The Effects of Particle LET and Fluence on the Complexity and Frequency of Clustered DNA Damage

Motivation

Clustered DNA-lesions are predominantly induced by ionizing radiation, particularly by high-LET particles, and considered as lethal damage. Quantification of this specific type of damage as a function of radiation parameters such as LET, dose rate, dose, and particle type can be informative for the prediction of biological outcome in radiobiological studies. This study investigated the induction and complexity of clustered DNA damage for three different types of particles at an LET range of 0.5-250 keV/μm.

Methods

Nanometric volumes (36.0 nm3) of 15 base-pair DNA with its hydration shell was modeled. Electron, proton, and alpha particles at various energies were simulated to irradiate the nanometric volumes. The number of ionization events, low-energy electron spectra, and chemical yields for the formation of °OH, H°, e aq - , and H2O2 were calculated for each particle as a function of LET. Single- and double-strand breaks (SSB and DSB), base release, and clustered DNA-lesions were computed from the Monte-Carlo based quantification of the reactive species and measured yields of the species responsible for the DNA lesion formation.

Results

The total amount of DNA damage depends on particle type and LET. The number of ionization events underestimates the quantity of DNA damage at LETs higher than 10 keV/μm. Minimum LETs of 9.4 and 11.5 keV/μm are required to induce clustered damage by a single track of proton and alpha particles, respectively. For a given radiation dose, an increase in LET reduces the number of particle tracks, leading to more complex clustered DNA damage, but a smaller number of separated clustered damage sites.

Conclusions

The dependency of the number and the complexity of clustered DNA damage on LET and fluence suggests that the quantification of this damage can be a useful method for the estimation of the biological effectiveness of radiation. These results also suggest that medium-LET particles are more appropriate for the treatment of bulk targets, whereas high-LET particles can be more effective for small targets.

Discovery of the radio-protecting effect of Ecliptae Herba, its constituents and targeting p53-mediated apoptosis in vitro and in vivo

Radiation protection drugs are often accompanied by toxicity, even amifostine, which has been the dominant radio-protecting drug for nearly 30 years. Furthermore, there is no therapeutic drug for radiation-induced intestinal injury (RIII). This paper intends to find a safe and effective radio-protecting ingredient from natural sources. The radio-protecting effect of Ecliptae Herba (EHE) was discovered preliminarily by antioxidant experiments and the mouse survival rate after ¹³⁷Cs irradiation. EHE components and blood substances in vivo were identified through UPLC‒Q-TOF. The correlation network of "natural components in EHE-constituents migrating to blood-targets-pathways" was established to predict the active components and pathways. The binding force between potential active components and targets was studied by molecular docking, and the mechanism was further analyzed by Western blotting, cellular thermal shift assay (CETSA), and ChIP. Additionally, the expression levels of Lgr5, Axin2, Ki67, lysozyme, caspase-3, caspase-8,8-OHdG, and p53 in the small intestine of mice were detected. It was found for the first time that EHE is active in radiation protection and that luteolin is the material basis of this protection. Luteolin is a promising candidate for RⅢ. Luteolin can inhibit the p53 signaling pathway and regulate the BAX/BCL2 ratio in the process of apoptosis. Luteolin could also regulate the expression of multitarget proteins related to the same cell cycle.

Establishment of a Method for Investigating Direct and Indirect Actions of Ionizing Radiation Using Scavenger-free Plasmid DNA

In this study, an improved method using scavenger-free plasmid DNA was established to accurately determine yields of DNA damage induced by direct and indirect actions of ionizing radiation. The scavenger-free plasmid DNA was obtained by dialysis over 5-7 days, and the DNA solvent was replaced with phosphate buffer to completely remove impurities, which could be scavengers of radicals produced as a result of water radiolysis. DNA samples of films and dilute aqueous solutions were used to separately evaluate contributions of the direct and indirect actions of X rays (150-160 kVp). The irradiated DNA was analyzed by agarose gel electrophoresis to quantify strand-break yields. The yields of single-strand breaks (SSBs), n(SSB), were determined to be (6.5 ± 2.0) × 10-10 and (3.1 ± 0.9) × 10-7 SSBs/Gy/Da for the film and solution samples, respectively, showing a significant contribution of hydroxyl radicals (•OH) compared with direct energy depositions from ionizing radiation to DNA. As observed in SSBs, the yields of double-strand breaks (DSBs), n(DSB), were (5.6 ± 1.1) × 10-11 and (1.3 ± 0.2) × 10-8 DSBs/Gy/Da for the film and solution samples, respectively. The yield ratio of DSBs to SSBs, that is, n(DSB)/n(SSB), was 0.091 ± 0.026 for the film samples, while it was much lower for the solution samples (0.045 ± 0.010), indicating that direct actions result in more localized strand breaks relative to indirect actions. Base excision repair enzymes, namely, endonuclease III (Nth) and formamidopyrimidine-DNA glycosylase (Fpg), were utilized after irradiations to convert base lesions and apurinic/apyrimidinic (AP) sites into strand breaks. The amounts of Nth and Fpg for the conversion were optimized to a few units per μg of DNA, although the optimal concentrations can differ among conditions.

