Peroxyl bimolecular decay

Scheme 10.—Base-induced Elimination of HOt, and Bimolecular Decay of the Peroxyl Radical at C-5 of D-Clucose. ...

Mieden OJ, Schuchmann MN, von Sonntag C (1993) Peptide peroxyl radicals base-induced O2 elimination versus bimolecular decay. A pulse radiolysis and product study. J Phys Chem 97 3783-3790... 

In peroxyl-free-radical chemistry, H02702 " elimination reactions play a major role (Chap. 8.4). In polymer free-radical chemistry, these reactions are of special interest, because they lead to a conversion of slowly diffusing polymer-derived radicals into the readily diffusing HCV/CV radicals. The H02 /02 "-elimina-tion typically proceeds from an a-hydroxyalkylperoxyl radical [reaction (22)]. In poly(vinyl alcohol), for example, such an structural element is formed by H-abstraction and subsequent 02 addition [reactions (18) and (19)]. The same structural element may also be formed during the bimolecular decay of peroxyl radicals which carry an H-atom in [3-position [reactions (20) and (21)]. 

The H02- elimination at reaction (22) is often slow, and at a high concentration of peroxyl radicals this reaction may compete with the bimolecular decay of the peroxyl radicals (leading to chain scission Ulanski et al. 1994). However in the presence of base, deprotonation speeds up the 02 elimination [reactions (23)... 

For DNA in cells and in the absence of O2, one has to take into account that the lifetime of the DNA radicals is not determined by their bimolecular decay but rather by their reaction with the cellular thiols, mainly GSH (Chap. 12.11). In the presence of 02, the situation becomes more complex, and the lifetime of the DNA peroxyl radicals is as yet not ascertained. It is expected to be consider-... 

In acid solutions, but also in neutral solutions at a high steady-state radical concentration, CV "-elimination becomes too slow to be of importance as compared to the bimolecular decay of the peroxyl radicals. This leads to a very different product distribution (Table 10.15). 

In the presence of O2, it is converted into the corresponding peroxyl radical [reaction (266)]. The bimolecular decay of this peroxyl radical with the other peroxyl radicals present is this system leads to erythrose [reaction (267)] in 15% yield, i.e., the p-fragmentation reaction of a short-lived oxyl radical intermediate is of minor importance. 

In the presence of GSH, 5 -d(T4AT7) and 5 -d(T4A) are formed [reactions (20) and (21)]. In the presence of 02, the primary radical is trapped by 02. In a subsequent step, the C(4 ) peroxyl radical is reduced to the corresponding hydroperoxide (the source of the reduction equivalent is as yet unknown potentially 02 generated in side reactions), and treatment with NH3 increases the yield of the glycolate which is also formed upon the bimolecular decay of the peroxyl radial [reactions (23)-(25)]. 

Yet, the corresponding geminal chlorohydrines have a lifetime of less than a few ps (Mertens et al. 1994). The related (i-Pr0)2P(0)0C(CH3)20H decays with k > 3 x 104 s"1, and the data on the peroxyl radical chemistry of trimethylphosphate (Schuchmann, von Sonntag 1984) will have to be reinterpreted in so far as the decay of (CH30)2P(0)CH20H into dimethylphosphate and formaldehyde must have occurred during the bimolecular decay of the peroxyl radicals, i.e. on the submillisecond time scale. 

The rate constant for the bimolecular decay of the a-tocopheroxyl radical is only 3.5x 102M-1 s . Therefore, its half-life is several hours in chloroform at ambient temperature. This implies that vitamin E free radical can react with a second peroxyl radical. In biological membranes, a-tocopherylquinone is generally believed to be the major end-product, but the mechanism of its production remains controversial. It may arise either from the decomposition of a-tocopherone, or from dismutation of a-tocopheroxyl radicals. However, the steady-state concentration of a-tocopherylquinone is usually too low to be measurable ex vivo when tissue homogenization and extraction are performed in the presence of pyrogallol and butylated hydroxytoluene, respectively,... 

In the bimolecular decay of peroxyl radicals, a short-lived tetroxide is an intermediate. When a hydrogen is present in /3-position to the peroxyl function, a carbonyl compound plus an alcohol and O2 [Russell mechanism, e.g. reaction (42)] or two carbonyl compound plus H2O2 (Bennett mechanism, not shown) may be formed in competition to a decay into two oxyl radicals plus O2 [e.g. reaction (43) for details of peroxyl radical chemistry in aqueous solution, see Refs. 2 and 39]. 

In acid solutions, but also in neutral solutions at high steady-state radical concentrations, the superoxide elimination becomes too slow compared with the bimolecular decay of these peroxyl radicals [reactions (28)-(31)]. This leads to a very different product distribution, as seen in Table 5. There is evidence that in their bimolecular decay peroxyl radicals can give rise to the formation of oxyl radicals which may undergo fragmentation (see, e.g., [37, 38]) [e.g., reaction (30)], leading to products with the pyrimidine cycle destroyed (e.g., l-N-formyl-5-hydroxyhydantoin. Other pyrimidine-ring cleavage reactions are conceivable but at present not supported by product data). 

He supposed that the decay proceeds as the decomposition of alkoxyl radicals formed in the bimolecular reaction of two tertiary peroxyl radicals (see Chapter 2). 

Peroxyl radicals which do not decay by one of the unimolecular processes discussed above must disappear bimolecularly. In contrast to many other radicals, they cannot undergo disproportionation. Hence they are left to decay via the recombination process, the results of which is a tetroxide intermediate [reaction (46) an exception may be their reaction with 02- cf. reaction (7)].,... 

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