Reactivity of prehydrated electrons toward nucleobases and nucleotides in aqueous solution

DNA damage induced via dissociative attachment by low-energy electrons (0 to 20 eV) is well studied in both gas and condensed phases. However, the reactivity of ultrashort-lived prehydrated electrons (e(pre)(-)) with DNA components in a biologically relevant environment has not been fully explored to date. The electron transfer processes of e(pre)(-) to the DNA nucleobases G, A, C, and T and to nucleosides/nucleotides were investigated by using 7-ps electron pulse radiolysis coupled with pump-probe transient absorption spectroscopy in aqueous solutions. In contrast to previous results, obtained by using femtosecond laser pump-probe spectroscopy, we show that G and A cannot scavenge e(pre)(-) at concentrations of <= 50 mM. Observation of a substantial decrease of the initial yield of hydrated electrons (e(hyd)(-)) and formation of nucleobase/nucleotide anion radicals at increasing nucleobase/nucleotide concentrations present direct evidence for the earliest step in reductive DNA damage by ionizing radiation. Our results show that e(pre)(-) is more reactive with pyrimidine than purine nucleobases/nucleotides with a reactivity order of T > C > A > G. In addition, analyses of transient signals show that the signal due to formation of the resulting anion radical directly correlates with the loss of the initial e(hyd)(-) signal. Therefore, our results do not agree with the previously proposed dissociation of transient negative ions in nucleobase/nucleotide solutions within the timescale of these experiments. Moreover, in a molecularly crowded medium (for example, in the presence of 6 M phosphate), the scavenging efficiency of e(pre)(-) by G is significantly enhanced. This finding implies that reductive DNA damage by ionizing radiation depends on the microenvironment around e(pre)(-).