Binding of Radiation-induced Phenylalanine Radicals to DNA: Influence on the Biological Activity of the DNA and on Its Sensitivity to the Induction of Breaks by γ-rays

Summary

When an aqueous solution of double-stranded DNA of bacteriophage PM2 containing phenylalanine and saturated with N₂O is irradiated with γ-rays, radiation-induced phenylalanine radicals are bound covalently. Under the conditions used, about 25 phenylalanine molecules may be bound per lethal hit. For single-stranded PM2 DNA, most of the phenylalanine radicals bound are non-lethal. Evidence is presented that, in double-stranded DNA, an appreciable fraction of the single-strand breaks is induced by phenylalanine radicals. Radiation products of phenylalanine and the phenylalanine bound to the DNA decrease the sensitivity of the DNA to the induction of single-strand breaks. There are indications that the high efficiency of protection by radiation products of phenylalanine is due to their positive charge, which will result in a relatively high concentration of these compounds in the vicinity of the negatively-charged DNA molecules.