Antioxidative effects of melatonin in protection against cellular damage caused by ionizing radiation

Ionizing radiation is classified as a potent carcinogen, and its injury to living cells is, to a large extent, due to oxidative stress. The molecule most often reported to be damaged by ionizing radiation is DNA. Hydroxyl radicals (*OH), considered the most damaging of all free radicals generated in organisms, are often responsible for DNA damage caused by ionizing radiation. Melatonin, N-acetyl-5-methoxytryptamine, is a well-known antioxidant that protects DNA, lipids, and proteins from free-radical damage. The indoleamine manifests its antioxidative properties by stimulating the activities of antioxidant enzymes and scavenging free radicals directly or indirectly. Among known antioxidants, melatonin is a highly effective scavenger of *OH. Melatonin is distributed ubiquitously in organisms and, as far as is known, in all cellular compartments, and it quickly passes through all biological membranes. The protective effects of melatonin against oxidative stress caused by ionizing radiation have been documented in in vitro and in vivo studies in different species and in in vitro experiments that used human tissues, as well as when melatonin was given to humans and then tissues collected and subjected to ionizing radiation. The radioprotective effects of melatonin against cellular damage caused by oxidative stress and its low toxicity make this molecule a potential supplement in the treatment or co-treatment in situations where the effects of ionizing radiation are to be minimized.     

Repair of oxidative DNA damage by amino acids

Guanyl radicals, the product of the removal of a single electron from guanine, are produced in DNA by the direct effect of ionizing radiation. We have produced guanyl radicals in DNA by using the single electron oxidizing agent (SCN)₂^–, itself derived from the indirect effect of ionizing radiation via thiocyanate scavenging of OH. We have examined the reactivity of guanyl radicals in plasmid DNA with the six most easily oxidized amino acids cysteine, cystine, histidine, methionine, tryptophan and tyrosine and also simple ester and amide derivatives of them. Cystine and histidine derivatives are unreactive. Cysteine, methionine, tyrosine and particularly tryptophan derivatives react to repair guanyl radicals in plasmid DNA with rate constants in the region of ~10⁵, 10⁵, 10⁶ and 10⁷ dm³ mol^–1 s^–1, respectively. The implication is that amino acid residues in DNA binding proteins such as histones might be able to repair by an electron transfer reaction the DNA damage produced by the direct effect of ionizing radiation or by other oxidative insults.     

Modeling the influence of histone proteins on the sensitivity of DNA to ionizing radiation

The DNA-binding proteins that are present in chromatin significantly affect the sensitivity of cells to ionizing radiation and to the radiation chemistry of DNA damage. The interaction between protein and DNA modifies the radiation chemistry of the latter. To model these processes, we have examined the effects of ionizing radiation on the minichromosome form of SV40 (which contains histone proteins arranged in nucleosomes) and also on plasmid DNA in the presence of lysozyme. Although high concentrations of lysozyme can bring about an extensive radioprotection by condensation of the plasmid, at lower levels it still produces significant radioprotective effects under conditions where this associative phase separation does not take place. The presence of histones or of lysozyme decreases the yield of modified guanines produced by ionizing radiation. Comparison with previous observations made with oligopeptides suggests that the mechanism responsible is electron donation to guanyl radicals in the DNA by tryptophan and tyrosine residues in the proteins. However, there was no evidence for DNA-protein crosslink formation.     

Radiation-induced long-lived extracellular radicals do not contribute to measurement of intracellular reactive oxygen species using the dichlorofluorescein method

The dichlorofluorescein method has become a standard technique for measuring reactive oxygen species (ROS) formed in cells by ionizing radiation. A recent report (Korystov et al., Radiat. Res. 168, 226-232, 2007) has suggested that the method is subject to an artifact in that it erroneously reports hydrogen peroxides generated in the extracellular medium as ROS formed intracellularly by ionizing radiation. It was hypothesized that radiation-induced extracellular peroxides enter cells in the minutes after radiation exposure and subsequently oxidize the intracellular dichlorofluorescin probe and that dichlorofluorescein fluorescence is not due to ROS formed intracellularly by ionizing radiation. We tested this hypothesis by measuring the contribution of long-lived radicals formed in medium by ionizing radiation on intracellular dichlorofluorescein fluorescence. We found no evidence that this artifact contributes significantly to intracellular dichlorofluorescein fluorescence. These results and those of Korystov et al. are discussed in view of cellular dichlorofluorescin leakage and radiation chemistry. We conclude that the dichlorofluorescein method is effective for quantifying intracellular ROS induced by ionizing radiation.     

