Products of the reactions of the dry and aqueous electron with hydrated DNA: hydrogen and 5,6-dihydropyrimidines

Molecular products resulting from the reaction of radiation-produced dry and aqueous electrons with DNA hydrated between 2.5 and 367 waters per nucleotide (Γ) are quantified in this work. Specifically, the yields of the 5,6-dihydropyrimidines (formed from the reaction of electrons with thymine and cytosine) and molecular hydrogen (H2) (formed in the reaction of intra-spur electrons) have been studied. We find the yields for the 5,6-dihydropyrimidines are highest (41-108 nmol/J) and the yields of hydrogen lowest (14-18 nmol/J) at low hydrations (Γ<22) where the scavenging of dry electrons in intra-spur reactions in the hydration water is greatest. At higher hydrations (Γ⩾22) an almost five-fold decrease in the yields of the 5,6-dihydropyrimidines and a corresponding increase in hydrogen yields are found. The average G-values for H2, when calculated based on the total mass of the hydrated DNA (DNA+added water), quickly approach the G-value for bulk water of 44-45 nmol/J with increasing hydration, plateauing at DNA hydrations above Γ=32. This result would suggest that the H2 produced in the irradiated hydrated DNA is derived mainly from intra-spur reactions in bulk water. However, an analysis of the H2G-values calculated based on the dry mass of DNA (i.e., an analysis of the contribution of the water alone to the H2 yields) suggests that H2 produced in irradiated hydrated DNA is derived not only from intra-spur reactions in bulk water (44-45 nmol/J) but also from DNA (61 nmol/J). This is an interesting finding that suggests fully hydrated DNA (Γ>33) forms H2 in competition with dihydropyrimidines.