PHOTOCLEAVAGE OF DNA IN THE PRESENCE OF SYNTHETIC WATER-SOLUBLE PORPHYRINS

In the presence of oxygen and visible light, various synthetic water-soluble porphyrins cleave pBR 322 plasmid supercoiled DNA (form I) producing relaxed (form II) and linear (form III) DNA corresponding to single-strand and double-strand breaks respectively. Large variations are observed in the efficiency of the porphyrins containing a diamagnetic metal or no metal at all. Singlet oxygen (¹O₂) seems to be involved in the mechanism of cleavage consistent with the inhibitory effect of the azide anion, N^–₃. The higher efficiency of cationic porphyrins (as compared to anionic ones) is due to their greater affinity for DNA as shown by experiments carried out at either high ionic strength or in the presence of the surfactant, sodium dodecyl sulfate.