Dioxygen, activation triplet

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

The reaction sequence at the heme active site starts with the binding of unactivated triplet dioxygen forming the so-called oxy-heme complexes. The iron center in 02-activating heme enz5maes is then thought to be converted into a peroxo anion species. It can be protonated to form a ferric hydroperoxo intermediate usually termed compormd 0 (183), which is a crucial reactive species in catalase and peroxidase enz5nne catalysis (Fig. 21). These hydroperoxo intermediates of hemoproteins are important... 

The arrows represent electron spins represents a singlet molecule with all electron spins paired t f represents a triplet molecule with two unpaired electrons and I (which we will see in Reaction 5.13) represents a doublet molecule, also referred to as a free radical, with one unpaired electron. The pathways that do not violate the spin restriction are all costly in energy, resulting in high activation barriers. For example, the reaction of ground-state triplet dioxygen,... 

The highest rate constant observed for intramolecular ET in AO is 1100 s , which still is considerably smaller than the turnover number of about 14,000 s . Thus, interaction between dioxygen and the trinuclear site is not sufficient to ensure maximal enzymatic activity. Under optimal conditions, the concentration of reducing substrate (e.g., ascorbate) is sufficiently high to maintain a steady state of fully reduced copper sites. Thus, an antithetical approach was very recently taken by Tollin and co-workers studying the reoxidation of fully reduced AO by a laser-generated triplet state of 5-deazariboflavin 41). Subsequent to the assumed one-electron oxidation of the reduced trinuclear cluster, a rapid, biphasic intramolecular ET occurs from Tl[Cu(I)] (and presumably) to the oxidized trinuclear center. The faster of the two observed rate constants (9500 and 1400 s, respectively) is comparable to the turnover number determined for AO under steady-state conditions and renders it likely that this is the rate-limiting step in catalysis. 

Beyond the relevance in this particular study, the peroxy-like defects can be reaction intermediates in other zeolites-based oxidations in industrial applications. On the basis of present results, it can be argued that more effective oxidizing media can be obtained by modifying zeolites and mesoporous aluminosilicates in order to allow an easier formation of peroxy-like structures. In this respect, the presented data may suggest a possible activation mechanism of the inert triplet state of dioxygen in the cavities of nitrite sodalite to the more reactive singlet O2 [26]. 

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