishi, 0., Academic Press, New York and London, 1974, Chapter 3. 

Fig. 10. The possible mechanism of oxygen activation during oxygenation of catechol. E symbolizes the enzyme protein. The sequence of chemical transformation within the complex is caused by rearrangement of electron orbitals. The dotted line between -O and -H are hydrogen bonds.

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