Photogeneration of superoxide and decarboxylated peptide radicals by carboquone, mitomycin C and streptonigrin. An electron spin resonance and spin trapping study

Visible light (405–615 nm) excitation of carboquone, mitomycin C, and streptonigrin dissolved in dimethylsulfoxide in the presence of oxygen generates superoxide anion radicals (O₂^?). The quantum yields for these reactions range from 4.2 times 10^?2 (carboquone, λ= 615 ± 10 nm) to 7.3 times 10^?6 (streptonigrin, λ=545 ± 10 nm). O₂^? radicals were spin trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance (ESR). The efficiency of DMPO to spin trap O₂^? in dimethylsulfoxide was determined and indicated that 91% of the O₂^? present in dimethylsulfoxide is trapped by DMPO. The oxidation of the photoexcited drug molecules occurs via a direct electron transfer to dissolved oxygen in solution. Ultraviolet irradiation (λ= 313 ± 10 nm) of the aminoquinone drug solutions (80% H₂O, 20% dimethylsulfoxide) in the presence of peptides results in the decarboxylation of the peptides. In this case the photoexcited drugs are reduced, abstracting an electron from the C-terminal carboxyl group of the peptides. The reaction is specific to the C-terminal amino acid of the peptide. The decarboxylated peptide radicals were spin trapped with 2-methyl-2-nitrosopropane (MNP) and identified by ESR.