Superoxide from elimination reactions

Reactions of potassium superoxide solubilized in apolar solvents with crown ethers (see Oxidation reactions, p. 356) are also frequently accompanied by elimination reactions. Thus, in DMSO solution, secondary alkyl bromides only yield olefins when treated with the K02 complex of dicyclohexyl-18-crown-6 (Johnson et al., 1978). Scully and Davis (1978) have studied the elimination of HC1 from N-chloramines with 18-crown-6-solubilized K02, KOH, and KOAc in ether solution (27). High yields of aldimines were obtained with K02,... 

The yield of superoxide radicals given in Table 1 amounts to some 60% of the hydroxyl-radical yield and most likely arises from the elimination of superoxide from peroxy radicals, the latter being formed in Reaction (15). From the careful design of these experiments, the yield of superoxide also represented the maximum yield of those hyaluronic-acid peroxy free radicals... 

Several interesting points arise from these data. The potential for the first one electron reduction of molecular oxygen is unfavourable and gives the unstable superoxide anion. This and the other possible intermediates (H2O2 and HO) are notably more reactive than free molecular oxygen and most biological systems appear to have taken steps to eliminate them. O2 is eliminated by superoxide dismutase" which catalyses the highly favourable reaction ... 

In basic solution, deprotonation at the hydroxyl group (typically) leads to a very rapid elimination of the superoxide radical anion [65, 69]. The deprotonation reaction was found to be the rate-limiting step in all of those cases where the kinetics were studied in sufficient detail. The peroxyl radical derived from a carbon-centered radical such as 53 apparently eliminates superoxide in a similar fashion [18] this probably happens following the pathway (39), (40). 

We recognized the need for methodology to measure SOD activity directly that would be more accessible to the bench-top scientist than is the method of pulse radiolysis, another direct measure. Consequently, we developed methodology to measure the catalytic dismutation of superoxide by stopped-flow kinetic analysis.By this technique, we directly monitor the decay of superoxide spectrophotometrically in the presence or absence of a putative SOD mimic at a given pH. Kinetic analysis of this decay can determine whether the complex is a SOD mimic (decay of superoxide becomes first-order in superoxide and first-order in complex see equations 1 and 2), or is inactive (decay of superoxide remains second-order for its self-dismutation see equation 3). At least a tenfold excess of superoxide over the putative SOD mimic is used in the stopped-flow assay, to eliminate contributions due to a stoichiometric reaction of the complex with superoxide. A catalytic rate constant for the dismutation of superoxide by the complex can be determined from the observed rate constants of superoxide decay as a function of catalyst concentration. ... 

Elimination of HOO- from peroxy radicals having a-hydrogens is another common pathway for the formation of superoxide. For example, ethanol reacts with OH by hydrogen atom transfer to give a mixture of several radicals dominated by 5, which rapidly reacts with oxygen to form a peroxy radical that eliminates OOH (with the concomitant production of acetaldehyde). This reaction is not as efficient with the peroxy radical of t-butanol, which has no a-hydrogens (von Sonntag, 1987). 

Because of this reaction, potassium superoxide is used as an oxygen source in masks worn by rescue workers ( FIGURE 22.15). For proper breathing in toxic environments, oxygen must be generated in the mask and exhaled carbon dioxide in the mask must be eliminated. Moisture in the breath causes the KO2 to decompose to O2 and KOH, and the KOH removes CO2 from the exhaled breath ... 

These homodimeric enzymes, that are present in both prokaryotic and eukaryotic organisms, contain one Cu ion and one redox-active cofactor topaquinone (TPQ) per monomer [5, 6]. They catalyze the oxidative deamination of primary amines [7-9]. The Cu(ll) ion is coordinated by three histidine residues and three water molecules (Fig. 11.1). The TPQ cofactor is not far from the Cu ion. The process can be divided into an initial reductive reaction followed by an oxidative step, based on the redox state of TPQ the Cu ion is thought to be involved in the formation of the TPQ semiquinone through reduction of Cu(II) to Cu(I). An alternative hypothesis has been recently proposed where the copper ion stays as Cu(II) and the one-electron reduction of O2 is carried out by a modified amino-resorcinol TPQ cofactor. The Cu(II) would provide electrostatic stabilization to the superoxide anion intermediate [10-12]. The reduction of molecular oxygen would result in weakly Cu-bormd hydroperoxide which is subsequently displaced by a water molecule, gets protonated and it is eliminated as hydrogen peroxide. 

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