Clustered DNA damage induced by gamma radiation in human fibroblasts (HF19), hamster (V79-4) cells and plasmid DNA is revealed as Fpg and Nth sensitive sites

The signature DNA lesion induced by ionizing radiation is clustered DNA damage. Gamma radiation-induced clustered DNA damage containing base lesions was investigated in plasmid DNA under cell mimetic conditions and in two cell lines, V79-4 (hamster) and HF19 (human), using bacterial endonucleases Nth (endonuclease III) and Fpg (formamidopyrimidine DNA glycosylase). Following irradiation with ⁶⁰Co γ-rays, induction of double-strand breaks (DSB) and clustered DNA damage, revealed as DSB by the proteins, was determined in plasmid using the plasmid-nicking assay and in cells by either conventional pulsed field gel electrophoresis or a hybridization assay, in which a 3 Mb restriction fragment of the X chromosome is used as a radioactive labeled probe. Enzyme concentrations (30–60 ng/µg DNA) were optimized to minimize visualization of background levels of endogenous DNA damage and DSB produced by non-specific cutting by Fpg and Nth in cellular DNA. ⁶⁰Co γ- radiation produces a 1.8-fold increase in the yields of both types of enzyme sensitive sites, visualized as DSB compared with that of prompt DSB in plasmid DNA. In mammalian cells, the increase in yields of clustered DNA damage containing either Fpg or Nth sensitive sites compared with that of prompt DSB is 1.4–2.0- and 1.8-fold, respectively. Therefore, clustered DNA damage is induced in cells by sparsely ionizing radiation and their yield is significantly greater than that of prompt DSB.     

The effect of alpha-phenyl-N-t-butylnitrone on ionizing radiation-induced apoptosis in U937 cells

Ionizing radiation induces the production of reactive oxygen species (ROS), which play an important causative role in apoptotic cell death. alpha-Phenyl-N-t-butylnitrone (PBN) is one of the most widely used spin-trapping compounds for investigating the existence of free radicals in biological systems. We investigated the effects of PBN on ionizing radiation-induced apoptosis in U937 cells. Upon exposure to 2 Gy of gamma-irradiation, there was a distinct difference between the control cells and the cells pre-treated with 2 mM PBN for 2 h in regard to apoptotic parameters, cellular redox status, mitochondria function and oxidative damage to cells. PBN effectively suppressed morphological evidence of apoptosis and DNA fragmentation in U937 cells exposed to ionizing radiation. The [GSSG]/[GSH+GSSG] ratio and the generation of intracellular ROS were higher and the [NADPH]/[NADP+ +NADPH] ratio was lower in control cells compared to PBN-treated cells. The ionizing radiation-induced mitochondrial damage reflected by the altered mitochondrial permeability transition, the increase in the accumulation of ROS, and the reduction of ATP production were significantly higher in control cells compared to PBN-treated cells. PBN pre-treated cells showed significant inhibition of apoptotic features such as activation of caspase-3, up-regulation of Bax and p53, and down-regulation of Bcl-2 compared to control cells upon exposure to ionizing radiation. This study indicates that PBN may play an important role in regulating the apoptosis induced by ionizing radiation presumably through scavenging of ROS.     

