The sulfite radical anion, SO3•‾, was easily generated by chemical (Ce(IV)-HSO3‾ in acidic medium or Ti(III) (EDTA)-H2O2-SO3 in alkaline medium) or biochemical (horseradish perexidase-H2O2) methods. The SO3 radical was identified by electron spin resonance (ESR) spectroscopy coupled with a rapid-mixing flow technique as a direct detection or a spin-trapping method as an indirect detection. The life-time of the SO3‾ radical is about 100 msec in aqueous solutions. The SO3 radical could not abstract a hydrogen atom from the saturated compounds, but easily add to the unsaturated compounds having C=C, C=S, C≡C and C=N bonds. In the case of addition to C=S bond, it is suggested that the SO3 radical is the active species for the chemical modification of biologically active nucleic acids by the bisulfite (HSO3‾). Further, the biological reactivities of the SO3 radical have been briefly summarized.
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