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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 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 H2 G-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.

ESR spectroscopy at low temperatures is employed to investigate electron transfer within DNA doped with randomly spaced electron traps. The traps were introduced by careful bromination of DNA in ice-cooled aqueous solution. The procedure is shown by NMR and GC/MS techniques to modify thymine, cytosine, and guanine 2‘-deoxyribosides, transforming them into 5-bromo-6-hydroxy-5,6-dihydrothymine, T(OH)Br, 5-bromocytosine, CBr, and 8-bromoguanine, GBr, derivatives. The bromination products formed in molar ratio close to T(OH)Br/CBr/GBr = 0.2:1:0.23 and serve as internal electron scavengers on γ-irradiation. Paramagnetic products that result from electron scavenging in DNA by T(OH)Br and CBr units at 77 K have been identified by ESR as the 6-hydroxy-5,6-dihydrothymin-5-yl (TOH•) radical and the 5-bromocytosine σ* radical anion, CBr•-. Our quantitative estimates show that electron scavenging by T(OH)Br in bromine-doped DNA is over an order of magnitude more efficient than the more abundant CBr traps. This indica...

The induction of base damage products in γ-irradiated DNA, hydrated between 2.5 and 32.8 moles of water per mole of nucleotide (Γ), was investigated using the gas chromatography/mass spectrometry-selected ion monitoring technique. In general, the yields of the measured base damage products were found to be dependent on the extent of the hydration when the DNA was irradiated under nitrogen. At low hydrations (Γ ≤ 13), the highest yields of the measured products were found for 7,8-dihydro-8-oxo-guanine, 5,6-dihydrothymine and, to a lesser extent, 2,6-diamino-4-oxo-5-formamidopyrimidine, products which are consistent with the base radicals found in low-temperature ESR studies. At higher hydrations (Γ ≤ 13), changes in DNA conformation and an increase in the attack of bulk water radicals on DNA play a significant role in the formation of radiation-induced DNA base damage products. Additional findings in our study include: (1) the sum of the yields of the products formed from electron-loss centers is greater t...