Radiation-induced Strand Breakage in Poly(Riboadenylic Acid). A Pulse Radiolysis Study on the Protective Action of Cysteamine Under Anoxic Conditions

Poly(riboadenylic acid) was irradiated in N2O-saturated dilute aqueous solution at pH 7.8 with single pulses (50-200 ns) of 16 MeV electrons. With the aid of Rayleigh light scattering measurements both the extent and rate of main-chain scission were measured in the absence and the presence of cysteamine (RSH). In both cases two modes of light scattering intensity (LSI) decrease were detected. From this finding it was concluded that two chemically different radical sites, giving rise to main-chain breakage, were produced by OH attack on the macromolecules. Cysteamine reduced the extent of LSI decrease and accelerated the rate of the slow mode of LSI decrease significantly. The fast mode of the LSI decrease is due to fragment diffusion and therefore the influence of cysteamine on the rate of decay of the short-lived radical could not be studied. The assessment of the relative importance of .OH scavenging (RSH + .OH----RS. + H2O) and of repair (PA. + RSH----PAH + RS.) with respect to protection yielded the following: in the case of the long-lived radical, repair contributes significantly at low RSH concentration (less than 10(-4) M); in the case of the short-lived radical, OH scavenging dominates and repair becomes noticeable only at [RSH] greater than 10(-4)M. The following data were evaluated in this work: 100 eV-yield of main-chain scission, G(sb) = 1.1; lifetimes of radicals: less than 50 microseconds (short-lived), 0.6 s (long-lived), kPA. + RSH = 3.4 X 10(6)M-1 S-1 (for long-lived radicals).