Radiolysis of nucleosides in aqueous solutions: base liberation by the base attack mechanism

On the radiolysis of uridine and some other nucleosides in aqueous solution, a pH-dependent liberation of uracil or the corresponding base was found. e-aq and HO-.2 gave no freed bases, although many oxidizing radicals, including OH, Cl-.2, Br-.2, (CNS)-.2 and SO-.4, did cause the release of unaltered bases, depending on the pH of the solutions. The base yields were generally high at pH greater than or equal to 11, with the exception of SO-.4, which gave a rather high yield of uracil (from uridine) even in the pH region of less than or equal to 11. The pattern of the base formation against pH varied with the reacting radical, but was the same for the different nucleosides. With regard to the OH radical system, bases are considered to be formed after H-abstraction of the sugar moiety at low pH. The other oxidizing radicals, however, may cause base release by attack at the base. It is possible that O-, present at high pH as the dissociated form of OH, may act partly as an oxidizing radical. A plausible mechanism of 31'-radical formation is discussed.