DNA lesions that signal the induction of radioresistance and DNA repair in yeast

DNA recombinational repair, and an increase in its capacity induced by DNA damage, is believed to be the major mechanism that confers resistance to killing by ionizing radiation in yeast. We have examined the nature of the DNA lesions generated by ionizing radiation that induce this mechanism, using two different end points: resistance to cell killing and ability of the error-free recombinational repair system to compete for other DNA lesions and thereby suppress chemical mutation. Under the various conditions examined in this study, the "maximum" inducible radiation resistance was increased approximately 1.5- to 3-fold and suppression of mutation about 10-fold. DNA lesions produced by low-LET gamma rays at doses greater than about 20 Gy given in oxygen were shown to be more efficient, per unit dose, at inducing radioresistance to killing than were lesions produced by neutrons (high-LET radiation). This suggests that DNA single-strand breaks are more important lesions in the induction of radioresistance than DNA double-strand breaks. Oxygen-modified lesions produced by gamma rays (low-LET radiation) were particularly efficient as induction signals. DNA damage due to hydroxyl radicals (OH.) derived from the radiolytic decomposition of H2O produced lesions that strongly induced this DNA repair mechanism. Similarly, OH. derived from aqueous electrons (e-aq) in the presence of N2O also efficiently induced the response. Cells induced to radioresistance to killing with high-LET radiation did not suppress N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-generated mutations as well as cells induced with low-LET radiation, supporting the conclusion that the type of DNA damage produced by low-LET radiation is a better inducer of recombinational repair. Surprisingly, however, cells induced with gamma radiation in the presence of N2O that became radioresistant to killing were unable to suppress MNNG mutations. This result indicates that OH. generated via e-aq (in N2O) may produce unusual DNA lesions which retard normal repair and render the system unavailable to compete for MNNG-generated lesions. We suggest that the repairability of these unique lesions is restricted by either their chemical nature or topological accessibility. Attempted repair of these lesions has lethal consequences and accounts for N2O radiosensitization of repair-competent but not incompetent cells. We conclude that induction of radioresistance in yeast by ionizing radiation responds variably to different DNA lesions, and these affect the availability of the induced recombinational repair system to deal with subsequent damage.     

Characterization of free radical-induced base damage in DNA at biologically relevant levels

DNA damage induced by oxygen radicals, e.g., hydroxyl radicals generated in living cells either by cellular metabolism or external agents such as ionizing radiations, appears to play an important role in mutagenesis, carcinogenesis, and aging. Elucidation of the chemical nature of such DNA lesions at biologically significant quantities is required for the assessment of their biological consequences and repair. For this purpose, a sensitive method using gas chromatography-mass spectrometry with the selected-ion-monitoring technique (GC-MS/SIM) was developed in the present work. DNA was exposed to hydroxyl radicals and hydrogen atoms produced by ionizing radiation in N2O-saturated aqueous solution. DNA samples were subsequently hydrolyzed with formic acid, trimethylsilylated, and analyzed by GC-MS/SIM. Characteristic ions from previously known mass spectra of DNA base products as their trimethylsilyl derivatives were recorded and the area counts of each ion were integrated. From these acquired data, a partial mass spectrum of each product was generated and then compared with those of authentic materials. This technique permitted the detection and characterization of a large number of free radical-induced based products of DNA, i.e., 5,6-dihydrothymine, 5-hydroxy-5,6-dihydrothymine, 5-hydroxymethyluracil, 5-hydroxyuracil, 5-hydroxycytosine, thymine glycol, 4,6-diamino-5-formamidopyrimidine, 8-hydroxyadenine, 2,6-diamino-4-hydroxy-5-formamidopyrimidine, and 8-hydroxyguanine, simultaneously in a single sample after radiation doses from 0.1 to 10 Gy. Detectable amounts of the base products were found to be as low as approximately 10 fmol per injection.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of α-phenyl-N-t-butylnitrone on ionizing radiation-induced apoptosis in U937 cells

Ionizing radiation induces the production of reactive oxygen species (ROS), which play an important causative role in apoptotic cell death. α-Phenyl-N-t-butylnitrone (PBN) is one of the most widely used spin-trapping compounds for investigating the existence of free radicals in biological systems. We investigated the effects of PBN on ionizing radiation-induced apoptosis in U937 cells. Upon exposure to 2 Gy of γ-irradiation, there was a distinct difference between the control cells and the cells pre-treated with 2 mM PBN for 2 h in regard to apoptotic parameters, cellular redox status, mitochondria function and oxidative damage to cells. PBN effectively suppressed morphological evidence of apoptosis and DNA fragmentation in U937 cells exposed to ionizing radiation. The [GSSG]/[GSH+GSSG] ratio and the generation of intracellular ROS were higher and the [NADPH]/[NADP⁺+NADPH] ratio was lower in control cells compared to PBN-treated cells. The ionizing radiation-induced mitochondrial damage reflected by the altered mitochondrial permeability transition, the increase in the accumulation of ROS, and the reduction of ATP production were significantly higher in control cells compared to PBN-treated cells. PBN pre-treated cells showed significant inhibition of apoptotic features such as activation of caspase-3, up-regulation of Bax and p53, and down-regulation of Bcl-2 compared to control cells upon exposure to ionizing radiation. This study indicates that PBN may play an important role in regulating the apoptosis induced by ionizing radiation presumably through scavenging of ROS.     

