Precedents for Metal-Activation of Dioxygen

There is no doubt that transition metal complexes are able to catalyze the oxygenation and oxidation of organic substrates which otherwise would only react slowly or not at all with molecular oxygen. In most of these cases, however, the reaction is inhibited by free radical scavengers and shows an initial induction period, which is undoubtedly due to an initiating step typified by the Haber-Weiss process (eq. 4) (9,15). 

Although there may be one or two exceptions, there is no evidence, as revealed by kinetics, effects of anti-oxidants, electron paramagnetic resonance spectral data, etc. for the belief that dioxygenases exploit this form of catalysis. Unfortunately, precedents for bona fide non-radical chain catalyses are hard to find. Nevertheless, several examples are known of oxidation of ligands and substrates by dioxygen co-ordinated to metal. 

How Would an Iron- or Copper-Protein Interact with Molecnlar 

Another type of mononuclear dioxygen complex, the 7t-peroxo triangular complex (7) is known to exist, but is confined to metals having more d-electrons than iron or copper. 

It is to be expected then that metallo-proteins will form dioxygen complexes of great diversity. For example, those containing the same metal ion such as the ferrous-containing dioxygenases will not automatically share the same mechanisms of catalysis. 

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