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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 radiosensitivities and the kinetics for removal of radiation-induced DNA damage were compared for proliferative (P) and quiescent (Q) cells of the lines 66 and 67 derived from a mouse mammary adenocarcinoma. As determined from cell survival assays, the 66 and 67 Q cells were more radiosensitive than their 66 and 67 P counterparts. The rank order of their radiosensitivity was: 67 Q greater than 66 Q greater than or equal to 67 P greater than 66 P. Induction of radiation damage in the DNA of these cells, as measured by the alkaline elution technique, was identical for 66 and 67 P and Q cells. The repair of this DNA damage was biphasic for 66 and 67 P and Q cells. The half-times for the fast and slow repair phases in 66 Q cells were identical to those previously measured in 67 Q cells. The half-times of the fast and slow repair phases in 66 P cells were also identical to those previously measured in 67 P cells. However, the half-times for the fast and slow repair phases in 66 and 67 Q cells were longer than those measured in their 66 and 67 P counterparts. The 66 cell data are consistent with our previously published hypothesis that Q cells are more radiosensitive than their corresponding P cells because they repair their radiation-induced DNA damage slower. However, our results are not consistent with hypotheses that attempt to explain the radiosensitivity differences between lines 66 and 67 solely on the basis of measurable induction and repair of DNA damage.

Little computational or experimental information is available on site-specific hydroxyl attack probabilities to DNA. In this study, an atomistic stochastic model of OH radical reactions with DNA was developed to compute relative OH attack probabilities at individual deoxyribose hydrogen atoms. A model of the self-complementary decamer duplex d(CCAACGTTGG) was created including Na(+) counter ions and the water molecules of the first hydration layer. Additionally, a method for accounting for steric hindrance from nonreacting atoms was implemented. The model was then used to calculate OH attack probabilities at the various C-H sites of the sugar moiety. Results from this computational model show that OH radicals exhibit preferential attack at different deoxyribose hydrogens, as suggested by their corresponding percentage solvent-accessible surface areas. The percentage OH attack probabilities for the deoxyribose hydrogens [1H(5')+2H(5'), H(4'), H(3'), 1H(2')+2H(2'), H(1')] were calculated as approximately 54.6%, 20.6%, 15.0%, 8.5% and 1.3%, respectively, averaged across the sequence. These results are in good agreement with the latest experimental site-specific DNA strand break data of Balasubramanian et al. [Proc. Natl. Acad. Sci. USA 95, 9738-9742 (1998)]. The data from this stochastic model suggest that steric hindrance from nonreacting atoms significantly influences site-specific hydroxyl radical attack probabilities in DNA. A number of previous DNA damage models have been based on the assumption that C(4') is the preferred site, or perhaps the only site, for OH-mediated DNA damage. However, the results of the present study are in good agreement the experimental results of Balasubramanian et al. in which OH radicals exhibit preferential initial attack at sugar hydrogen atoms in the order 1H(5')+2H(5')H(4')H(3')1H(2')+2H(2')H(1').

Purpose: One of the most confounding issues in the treatment of cancer is that two patients with the same diagnosis respond differently to the same treatment. The ultimate goal of this research effort is to better understand why this occurs. As a first step, the study presented here is performed to develop a system of computer codes to model the complex reactions between ionizing radiation and DNA. Method and Materials: A mathematical atomistic model of a 167 base-pair B-DNA molecule was constructed using commercially available software packages. The probability of each possible fate of an •OH approaching to this 167 base-pair molecule was determined by implementing the “near-approach” DNA-radiation chemistry computational model described by Aydogan et al. (Radiat Res 157(1), 38–44, 2002) and compiled into an outcomes database. The secondary electron spectrum generated at depth in tissue by a Co-60 beam was modeled using the code MCNP 4.B. Two microdosimetry codes, OREC and RADLYS, originally developed at Oak Ridge National Laboratory, were adapted for this research. Attack sites were logged and single strand break (ssb) and double strand break (dsb) rates were calculated. Results: At a dose of 100 Gy, the concentration scaled dsb per molecule was only 0.4 which is in good agreement with the dsb data which is 0.43 dsb/ molecule from the computational effort described in this report. Conclusion: A Monte Carlo calculation model has been shown to simulate single and double strand breaks using a novel computational approach presented authors previously that reduces the computation time and eliminates unnecessary repetition of the computationally expensive near chemistry simulation in DNA damage. Experimental comparison of the single and double strand break yields calculated in this study have shown the accuracy of the DNA damage simulation model presented.

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...

The release of unaltered bases from irradiated DNA, hydrated between 2.5 and 32.7 mol of water per mole of nucleotide (Γ), was investigated using HPLC. The objective of this study was to elucidate the yield of the four DNA bases as a function of dose, extent of hydration, and the presence or absence of oxygen. The increase in the yield of radiation-induced free bases was linear with dose up to 90 kGy, except for the DNA with Γ = 2.5, for which the increase was linear only to 10 kGy. The yield of free bases as a function of Γ was not constant in either the absence or the presence of oxygen over the range of hydration examined. For DNA with Γ between 2.5 and 15, the yield of free bases was nearly constant under nitrogen, but decreased under oxygen. However, for DNA with Γ > 15, the yield increased rapidly under both nitrogen and oxygen. The yield of free bases was described by a model that depended on two factors: 1) a change in the DNA conformation from a mixture of the A and C conformers in vacuum-dried D...