DNA-radicals as structural monitors of direct radiation action: Sensitization by incorporation of 5-halouracils

Summary A survey over the development of studies and the present state of results concerning free radical formation in DNA by sparsely ionizing radiation as studied by electron spin resonance (ESR)-spectroscopy is presented. The emphasis is laid on solid state studies dealing with the determination of the chemical structure of radicals and of radical reactions rather than on quantitative aspects. As a system with close potential relation between model system studies and the in vivo response to radiation the question of radiosensitization of cells by incorporation of 5-halouracil derivatives into their DNA replacing thymine residues is discussed in some detail.Dedicated to Prof. Dr. Dr. U. Hagen on the occasion of his 65th birthday     

Non-homologous end-joining defect in fanconi anemia hematopoietic cells exposed to ionizing radiation

Fanconi anemia is a genetically heterogeneous recessive disease characterized mainly by bone marrow failure and cancer predisposition. Although it is accepted that Fanconi cells are highly sensitive to DNA crosslinking agents, their response to ionizing radiation is still unclear. Using pulsed-field gel electrophoresis, we have observed that radiation generates a similar number of DNA double-strand breaks in normal and Fanconi cells from three (FA-A, FA-C and FA-F) of the 11 complementation groups identified. Nonsynchronized as well as nonproliferating Fanconi anemia cells showed an evident defect in rejoining the double-strand breaks generated by ionizing radiation, indicating defective non-homologous end-joining repair. At the cellular level, no difference in the radiosensitivity of normal and FA-A lymphoblast cells was noted, and a modest increase in the radiosensitivity of Fanca-/- hematopoietic progenitor cells was observed compared to Fanca+/+ cells. Finally, when animals were exposed to a fractionated total-body irradiation of 5 Gy, a similar hematopoietic syndrome was observed in wild-type and Fanca-/- mice. Taken together, our observations suggest that Fanconi cells, in particular those having nonfunctional Fanconi proteins upstream of FANCD2, have a defect in the non-homologous end-joining repair of double-strand breaks produced by ionizing radiation, and that compensatory mechanisms of DNA repair and/or stem cell regeneration should limit the impact of this defect in irradiated organisms.     

Radiation-induced crosslinks between thymine and phenylalanine

OH radicals generated by ionizing radiation in aqueous solutions of thymine (T) and phenylalanine (Phe) induce crosslinking between thymine and phenylalanine. The crosslinked products were isolated and characterized by capillary gas chromatography-mass spectrometry. They are formed via OH radical adducts to thymine and phenylalanine and the reaction between dissimilar radicals is greatly favoured (T-Phe:Phe-Phe:T-T = 0.46:0.14:0.05). The reaction mechanism presented may serve as a model for radiation or any free radical-induced crosslinks between DNA and proteins.     

DNA conformation of Chinese hamster V79 cells and sensitivity to ionizing radiation

Chinese hamster V79 cells grown for 20 h in suspension culture form small clusters of cells (spheroids) which are more resistant to killing by ionizing radiation than V79 cells grown as monolayers. This resistance appears to be due to the greater capacity of cells grown in contact to repair radiation damage. Attempts to relate this "contact effect" to differences in DNA susceptibility or DNA repair capacity have provided conflicting results. Two techniques, alkaline sucrose gradient sedimentation and alkaline elution, show no difference in the amounts of radiation-induced DNA single-strand breakage or its repair between suspension or monolayer cells. However, using the alkali-unwinding assay, the rate of DNA unwinding is much slower for suspension cells than for monolayer cells. Interestingly, a decrease in salt concentration or in pH of the unwinding solution eliminates these differences in DNA unwinding kinetics. A fourth assay, sedimentation of nucleoids on neutral sucrose gradients, also shows a significant decrease in radiation damage produced in suspension compared to monolayer cultures. It is believed that this assay measures differences in DNA conformation (supercoiling) as well as differences in DNA strand breakage. We conclude from these four assays that the same number of DNA strand breaks/Gy is produced in monolayer and spheroid cells. However, changes in DNA conformation or packaging occur when cells are grown as spheroids, and these changes are responsible for reducing DNA damage by ionizing radiation.     

Bistranded oxidized purine damage clusters: induced in DNA by long-wavelength ultraviolet (290-400 nm) radiation?

