Radiation chemistry of carbohydrates. Part 16. Kinetics of HO2˙ elimination from peroxyl radicals derived from glucose and polyhydric alcohols

Abstract

Oxygenated aqueous solutions of H₂O₂ at pH 5.5 containing ethylene glycol, glycerol, meso-erythritol, D-xylitol, D-glucitol,myo-inositol, D-glucose, methyl α-D-glucoside, or bis-1-methylethyl ether as substrates were subjected to flash photolysis and the rise of conductivity measured as a function of time. Except for bis-1-methylethyl ether all substrates showed transient conductivity which decayed within several hundred ms. This transient conductivity is attributed to the formation of HO₂˙ which dissociates into H⁺ and O₂˙^–. The precursors of HO₂˙ are α-hydroxyperoxyl radicals derived from the various substrates. From the kinetic analysis it follows that the peroxyl radical of ethylene glycol shows one first-order elimination process (k 190 s^–1). Two first-order processes are observed with the mixture of the peroxyl radicals of glycerol, D-xylitol, meso-erythritol, D-glucitol (k ca. 200 ca. 3 000 s^–1), myo-inositol (k 470 and k 2 700 s^–1), and methyl α-D-glucoside (k 480 and 2 000 s^–1). Three parallel first-order processes are shown by the peroxyl radicals of D-glucose (k 400, 2 700, and >70 000 s^–1). The simple assumption that the slow and fast elimination in the polyhydric alcohols is due to eliminations from the α, ω, and other positions is not sufficient to explain the results quantitatively. Neighbouring hydroxy-groups might influence the rate of elimination. The very fast component in D-glucose is ascribed to the peroxyl radical at C-1 of D-glucose. Peroxyl radicals at C-2, C-3, and C-4 are thought to eliminate at both a slow and a fast rate as do those of myo-inositol, that at C-5 not on the time scale of this experiment, whereas that at C-6 is expected to contribute to the slow elimination rate.