Radiation-induced molecular size changes in DNA in the presence of p-nitroacetophenone. Pulse radiolysis in conjunction with light-scattering measurements.

Native calf thymus DNA, denatured DNA, and synthetic nucleic acids (poly A, poly C, poly U, and poly G) crosslink upon irradiation with 16 MeV electrons in O2-free dilute aqueous solutions containing 0.5mol/l t-butanol and p-nitroacetophenone (PNAP) or nitrobenzene at concentrations between 10(-4) to 10(-3) mol/l. The occurrence of crosslinking was inferred from the increase of the light-scattering intensity after the irradiation with a 100ns electron pulse. The mechanism of crosslinking is based on the indirect action of radiation, typical 100eV yields G(X) being between 10(-5) and 10(-4). The attack of unsaturations in pendant bases by anion radicals PNAP- or by dimer anion radicals (PNAP)2 radical leading to pendant macroradicals appears to be feasible. Combination of the latter causes crosslinkages. Rate constants and yields of crosslinking depend strongly on the ionic strength. Evidence for counteracting kinetic salt- and steric hindrance effects has been obtained.