Bistranded clustered DNA damages involving oxidized bases, abasic sites, and strand breaks are produced by ionizing radiation and radiomimetic drugs, but it was not known whether they can be formed by other agents, e.g., nonionizing radiation. UV radiation produces clusters of cyclobutyl pyrimidine dimers, photoproducts that occur individually in high yield. Since long-wavelength UV (290-400 nm) radiation induces oxidized bases, abasic sites, and strand breaks at low yields, we tested whether it also produces clusters containing these lesions. We exposed supercoiled pUC18 DNA to UV radiation with wavelengths of >290 nm (UVB plus UVA radiation), and assessed the induction of bistranded clustered oxidized purine and abasic clusters, as recognized by Escherichia coli Fpg protein and E. coli Nfo protein (endonuclease IV), respectively, as well as double-strand breaks. These three classes of bistranded clusters were detected, albeit at very low yields (37 Fpg-OxyPurine clusters Gbp(-1) kJ(-1) m(2), 8.1 double-strand breaks Gbp(-1) kJ(-1) m(2), and 3.4 Nfo-abasic clusters Gbp(-1) kJ(-1) m(2)). Thus, these bistranded OxyPurine clusters, abasic clusters, and double-strand breaks are not uniquely induced by ionizing radiation and radiomimetic drugs, but their level of production by UVB and UVA radiation is negligible compared to the levels of frequent photoproducts such as pyrimidine dimers.     

Free-radical disorders in the tissues of mice exposed to gamma irradiation and neutrons in vitro. The radiochemical yields

The ESR method was used to study free-radical disturbances in tissues of mouse liver and spleen exposed in vitro to gamma-radiation and fission neutrons. It was shown that both types of radiation induced damages to basic chemical components of the cell, namely, DNA, lipids, proteins, and water. The radiochemical yields of each radical registered were obtained and RBE of neutrons were evaluated with a reference to the formation in tissues of radicals of each type. Membrane lipids were shown to be markedly injured by neutrons.     

Peptide repair of oxidative DNA damage

Guanyl radical species are produced in DNA by electron removal caused by ionizing radiation, photoionization, oxidation, or photosensitization. DNA guanyl radicals can be reduced by electron donation from mild reducing agents. Important biologically relevant examples are the redox active amino acids cysteine, cystine, methionine, tryptophan, and tyrosine. We have quantified the reactivity of derivatives of these amino acids with guanyl radicals located in plasmid DNA. The radicals were produced by electron removal using the single electron oxidizing agent (SCN)(2)(*)(-). Disulfides (cystine) are unreactive. Thioethers (methionine), thiols (cysteine), and phenols (tyrosine) react with rate constants in the range 10(4)-10(6), 10(5)-10(6), and 10(5)-10(6) dm(3) mol(-1) s(-1), respectively. Indoles (tryptophan) are the most reactive with rate constants of 10(7)-10(8) dm(3) mol(-1) s(-1). Selenium analogues of amino acids are over an order of magnitude more reactive than their sulfur equivalents. Increasing positive charge is associated with a ca. 10-fold increase in reactivity. The results suggest that amino acid residues located close to DNA (for example, in DNA binding proteins such as histones) might participate in the repair of oxidative DNA damage.     

Chromatin organization contributes to non-randomly distributed double-strand breaks after exposure to high-LET radiation

The influence of higher-order chromatin structure on the non-random distribution of DNA double-strand breaks induced by high-LET radiation was investigated. Five different chromatin structures (intact cells, condensed and decondensed chromatin, nucleoids and naked genomic DNA) from GM5758 cells or K562 cells were irradiated with (137)Cs gamma-ray photons and 125 keV/microm nitrogen ions (16-25 MeV/nucleon). DNA was purified with a modified lysis procedure to avoid release of heat-labile sites, and fragment size distributions and double-strand break yields were analyzed by different pulsed-field gel electrophoresis protocols. Whereas double-strand breaks in photon-irradiated cells were randomly distributed, irradiation of intact K562 cells with high-LET nitrogen ions produced an excess of non-randomly distributed DNA fragments 10 kb-1 Mbp in size. Complete removal of proteins eliminated this non-random component. There was a gradual increase in the yield of double-strand breaks for each chromatin decondensation step, and compared to intact cells, the yields for naked DNA (in buffer without scavengers) increased 83 and 25 times after photon and nitrogen-ion irradiation, respectively. The corresponding relative biological effectiveness decreased from 1.6-1.8 for intact cells to 0.49 for the naked DNA. We conclude that the organization of DNA into chromatin fiber and higher-order structures is responsible for the majority of non-randomly distributed double-strand breaks induced by high-LET radiation. However, our data suggest a complex interaction between track structure and chromatin organization over several levels.     

