Hydrogen-atom abstraction bound

Nature has designed the active sites in the enzymes GO and glyoxal oxidase in order to perform a hydrogen-atom abstraction reaction from the a-carbon atom of an O-bound alcoholate (or aldehyde) in the rds. The essence of this chemistry is depicted in Fig. 8. 

The major intermolecular reaction of triplet aryl nitrenes in solution is hydrogen atom abstaction to form primary amines. For a photoaffinity reagent bound to a receptor, this would result in a failure to couple. However, it is possible that the intramolecular photochemistry of aryl azides is more relevant, and here numerous examples of insertion by triplets have been noted. Presumably, these are two step processes hydrogen atom abstraction, followed by radical coupling (cf. Figs. 2.1 and 2.3). 

Radical cations are probably involved in the photooxidation of olefins [1-7, 24, 50-54, 56, 61, 66] (Eqs. 2 and 5). Many oxidizable arenes and dienes are, similarly, oxidatively cleaved or rearranged by routes consistent with initial formation of a radical cation. As we saw above, the observed oxidative cleavage product probably derives from the combination of a surface-bound radical cation with superoxide or adsorbed oxygen. With alkanes or simply substituted alkenes, however, the capture of a photogenerated hole is often thermodynamically forbidden. Thus, instead of radical cations, radicals formed by hydrogen atom abstraction by an activated oxygen species dominate the observed chemistry. With alkanes, oxygenation at sites... 

Electron- and Energy-Transfer Reactions Involving Bound Complexes DNA Cleavage by Hydrogen-Atom Abstraction DNA Cleavage by Hydride and Oxo Transfer... 

First, the flexibility of the iron-sulfur center gives it the potential to exist under different [2Fe-2S], [3Fe-4S], [4Fe-4S] forms. Whether this has functional importance remains to be determined. Very recently the Fnr protein, the pyruvate formate lyase and the biotin synthase were shown to share this property [48-50]. Second, its reducing power allows it to inject electrons into protein-bound AdoMet, as a prerequisite for the formation of an essential glycyl radical. Third, this radical is used for ribose activation, by hydrogen atom abstraction. It is likely that this reaction is mediated by a cysteinyl radical [53]. Fourth, formate is used as the hydrogen donor for ribose reduction [38]. 

We will see that similar conclusions with respect to the mechanism (possibility of insertion as well as hydrogen atom abstraction) may be made for reactions of oxygen atoms bound to metal ( oxenes ) with alkanes in enzymatic and model chemical reactions. The excited nitrogen atom, N( D), reacts analogously [8f,g]. 

A putative Cu -oxo species, formed from an acido-basic catalysis or from the heterolysis of the 0-0 bound of a Cu -hydroperoxo intermediate as in Eq. (23) of Fig. 15, was also proposed although its orbital populating seems unfavorable 1ST). Eqs. (24), (25) summarize the pathway that could be involved with this species during DNA oxidation events. A hydrogen atom abstraction on the DNA by the Cu -oxo species would produce a radical on DNA and a Cu -hydroxo species [Eq. (24)]. Then an eventual electron transfer between them may allow the oxidation of the radical to a cation and the regeneration of the initial Cu complex. 

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