Oxidation vs. fragmentation in radiosensitization. Reactions of α-alkoxyalkyl radicals with 4-nitrobenzonitrile and oxygen. A pulse radiolysis and product analysis study

Abstract

x-Monoalkoxyalkyl radicals produced from 1,4-dioxane ( 1 OO%), 1,3-dioxane (56%), tetrahydrofuran (92%) and dimethyl ether (100%) by H-abstraction by hydroxyl radicals generated in the radiolysis of water were found to react with 4-nitrobenzonitrile (NBN) by addition to give N-alkoxyaminoxyl-type radicals, which have absorption maxima at about 310 nm and decay very slowly (k = 0.4 -1.0 s-l). On the other hand, the reaction of the a-dialkoxyalkyl radical, 1,3-dioxan-2-yl 3 [from the reaction of hydroxyl radicals with 1,3dioxane (32%)] with NBN leads to the rapid formation of the radical anion NBN'-.The N-alkoxyaminoxyl-type radicals (A in the case of 1,4-dioxane and D in the case of dimethyl ether) react with ascorbate (k z 2 x lo4 dm3 mol-' s-'). They have a very low reactivity with oxygen (k < lo3 dm3 mol-' s-' in the case of tetrahydrofuran). On the other hand, they are rapidly reduced by NBN'-(k z lo9 dm3 mol-' s-' as observed with A and with B derived from 1,3-dioxane).The products [G values (in parenthesis) in units of lop7 mol J-'1 in the y-radiolysis of N,O-saturated solution of 1,4-dioxane in the presence of NBN are 1,4-dioxan-2-0ne (0.3), 2-hydroxy-l,4-dioxane (2.5), ethane-1,2-diol monoformate (2. l), ethane-1,2-diol diformate (0.7), formaldehyde (2. I), 4-nitrosobenzonitrile and other reduction products of 4-nitrobenzonitrile. These products are accounted for as resulting from the fragmentation of the aminoxyl radical A by (a) heterolysis of the C-0 bond (45%, leading to the one-electron oxidation of the 1,4-dioxan-2-y1 radical) and (b) homolysis of the N-0 bond (55%, leading to the formation of the 1,4-dioxan-2-0xyl radical which undergoes further fragmentation).The products from the reaction of methoxymethyl radicals with NBN under y-radiolysis conditions are formaldehyde (5.71, methanol (2.5) and methyl formate (1.3). It is concluded that also in this case the decay of the aminoxyl radical D occurs by two pathways: the heterolysis route (46%) and the homolysis route (54%).In the presence of oxygen the 1,4-dioxan-2-y1 radicals are converted into the corresponding peroxyl radicals. Their bimolecular decay (2k = 2.0 x lo8 dm3 mol-' s-l) yields the same products as in the case of NBN (albeit with a different product distribution and the formation of some peroxides): 1,4-dioxan-2-0ne (0.4), 2-hydroxy-l,4-dioxane (0.4), ethane-l,2-diol monoformate (0.6), ethane-l,2-diol diformate (2.8) and formaldehyde (0.6).These results indicate that fragmentation reactions involving the carbon-skeleton of organic radicals are important not only in the case of peroxyl radicals but they can also be induced by nitroaromatic sensitizers. In cells, reduction of the long-lived sensitizer adduct radicals by reducing agents such as ascorbate to give (toxic) hydroxylamine type products may compete with the homolytic or heterolytic fragmentation of the N-alkoxyaminoxyl radicals.

Yields of hydrogen peroxide from the reaction of hydroxyl radical with organic compounds in solution and ice

“…Unimolecular elimination of HO q 2 radicals from α-hydroxyperoxyl radicals (i.e., Reaction R6) represents a significant pathway for HOOH production in many of these past experiments. However, the results also suggest that bimolecular reactions (e.g., Reaction R11 followed by Reactions R12-R16) can compete with unimolecular elimination (Schuchmann and von Sonntag, 1984) and that superoxide can cross-terminate with other peroxyl radicals (Pan et al, 1993b). Work by Stemmler and von Gunten (2000) (Stemmler and von Gunten, 2000).…”

“…The kinetics and product yields from the aqueous reactions of q OH with a range of organic compounds have been studied using gamma (γ ) radiation and pulse radiolysis (Christensen and Gustafsson, 1972;Nese et al, 1995;Pan et al, 1993a, b;Piesiak et al, 1984;Schuchmann and von Sonntag, 1984;Schuchmann et al, 1990Schuchmann et al, , 1995Schuchmann and von Sonntag, 1977, 1979Schuchmann et al, 1985;Stemmler and von Gunten, 2000;Ulanski et al, 1996;. Overall HOOH yields in these experiments (defined as the rate of HOOH production divided by the rate of q OH production) ranged from 0.02 to 0.57, with an average yield of 0.38.…”

Hydroxyl-radical-induced reactions of poly(vinyl methyl ether): a pulse radiolysis, EPR and product study in deoxygenated and oxygenated aqueous solutions

“…diisopropyl ether 30 and 1,3-dioxane. 31 It has been reported that the absorption spectrum of the PVME-derived radicals shows a maximum at 310 nm. 20 We reproduced this result with a non-purified commercial sample, and conclude that low-molecular-weight impurities contained in the commercial material must give rise to the absorption maximum at 310 nm.…”

Yields of hydrogen peroxide from the reaction of hydroxyl radical with organic compounds in solution and ice

“…The kinetics and product yields from the aqueous reactions of q OH with a range of organic compounds have been studied using gamma (γ ) radiation and pulse radiolysis (Christensen and Gustafsson, 1972;Nese et al, 1995;Pan et al, 1993a, b;Piesiak et al, 1984;Schuchmann and von Sonntag, 1984;Schuchmann et al, 1990Schuchmann et al, , 1995Schuchmann and von Sonntag, 1977, 1979Schuchmann et al, 1985;Stemmler and von Gunten, 2000;Ulanski et al, 1996;. Overall HOOH yields in these experiments (defined as the rate of HOOH production divided by the rate of q OH production) ranged from 0.02 to 0.57, with an average yield of 0.38.…”