Repair of damage in double-stranded phi X174 (RF) DNA due to radiation-induced water radicals

Experiments in which the yields of radiation-induced OH and H radicals were varied, showed that both types of water radicals inactivate phi X174 RF DNA to about the same extent as measured by transfection of the (irradiated) DNA to E. coli wild-type spheroplasts. On the other hand, using spheroplasts prepared from E. coli strains, deficient in one of the proteins involved in excision DNA repair (uvrA- or uvrC-) or in post-replication repair (recA-), clear differences between damage originating from OH or H radical attack were found. Part of the radiation damage due to H radicals appeared to be repairable by an uvrA-gene-dependent repair mechanism, whereas this repair pathway does not play an important role in the case of OH radical damage. The reverse applies to uvrC-gene-dependent repair, which only affects OH radical damage (obtained under anoxic conditions), but has no influence on damage due to H radicals. Irradiation of double-stranded phi X174 (RF) DNA in the presence of oxygen however, yields damage--due to OH radicals only--which appeared not to be sensitive to either uvrC- or uvrA-gene-dependent repair. Furthermore, post-replication repair (recA) has only very little effect on the amount of inactivation by H or OH radicals, when irradiation is carried out under anoxic conditions. We did not find significant inactivation due to hydrated electrons, whether the biological activity was determined by use of wild-type spheroplasts or of strains deficient in excision or post-replication repair proteins.     

Ability of melanins to protect against the radiolysis of thymine and thymidine

Individuals with black skin rarely get skin cancer, and melanomas, tumors arising from pigmented cells, are generally resistant to radiation therapy. The role of melanin in these two phenomena has not been defined, but oxygen-radical species have been implicated in both effects. These studies were undertaken to determine the ability of various melanins to compete for ionizing radiation-produced radicals which destroy nucleic acid bases. The ability of Sigma eumelanin (S-eumelanin) to protect against the radiolysis of thymidine in buffered solutions was compared to the protective ability of seven amino acids, including melanin precursors; bovine serum albumin, as a model protein; ficoll, as a model polysaccharide; and DNA. Both proteins and polysaccharides are known to scavenge hydroxyl radicals in cells. The concentration of thymidine after exposure to gamma radiation was determined by High Performance Liquid Chromatography (HPLC) analysis after removal of insoluble melanin by acid precipitation. S-eumelanin was more effective at competing with thymidine for free radicals than bovine serum albumin, Ficoll, or DNA, but less effective than certain of the small molecules. Several of the above compounds were also examined for ability to protect against thymine radiolysis. In addition, melanins from other sources were compared to S-eumelanin. Of these, enzymatically synthesized phaeomelanin was the most effective. The results indicate that melanins can compete for base- and nucleoside-damaging free radicals more effectively than other cellular macromolecules. Of the small molecules, the phenolic compounds had the greatest scavenging ability. In vivo, melanins are found in melanosomes bound to protein. Therefore, the relevance of these findings to the photo- and radiobiology of melanins in vivo has yet to be determined.     

Electron spin resonance study of DNA irradiated with an argon-ion beam: evidence for formation of sugar phosphate backbone radicals

In this study, the effects of high-LET radiation on DNA were investigated and compared with the effects of gamma radiation. Hydrated DNA samples at 77 K were irradiated with argon-ion beams ((36)Ar or (40)Ar beam at energies between 60 and 100 MeV/nucleon). The individual free radicals formed were identified and their yields were investigated by electron spin resonance spectroscopy. Argon-ion irradiation resulted in lower yields of base ion radicals and higher yields of neutral radicals than gamma irradiation. A hitherto unknown species was assigned to the radical formed by C-O bond rupture at the deoxyribose C3', resulting in a sugar carbon-centered radical. A previously characterized phosphorus-centered radical was also found. The formation of each of these species was accompanied by an immediate strand break. G values, k values, and analyses for the individual yields of neutral radicals and ion radical composition for argon-ion-irradiated hydrated DNA are reported and compared to those found previously for gamma-irradiated DNA. The lower G values and k values for ion radicals and the higher fraction of neutral radicals found for argon-ion-irradiated DNA are attributed to differences in track structure inherent in the two radiations.