Bimolecular decay

Rousseau-Richard, C. Richard, C. Martin, R. Kinetics of bimolecular decay of a-ocopheroxyl free radicals studied by ESR. FEBS Lett. 1988, 233, 307-310. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

A careful study of the bimolecular decay of HO O in the absence of metal ions has shown [84] that the reactions involved are reactions (59) and (60) ... 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

Many reactive intermediates can decay via self-reactions, giving dimers or disproportionation products, as is the case of free radicals and carbenes. When these self-reactions are not the ones under study, it is desirable to keep the transient concentration low enough to minimize this type of interference. For example, for a radical that dimerizes with fet = 3 x 10 M s and a concentration c of lO M, its first half-life (ti/2 = 1/kc) would be 33 ps. Note that excited triplet states also undergo bimolecular decay by triplet-triplet... 

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

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

The bimolecular decay of the ascorbate radical is much more complex than shown in the overall reaction (86). In fact, it is in equilibrium with a dimer [equilibrium (88), K 103 dm3 mol-1, kieverse 105 s"1] which either may react with a proton [reaction (89), k = 1010 dm3 mol-1 s"1] or with water [reaction (90), k 40 s"1] (Bielski et al. 1981). 

Anderson RF (1983a) Energetics of the one-electron reduction steps of riboflavin, FMN and FAD to their reduced forms. Biochim Biophys Acta 722 158-162 Anderson RF (1983b) The bimolecular decay rates of the flavosemiquinones of riboflavin, FMN and FAD. Biochim Biophys Acta 723 78-82... 

IrCl62 for complete (i.e., sufficiently rapid with respect to the bimolecular decay of the radicals) oxidation (Fang et al. 1996). 

As mentioned above, these reactions are potentially of some heuristic importance also for DNA in so far as little is known about the fate of G (notably its bimolecular decay Chap 10.2). Both G- and phenoxyl radicals share quite some properties they are aromatic with high spin density at oxygen, reasonably strong oxidants, do not react with 02 at an appreciable rate, but quite readily with 02-. 

Bothe E, Schulte-Frohlinde D (1978) The bimolecular decay of thea-hydroxymethylperoxyl radicals in aqueous solution. Z Naturforsch 33b 786-788... 

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... 

The kinetics of the bimolecular decay of poly(vinyl alcohol) (Ulanski et al. 1994) and poly(vinyl methyl ether) radicals (Janik et al. 2000b) have been studied in some detail (cf. Fig. 9.2). The OH radicals formed during the pulse generate on the (coil-shaped) polymer a non-random distribution of radicals. First, the radicals which are very close to one another recombine. The intrinsic bimolecular rate constant for such a process can be much faster than that of the decay of an equal concentration of randomly distributed low molecular weight radicals. As the number of close-by radicals decreases, the intrinsic rate constant drops, and the lifetime of the polymer radicals increases considerably. Now, the bimolecular decay of the polymer radicals becomes much slower than that of the corresponding low molecular weight radicals. While in the case of low molecular weight radicals the bimolecular rate constant is independent of the... 

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. 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

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