A photochemical system for generating free radicals: superoxide, phenoxyl, ferryl and methyl.

The photosensitizer flavin mononucleotide (FMN), in conjunction with the reducing agents diethylenetriaminepentaacetic acid (DTPA), hydrazine and hydroxylamines derived from nitroxides, generates superoxide radicals in a strictly light-dependent reaction in aerobic solution. Addition of superoxide dismutase (SOD) converts this system to a hydrogen peroxide generator. In the presence of horseradish peroxidase the latter system becomes a phenoxyl radical generator with appropriate phenolic substrates. Under anaerobic conditions FMN, hydrogen peroxide and an iron chelate generate ferryl and when this system is combined with dimethylsulfoxide, methyl radicals are produced. All the radicals can be generated with little contamination from other radicals, in high yields and the reaction can be terminated immediately upon cessation of illumination. Useful applications of this photochemical system include ESR studies of transient free radical species.