Kinetics of Radiation-induced Strand Break Formation in Single-stranded Pyrimidine Polynucleotides in the Presence and Absence of Oxygen; a Time-resolved Light-scattering Study

Summary

Time-resolved reductions in the light-scattering intensity (LSI) of aqueous oxic and anoxic solutions of poly-C and poly-U at pH 7·8, following pulse-irradiation, have been studied as indicies of strand break formation. With doses of 3–24 Gy per pulse, a number of kinetically distinct strand breakage components have been detected. A comparison of the LSI responses obtained from irradiations conducted under N₂O with those conducted under air or O₂ show no marked difference in the overall extent of LSI change. However, the immediate and fast (t_½ ≤ 50 μs) reduction in LSI, accounting for about 18–19% of the pyrimidine polynucleotide's total LSI response in oxic solution, is reduced in the absence of oxygen, to about 12% of the total LSI response found with poly-C and to about 9% for poly-U. For poly-C there is a five-fold enhancement in the rate of major strand breakage under anoxia [k₁(N₂O) = 7·9 s⁻¹] whereas for poly-U a more modest enhancement (about two-fold) is observed. These enhanced rates are mirrored by those for the losses of the principal optical anoxic absorptions (observed pulse radiolytically) that are assigned to the pyrimidine 6-yl base radicals. Such findings support a proposal that the rate-limiting step of major strand breakage for pyrimidine polynucleotides is a base radical mediated hydrogen atom abstraction raction (Lemaire et al. 1987, Hildenbrand and Schulte-Frohlinde 1989). Irradiation of poly-C and poly-U in N₂O/O₂ (4:1, v/v) saturated solutions yields LSI changes much larger than those noted under N₂O and air (or O₂), which are in turn approximately double the responses observed under N₂. This indicates that the major strand breaking species of water radiolysis is the OH-radical and that there is an oxygen enhancement of single strand breakage of about 1·9 for poly-C and 1·6 for poly-U.