Radiation-Induced DNA Damage as a Function of Hydration. II. Base Damage from Electron-Loss Centers

Abstract

The induction of base damage products in gamma-irradiated DNA, hydrated between 2.5 and 32.8 moles of water per mole of nucleotide (tau), 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 (tau < or = 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 (tau < or = 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 than the sum of the yields of the products formed from electron-gain centers, indicating that there might be other electron-gain products which have not been identified; (2) the combined yield for the base damage products and the release of unaltered bases at tau < or = 13 is constant, implying that radiation damage in the tightly bound water molecules of the primary hydration layer causes DNA damage (quasi-direct effect) that is similar to the damage caused by direct ionization of the DNA (direct effect); and (3) the yields of the individual base damage products that were formed from electron-loss centers can be modeled on the basis of both the known reactions that lead to the formation of the initial charged base radicals in irradiated DNA, and the known reactions that involve the conversion of these initial DNA radicals into their respective nonradical end products.

Protection of DNA against Direct Radiation Damage by Complex Formation with Positively Charged Polypeptides

“…The films grown over saturated NaOH are referred to as dry films. Continuous vacuuming of dry DNA-PL films (over 48 h) did not lead to any weight change, so the dry DNA-PL, DNA-PR or DNA-PLT films were ascribed the lowest hydration level considered for DNA under these conditions (Γ = 2.5) (22,23). It should be noted, however, that the exact lowest hydration level for the DNA-PCP films was not measured.…”

The Influence of Packing on Free Radical Yields in Solid-State DNA: Film Compared to Lyophilized Frozen Solution

“…The extent of hydration is expressed in terms of moles of water per mole of nucleotide and is represented by Γ. In the calculation of Γ, in both films and lyophilized samples, two assumptions were made: (1) 2.5 mol of water per nucleotide could not be removed by vacuum drying (8,9), and (2) roughly 6% of the dry weight of DNA was attributable to excess salt (25).…”

“…[1][2][3][4][5]. Several variables influence the extent of direct-type damage experienced by DNA, including DNA hydration (6)(7)(8)(9)(10)(11)(12)(13)(14), conformation (6,8,9,11,14) and packing (10)(11)(12)15,16). The latter, DNA packing, reflects the aggregation of DNA molecules with each other, as well as the molecular order of the surrounding DNA hydration waters.…”

On the Chemical Yield of Base Lesions, Strand Breaks, and Clustered Damage Generated in Plasmid DNA by the Direct Effect of X Rays

“…The model is based on a rather large body of work that has been presented previously (20) and reviewed recently (9,21,22). Two of the major base products, stemming from one-electron oxidation of the DNA base stack, are 8oxoGua and fapyGua (23). The precursor for both is Gua(C8 +OH) • , which is formed by hydroxide addition to the guanine radical cation (reactions 7 and 8) (24,25).…”