Electron Spin Resonance Studies of Free Radicals in Solution III: pH Dependence of Thiyl Free Radical of Cysteine

An electron spin resonance flow technique was utilized to follow the concentration of thiyl free radicals (RS·) generated upon the oxidation of cysteine with ceric ion over the 0.25–7.5pH range. The thiyl free radical concentration was seen to decrease with increasing pH in a manner dependent on the extent of ionization of cysteine. The formation of thiyl free radicals was favored by the proportion of thiol present in the positively charged form. The diminution of RS· spin concentration at physiological pH was correlated with the increasing prevalence of singly and doubly negative thiolate ions. At acidic pH’s below 1.5, thiyl free radical formation was depressed by solutions of high ionic strength. The utility of this technique in studying the potential role of thiyl free radicals in radioprotective processes is indicated.