Understanding the FLASH effect to unravel the potential of ultra-high dose rate irradiation

A reemergence of research implementing radiation delivery at ultra-high dose rates (UHDRs) has triggered intense interest in the radiation sciences and has opened a new field of investigation in radiobiology. Much of the promise of UHDR irradiation involves the FLASH effect, an in vivo biological response observed to maintain anti-tumor efficacy without the normal tissue complications associated with standard dose rates. The FLASH effect has been validated primarily, using intermediate energy electron beams able to deliver high doses (>7 Gy) in a very short period of time (<200 ms), but has also been found with photon and proton beams. The clinical implications of this new area of research are highly significant, as FLASH radiotherapy (FLASH-RT) has the potential to enhance the therapeutic index, opening new possibilities for eradicating radio-resistant tumors without toxicity. As pioneers in this field, our group has developed a multidisciplinary research team focused on investigating the mechanisms and clinical translation of the FLASH effect. Here, we review the field of UHDR, from the physico-chemical to the biological mechanisms.

The protective effect of oleuropein against radiation-induced cytotoxicity, apoptosis, and genetic damage in cultured human lymphocytes

PURPOSE

The aim of this study was to evaluate the effects of oleuropein radiation protection and to find an effective radioprotector.

MATERIALS AND METHOD

Human mononuclear cells were treated with oleuropein at the concentration of 100 μM (optimum concentration), incubated for 24 h, and then exposed to 2 Gy gamma-rays. The anti-radiation effect of oleuropein was assessed by MTT assay, flow cytometry, comet assay, and micronucleus (MN) assay.

RESULTS

It was found that pretreatment with oleuropein (25, 50, 75, 100, 200, 400, and 800 nM, and 1, 5, 10, 15, 20, 25, 30, 40, 50, 75, 100, 125, 150, 175, and 200 µM) significantly increased the percentage of cell viability compared to the irradiated group (p < .001). Moreover, oleuropein treatment with the above concentrations defined without gamma-ray did not show any cytotoxicity effect in human mononuclear cells. The LD_50/24h dose was calculated as 2.9 Gy, whereas by 200, 150, 50, and 100 µM oleuropein prior to radiation (1, 2,and 4 Gy), radiation LD_50/24h increased to 3.36, 3.54, 3.81, and >4 Gy, in that order. A very noticeable dose-modifying factor (DMF) of 1.16, 1.23, 1.31, and 1.72 was observed for 200, 150, 50, and 100 µM, in order. Therefore, 100 µM of oleuropein was selected as the desirable dose for radio-protection trial, and 2 Gy gamma-rays were used for further research. Human mononuclear cells treatment with oleuropein (100 µM) prior to 2 Gy gamma-rays significantly decreased apoptosis, genomic damage, and MN occurrence in human mononuclear caused by gamma-radiation (p < .001). Furthermore, treatment with oleuropein (100 µM) without radiation did not lead to apoptosis, genotoxicity, or clastogenic effects caused by oleuropein in human mononuclear cells.

CONCLUSION

The results revealed that oleuropein is able to significantly reduce cytotoxicity, apoptosis, genotoxic, and clastogenic effects of gamma-rays.

