Oxygen, activation mechanism

Studies on the oxygen activation mechanisms by new heme enzymes using hemoprotein mutants and synthetic heme models 96YGK1046. 

Dealing with type II processes, the question arises whether the substrate or the oxygen is activated when it enters the termination reaction which leads to the final produces) AOa. Therefore, we may distinguish an acceptor-activation mechanism from an oxygen-activation mechanism. Furthermore, in each of these mechanisms the activated species may be either the electronically excited A or 02, respectively, or the electronically excited complexes of the primarily excited species with A or 02, respectively (Chart I). 

The main types of substrates investigated so far are polycyclic aromatic compounds, aryl substituted carbo- and heterocyclic pentadienes, cyclic 1,3-dienes, furans, and olefins. It has turned out that type II photooxygenation of these compounds in solution occurs via the oxygen-activation mechanism. 

In the years to follow, Gaffron s acceptor-activation mechanism was omitted from discussionf since all results obtained by different investigators were only in agreement with an oxygen-activation mechanism (see refs. 26,36,47 see also refs. 2 and 3 for discussion). However, the nature of the activated oxygen species remained obscure. In order to explain the activation process and the activated oxygen species, four different mechanisms were discussed. 

Until recently, no direct proof for or against one of the two remaining mechanisms, the energy-transfer mechanism and the complex formation mechanism, was available (see Chart I, under II. Oxygen-Activation Mechanism). 

Figure 9 Common oxygen activation mechanism proposed for nonheme diiron enzymes.

Figure 20 Proposed roles of the iron center and the pterin cofactor in the oxygen activation mechanism for pterin-dependent hydroxylases.

Kitagawa, Teizo and Ogura, Takashi, Oxygen Activation Mechanism at the Binuclear Site of Heme-Copper Oxidase Superfamily as Revealed by... 

To check the oxygen activation mechanism the comlex (CuPcO)2 has recently been prepared and its properties as a catalyst compared with those of ordinary Cu-Phthalocyanine 8fi). It was found that (CuPcO)2 was superior for the oxidation of a-methylstyrene, which occurs by an addition mechanism, but for the ordinary abstraction mechanism in the oxidation of cumene the copper phthalocyanine was superior. 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

An oxygen activation mechanism has also been postulated187 for the autoxidation of cumene at 35°C in the presence of the Pd(0) complex, (Ph3P)4 Pd ... 

Kropf and co-workers200-207b carried out detailed investigations of the autoxidation of alkyl aromatic hydrocarbons, such as cumene, catalyzed by metal phthalocyanines. They concluded that in the initial stages of reaction, in which the concentration of alkyl hydroperoxides is quite low, initiation occurred by an oxygen activation mechanism, e.g.,... 

An oxygen activation mechanism is favored by Ochiai for the autoxidation of cyclohexene catalyzed by copper phthalocyanine.197a,b Kamiya also proposed that the CuPc—02 complex initiates the autoxidation of a-methylstyrene by an addition mechanism196 ... 

The elucidation of the oxygen-activation mechanism in biological systems is still a most important target, although enormous efforts have... 

By contrast, there is a wealth of mechanistic information available in biochemistry and homogenous catalysis concerning oxygen activation mechanisms on first-row transition metals. It is therefore appropriate to provide a brief overview of this field here. However, this is a very large and active research field hence, it is not possible to do full justice to this subject here. For more extensive overviews, see references [50-60] in Section 4.5.6. 

Kitagawa, T., and Ogura, T., 1997, Oxygen activation mechanism at the binuclear site of heme-copper oxidase superfamily as revealed by time-resolved resonance Raman spectroscopy, in Progress in Inorganic Chemistry, volume 45(K. D. Karlin ed.), John Wiley of Son, Inc, New Yorkpp. 4319479. 

Some typical monooxygenation reactions catalyzed by P-450 are listed in Table I (75). Actually, the enzyme produces a strong oxidant in these reactions, but the precise reactive species in these oxygenations is still obscure. In the following sections, we will describe, step by step, what is known of the oxygen activation mechanism of P-450. 

P-450 have also been discussed, especially homolysis and heterolysis of the acylperoxo-iron(lll) complex. In both cases, we consider the active species to be oxoferryl porphyrin cation radicals however, many different reactivities exist between peroxidases and P-450. Ortiz de Montellano et al. have proposed that the position of substrates in the active site might depend on the spatial characteristics of the individual enzymes and influence the detailed course of the reaction (139). These propositions should be carefully examined. Scheme XXV illustrates all of the intermediates that have been observed and/or proposed in the oxygen activation mechanism by P-450 and that have been prepared by the model systems. 

In heme enz)mies that are able to activate molecular dioxygen, the porphyrin and proximal ligand provide the heme iron with five coordination sites. Thus, the side-on coordination of peroxide, which is common for nonheme metal-loenzymes and oxygen activation mechanisms, is not usually observed in heme enzymes and is also rare in model metalloporphyrins -... 

Interest in the utilization of such complexes for the selective oxygenation of hydrocarbons was aroused by two publications. Collman and coworkers [14] reported in 1967 that the oxidation of cyclohexene in the presence of low-valent complexes of Ir, Rh and Pd afforded a mixture of cyclohexen-2-one and cyclohexene oxide (reaction 9). It was proposed that the reaction involved an oxygen activation mechanism. 

Fe(N4PyXi OOH)f and 495 and 817 cm for Fe(N4PyXiJ -02)j". The former has activity for the hydroxylation of cyclohexane, but the latter looses it, supporting the hypothesis that protonation of the peroxo species activates it for participation in the oxygen activation mechanisms by iron enzymes. Similar results for identification of two peroxo complexesas shown in Fig. 16 were reported by Simaan et al.. ... 

While the active species responsible for the oxidations in the enzymatic system is still obscure. Scheme 1 summarizes a recent understanding of the oxygen activation mechanism by P-450 [1, 2]. 

Scheme 1 Proposed oxygen activation mechanism by cytochrome P-450. 223...

In addition, all conceivable intermediates proposed for the oxygen activation mechanism by P-450 were prepared by employing Mn porphyrin complexes [156]. Especially, X-ray structure of peroxo-Mn (TPP) served to understand a possible structure of the corresponding Fe(III) complexes [35]. 

---