Induction of Free Radicals in Dna by Proflavine and Visible Light: Influence of Oxygen and Ionic Strength

Abstract —The photosensitization of native DNA is observed as an induction of free radicals in the DNA moiety of proflavine‐DNA complexes. The intensity of the electron paramagnetic resonance spectra (at 77 K) is a measure of the number of free radicals present in frozen solutions of DNA‐proflavine complexes after irradiation with visible light (Λ > 320 nm). In the absence of O 2 , the photosensitization is significant but very low; it increases slightly with increasing NaCl ionic strength; it appears to be due to intercalated dye molecules and the qualitative analysis of the spectra obtained shows that mainly thymidine is involved. The reaction measured after saturation with O 2 is the same as the reaction in air but is quantitatively higher; the free radicals observed are peroxides. This induction of free radicals appears to be due to the intercalated dye molecules, each molecule acting independently. The important observation is a very sharp and large (around a hundred‐fold) increase in the photosensitizing efficiency of the bound dye molecules occurring in NaCl between μ, # 0–25 and μ= 0–5 and in MgCl 2 between μ# 0–01 and μ=0–1.