Desiccation and radiation stress tolerance in cyanobacteria

Cyanobacteria are among the oldest living organisms on this planet, existing since more than 3 billion years. They are ideal organisms for investigating biological processes such as photosynthesis, respiration, circadian rhythm, photoregulation of gene expression, developmental gene rearrangements, and specialized cell differentiation. They are nearly ubiquitous in distribution, have colonized a wide range of ecosystems including soil, air, dry rock, and aquatic systems, and even occupy extreme niches that are inaccessible to other organisms. Such wide ecological distribution reflects their capacity to acclimate to extreme environments. They show great adaptive abilities and have survived various adverse physiological growth conditions like desiccation, high temperatures, extreme pH, cold, osmosis, salt, light, nitrogen, and high salinity. Their ancient origin and surviving through numerous stresses during evolution indicates their remarkable capabilities to survive and prevail under different environmental and man-made stresses. It has been hypothesized that similar and overlap stress response mechanisms help them to survive different stresses. It has been stated that responses against stresses like radiation has been accidental-exhibited because of similar response against desiccation stress, which has prevailed more during evolution. These overlaps and similarities in stress responses have been instrumental in making these organisms a large class of biological entities today. Present review discuss about stress tolerance in cyanobacteria against two extreme stresses - desiccation and gamma radiation. It also discuss the commonality and underlying molecular mechanisms in these two stress responses.

The Use of Graphene and Its Derivatives for Liquid-Phase Transmission Electron Microscopy of Radiation-Sensitive Specimens

One of the key challenges facing liquid-phase transmission electron microscopy (TEM) of biological specimens has been the damaging effects of electron beam irradiation. The strongly ionizing electron beam is known to induce radiolysis of surrounding water molecules, leading to the formation of reactive radical species. In this study, we employ DNA-assembled Au nanoparticle superlattices (DNA-AuNP superlattices) as a model system to demonstrate that graphene and its derivatives can be used to mitigate electron beam-induced damage. We can image DNA-AuNP superlattices in their native saline environment when the liquid cell window material is graphene, but not when it is silicon nitride. In the latter case, initial dissociation of assembled AuNPs was followed by their random aggregation and etching. Using graphene-coated silicon nitride windows, we were able to replicate the observation of stable DNA-AuNP superlattices achieved with graphene liquid cells. We then carried out a correlative Raman spectroscopy and TEM study to compare the effect of electron beam irradiation on graphene with and without the presence of water and found that graphene reacts with the products of water radiolysis. We attribute the protective effect of graphene to its ability to efficiently scavenge reactive radical species, especially the hydroxyl radicals which are known to cause DNA strand breaks. We confirmed this by showing that stable DNA-AuNP assemblies can be imaged in silicon nitride liquid cells when graphene oxide and graphene quantum dots, which have also recently been reported as efficient radical scavengers, are added directly to the solution. We anticipate that our study will open up more opportunities for studying biological specimens using liquid-phase TEM with the use of graphene and its derivatives as biocompatible radical scavengers to alleviate the effects of radiation damage.

Short DNA Fragments Are a Hallmark of Heavy Charged-Particle Irradiation and May Underlie Their Greater Therapeutic Efficacy

Growing interest in proton and heavy ion therapy has reinvigorated research into the fundamental biological mechanisms underlying the therapeutic efficacy of charged-particle radiation. To improve our understanding of the greater biological effectiveness of high-LET radiations, we have investigated DNA double-strand breaks (DSBs) following exposure of plasmid DNA to low-LET Co-60 gamma photon and electron irradiation and to high-LET Beryllium and Argon ions with atomic force microscopy. The sizes of DNA fragments following radiation exposure were individually measured to construct fragment size distributions from which the DSB per DNA molecule and DSB spatial distributions were derived. We report that heavy charged particles induce a significantly larger proportion of short DNA fragments in irradiated DNA molecules, reflecting densely and clustered damage patterns of high-LET energy depositions. We attribute the enhanced short DNA fragmentation following high-LET radiations as an important determinant of the observed, enhanced biological effectiveness of high-LET irradiations.

