Reactivity of perhydroxyl (HOO.) with 1,4-cyclohexadiene (model for allylic groups in biomembranes)

The electron-transfer reduction of molecular oxygen yields superoxide ion (O2.-), which reacts with proton sources to form HO2.. In water the latter species disproportionates via reaction with O2.- (kbi, 10(8) M-1 s-1) and itself (kd, 10(6) M-1 s-1). The rate constants (kd) for the homolytic disproportionation process (HO2. + HO2.----H2O2 + O2), which have been determined from stopped-flow spectrophotometric decay data for HO2. at 25 degrees C, are (1.7 +/- 0.5) x 10(4) M-1 s-1 in dimethyl sulfoxide (Me2SO), (5.3 +/- 0.5) x 10(4) M-1 s-1 in dimethylformamide (DMF), and approximately 10(7) M-1 s-1 in acetonitrile. With limiting fluxes of protons to control the rate of formation of HO2. from O2.-, the rate of decay of HO2. is enhanced by reaction with the allylic hydrogens of excess 1,4-cyclohexadiene (RH). On the basis of such data the apparent second-order process (HO2. + RH----R. + H2O2) has a rate constant (kox) of (1.6 +/- 0.6) x 10(2) M-1 s-1. The reactivity of HO2. decreases as its solvation energy increases.