Release of K+ and H+ from Poly U in Aqueous Solution upon γ and Electron Irradiation.Rate of Strand Break Formation in Poly U

Conductivity changes were found which followed the reaction of radiolytically generated OH radicals with the potassium salt of polyuridylic acid (poly U) in aqueous solution. After ⁶⁰Co-γ-irradiation the observed increase of conductivity at pH = 6.8 was shown to consist of the liberation of K⁺ ions from the stock of K⁺ ions electrostatically bound to the polyanion. The initial G(K⁺) is 36 and hence 6 times higher than the G value of OH radicals in N₂O saturated solutions. At a poly U concentration of 60 mgl⁻¹ half of the ion release occurred at 12 J kg⁻¹ and nearly all ions are released at 40 J kg⁻¹. The liberation of K⁺ is explained to be a consequence of the formation of chain breaks leading to an increase of the degree of dissociation. The rate of the ion release was studied under pulse radiolysis conditions. Because of the high G-vlaue of counterion liberation and the use of conductivity as analytical quantity the method is very sensitive. With 6 mgl⁻¹ poly U the rate could be measured even at a dose per pulse of 0.25 J kg⁻¹. The kinetics of the ion release can be described in terms of two parallel first order reactions of comparable contribution with an average rate constant of 0.8 s⁻¹ at 20 °C, 60 mgl⁻¹ poly U and pH = 6.8 with a small contribution of slower components. In more acidic solutions, besides K⁺ ions H⁺ are also liberated since at low pH values bound K⁺ is replaced by H⁺. The rate of the ion release was found to increase with increasing replacement of K⁺ by H⁺ (kₒbₛ = 100 s⁻¹ at pH = 3.4, 60 mgl⁻¹ poly U and T = 18 °C). With potassium ion free polyuridylic acid the corresponding rate constant amounted to 220 s⁻¹, nearly independent of pH. From the temperature dependence activation parameters for the ion release were derived (Eₐ = 57 kJ mol⁻¹, A = 1.0 x 10¹⁰ s⁻¹ at pH = 6.8). Addition of p-benzoquinone at pH = 3.7 and dithiothreitol (DTT) at pH 6.8 were found to decrease the size of the conductivity changes and to increase the rate. The results show that p-benzoquinone and DTT react with poly U radicals and that these reactions prevent chain breaks and ion liberation. It is concluded that the rate determining step of the conductivity increase is the formation of strand breaks by a cleavage of poly U radicals and that this reaction is pH dependent. The pH dependence and the observed value for the activation energy was found to be in agreement with the behaviour of a model system for the earlier postulated C-4′ mechanism for strand break formation of polynucleotides and DNA.