DNA strand break dependence on Tris and arginine scavenger concentrations under ultra-soft X-ray irradiation: the contribution of secondary arginine radicals

In this study, we used a bench-top cold-cathode ultra-soft X-ray (USX) generator to expose aqueous DNA plasmid solutions to low-LET radiation under various scavenging conditions. Single- and double-strand breaks were assessed using classic gel electrophoresis quantification of linear, circular and supercoiled plasmid DNA topologies. With their very low penetration range in water, USX can only interact with matter up to short distances, of the order of 50 μm. We validated a stirring procedure which makes it possible to expose 100 µL of aqueous samples (2 mm thick). The scavenging of OH radicals by Tris buffer was studied at ambient temperature under aerobic conditions and compared to data gathered in the literature. A very good agreement was found with the rare data dealing with DNA plasmid exposed to Al Kα photons at low temperature (T ≤ 277 K), which therefore validated the experimental procedure. The yields for DNA single-strand breaks determined during this study enabled the ratio of indirect to direct effects to be determined at 96.2%, in good agreement with the value of 97.7% stemming from a study based on γ-ray irradiation of frozen solutions of plasmid DNA. Then, arginine was used both to create a "biological-like" chemical environment around the DNA plasmids and as an OH radical scavenger, in vitro. Although arginine has a greater scavenging (protecting) power than Tris, surprisingly, it led to higher rates of strand breakage. Based on the specific binding modes of arginine to DNA, we suggest that the side effects observed are due to the presence of arginine near to, but also inside, the DNA double helix.

Reactivity of Nucleic Acid Radicals

Nucleic acid oxidation plays a vital role in the etiology and treatment of diseases, as well as aging. Reagents that oxidize nucleic acids are also useful probes of the biopolymers' structure and folding. Radiation scientists have contributed greatly to our understanding of nucleic acid oxidation using a variety of techniques. During the past two decades organic chemists have applied the tools of synthetic and mechanistic chemistry to independently generate and study the reactive intermediates produced by ionizing radiation and other nucleic acid damaging agents. This approach has facilitated resolving mechanistic controversies and lead to the discovery of new reactive processes.

Double-strand breaks from a radical commonly produced by DNA-damaging agents

Double-strand breaks are widely accepted to be the most toxic form of DNA damage. Molecules that produce double-strand breaks via a single chemical event are typically very cytotoxic and far less common than those that form single-strand breaks. It was recently reported that a commonly formed C4′-radical produces double-strand breaks under aerobic conditions. Experiments described herein indicate that a peroxyl radical initiates strand damage on the complementary strand via C4′-hydrogen atom abstraction. Inferential evidence suggests that a C3′-peroxyl radical induces complementary strand damage more efficiently than does a C4′-peroxyl radical. Complementary strand hydrogen atom abstraction by the peroxyl radical is efficiently quenched by thiols. This mechanism could contribute to the higher than expected yield of double-strand breaks produced by ionizing radiation.

DNA damage by OH radicals produced using intense, ultrashort, long wavelength laser pulses

We probe femtosecond laser induced damage to aqueous DNA, relying on strong-field interaction with water wherein electrons and free radicals are generated in situ; these, in turn, interact with DNA plasmids under physiological conditions, producing nicks. Exposure to intense femtosecond pulses of 1350 and 2200 nm light induces single strand breaks and double strand breaks (DSBs) in DNA. At the longer wavelength (and at higher intensities), rotationally hot OH radicals induce DSBs, producing linear DNA. Strand breaks occur due to single or multiple OH hits on DNA. With 2200 nm light, DSBs are formed mostly by the action of two OH radicals; use of OH scavengers establishes that the probability of a two-hit event reduces much faster than a one-hit event as scavenger concentration is increased. Thermal effects do not induce DSBs with 2200 nm light.

Famotidine as a radioprotector for rectal mucosa in prostate cancer patients treated with radiotherapy: phase I/II randomized placebo-controlled trial

BACKGROUND AND PURPOSE

Acute bowel toxicity significantly affects the quality of life of patients treated with pelvic radiotherapy. This study was performed to assess whether pretreatment with famotidine can reduce acute radiation toxicities in patients undergoing radiotherapy for prostate cancer.

PATIENTS AND METHODS

Between April 2012 and February 2013, 36 patients undergoing radiotherapy for prostate cancer were enrolled to receive either placebo or famotidine. The patients received external-beam radiotherapy up to 70 Gy at daily fractions of 1.8-2 Gy (5 days/week). Oral famotidine 40 mg (80 mg/day) or placebo was administered twice daily (4 and 3 h prior to each radiotherapy fraction). Bowel and bladder acute toxicities were evaluated weekly during radiotherapy and once thereafter according to RTOG grading criteria.

