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 (F) 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 (H) (formed in the reaction of intra-spur electrons) have been studied. We find the Yields for (he 5.6-dihydropyrimidines are highest (41-108 nmol/J) and the yields of hydrogen lowest (14-18 nmol/J) at low hydrations (Gamma<22) where the scavenging of dry electrons in intra-spur reactions in the hydration water is greatest. At higher hydrations (Gammagreater than or equal to22) 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 H,. 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 Gamma = 32. This result would suggest that the H-2 produced in the irradiated hydrated DNA is derived mainly from intra-spur reactions in bulk water. However, an analysis of the H G-values calculated based on the dry mass of DNA (i.e., an analysis of the contribution of the water alone to the H-2 Yields) suggests that H-2 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 (Gamma>33) forms H-2 in competition with dihydropyrimidines. (C) 2004 Elsevier Ltd. All rights reserved.