RESULTS

Famotidine was well tolerated. No grade III or higher acute toxicities were noted in the two groups. Grade II rectal toxicity developed significantly more often in patients receiving placebo than in patients receiving famotidine (10/18 vs. 2/16, p=0.009). Moreover, no rectal bleeding occurred in the famotidine group, while 5 patients in the placebo group experienced rectal bleeding during treatment (p=0.046). The duration of rectal toxicity in the radiotherapy course was also reduced in the famotidine group (15.7 vs. 25.2 days, p=0.027). No significant difference between the two groups was observed in terms of urinary toxicity.

CONCLUSION

We demonstrated for the first time that famotidine significantly reduces radiation-induced injury on rectal mucosa representing a suitable radioprotector for patients treated with radiotherapy for prostate cancer.

Reduction in radiation-induced lymphocytopenia by famotidine in patients undergoing radiotherapy for prostate cancer

BACKGROUND

Ionizing radiation causes a series of hematological alterations especially profound lymphocytopenia during and after the radiotherapy course. To investigate whether famotidine can reduce hematologic toxicity in patients treated with radiotherapy for prostate cancer.

METHODS

A total of 36 patients undergoing radiotherapy for prostate cancer were randomized to receive either placebo or famotidine tablets. Participants were pretreated with 40 mg of oral famotidine or placebo tablets twice daily, 4 and 3 hr before each radiotherapy fraction. The patients received external-beam radiotherapy up to 70 Gy. Complete blood counts with differential, platelet counts, and hemoglobin levels were obtained at baseline, biweekly during the treatment and once 4 weeks after the end of radiotherapy course. Magnitude of changes from baseline in the hematological parameters was determined and compared using Repeated Measures ANOVA.

RESULTS

Famotidine was well tolerated. A total of 112 blood samples were evaluated. A significant reduction in radiation-induced lymphocytopenia was noted in patients receiving famotidine than in patients receiving placebo (P = 0.006). No significant difference was observed between two groups for the decline in platelets, erythrocytes and leucocytes. For both groups, neutrophil, monocyte, eosinophil, and hemoglobin levels did not change significantly during the treatment.

CONCLUSIONS

Our results indicate that famotidine could result in a significant reduction in radiation-induced lymphocytopenia and may consequently increase radiotherapy efficacy as well as survival times. This radioprotective effect may be chiefly associated with its antioxidant and radical scavenging properties. Further studies are required to confirm these encouraging results.

Photoinduced damage resulting from fluorescence imaging of live cells

The widespread application of fluorescence microscopy to study live cells has led to a greater understanding of numerous biological processes. Many techniques have been developed to uniquely label structures and track metabolic pathways using fluorophores in live cells. However, the photochemistry of nonnative compounds and the deposition of energy into the cell during imaging can result in unexpected and unwanted side effects. Herein, we examine potential live cell damage by first discussing common imaging considerations and modalities in fluorescence microscopy. We then consider several mechanisms by which various photochemical and photophysical phenomena cause cellular damage and introduce techniques that have leveraged these phenomena to intentionally create damage inside cells. Reviewing conditions under which intentional damage occurs can allow one to better predict when unintentional damage may be important. Finally, we delineate ways of checking for and reducing photochemical and photophysical damage.

DNA double strand cleavage via interstrand hydrogen atom abstraction

Double strand breaks (DSBs) are the most deleterious form of DNA damage. Natural products that produce them are potent cytotoxic agents. Designing molecules that produce DSBs via a single chemical event is challenging. We determined that formation of a C4'-nucleotide radical in duplex DNA under aerobic conditions gives rise to a DSB. The original radical yields a strand break containing a peroxyl radical, which initiates opposite strand cleavage via C4'-hydrogen atom abstraction. This mechanism provides the impetus to design DNA damaging agents that produce DSBs by abstracting a single hydrogen atom from the biopolymer.

Direct and bystander radiation effects: a biophysical model and clinical perspectives

In planning treatment for each new patient, radiation oncologists pay attention to the aspects that they control. Thus their attention is usually focused on volume and dose. The dilemma for the physician is how to protract the treatment in a way that maximizes control of the tumor and minimizes normal tissue injury. The initial radiation-induced damage to DNA may be a biological indicator of the quantity of energy transferred to the DNA. However, until now the biophysical models proposed cannot explain either the early or the late adverse effects of radiation, and a more general theory appears to be required. The bystander component of tumor cell death after radiotherapy measured in many experimental works highlights the importance of confirming these observations in a clinical situation.

Quantification of dye-mediated photodamage during single-molecule DNA imaging

Single-molecule fluorescence imaging of DNA-binding proteins has enabled detailed investigations of their interactions. However, the intercalating dyes used to visually locate DNA molecules have the undesirable effect of photochemically damaging the DNA through radical intermediaries. Unfortunately, this damage occurs as single-strand breaks (SSBs), which are visually undetectable but can heavily influence protein behavior. We investigated the formation of SSBs on DNA molecules by the dye YOYO-1 using complementary single-molecule imaging and gel electrophoresis-based damage assays. The single-molecule assay imaged hydrodynamically elongated lambda DNA, enabling the real-time detection of double-strand breaks (DSBs). The gel assay, which used supercoiled plasmid DNA, was sensitive to both SSBs and DSBs. This enabled the quantification of SSBs that precede DSB formation. Using the parameters determined from the gel damage assay, we applied a model of stochastic DNA damage to the time-resolved DNA breakage data, extracting the rates of single-strand breakage at two dye staining ratios and measuring the damage reduction from the radical scavengers ascorbic acid and β-mercaptoethanol. These results enable the estimation of the number of SSBs that occur during imaging and are scalable over a wide range of laser intensities used in fluorescence microscopy.

Human umbilical cord blood-derived mesenchymal stem cells undergo cellular senescence in response to oxidative stress

Since human mesenchymal stem cells (MSCs) are therapeutically attractive for tissue regeneration and repair, we examined the physiological responses of human umbilical cord blood-derived MSCs (hUCB-MSCs) to genotoxic stress. We found that that sublethal doses of reactive oxygen species (ROS) and ionizing radiation cause DNA damage and reduce DNA synthesis and cell proliferation in hUCB-MSCs, resulting in cellular senescence. In contrast, these physiological changes were limited in human fibroblast and cancer cells. Our data show that reduced activities of antioxidant enzymes, which may occur due to low gene expression levels, cause hUCB-MSCs to undergo cellular senescence in response to oxidative stress and ionizing radiation. Resistance of hUCB-MSCs to oxidative stresses was restored by increasing the intracellular antioxidant activity in hUCB-MSCs via exogenous addition of antioxidants. Therefore, the proliferation and fate of hUCB-MSCs can be controlled by exposure to oxidative stresses.

Track structures, DNA targets and radiation effects in the biophysical Monte Carlo simulation code PARTRAC

This review describes the PARTRAC suite of comprehensive Monte Carlo simulation tools for calculations of track structures of a variety of ionizing radiation qualities and their biological effects. A multi-scale target model characterizes essential structures of the whole genomic DNA within human fibroblasts and lymphocytes in atomic resolution. Calculation methods and essential results are recapitulated regarding the physical, physico-chemical and chemical stage of track structure development of radiation damage induction. Recent model extension towards DNA repair processes extends the time dimension by about 12 orders of magnitude and paves the way for superior predictions of radiation risks.

What fraction of DNA double-strand breaks produced by the direct effect is accounted for by radical pairs?

The purpose of this investigation was to determine what fraction of double strand breaks (dsb's), generated by the direct effect of ionizing radiation on DNA, can be accounted for by radical pairs. A radical pair is defined as two radicals trapped within a separation distance of <3 nm. Q-band EPR was used to measure the yield of radical pairs in calf thymus DNA films X-irradiated at 4 K. The EPR spectrum of DNA showed no evidence of radical pairs. To determine the relative sensitivity for radical pair detection via Q-band EPR, we measured the yield of radical pairs in single crystals of thymine, G(rp-Thy). Under the same conditions employed for DNA, G(rp-Thy) was approximately 8 nmol/J. The value of G(rp-Thy), in conjunction with the measured signal-to-noise, was used to calculate an upper limit for the yield of radical pairs in DNA, G(max)(rp-DNA) < 0.7-1.4 nmol/J. The upper limit, G(max)(rp-DNA), was compared with the yield of dsb's, G(total)(dsb) = 10 nmol/J, previously measured in pUC18 DNA films by Purkayastha, S.; Milligan, J. R.; Bernhard, W. A. Radiat. Res. 2007, 168, 357. We found that G(total)(dsb) > 2 x G(max)(rp-DNA), implying that a significant fraction of dsb's were not derived from a pair of trappable radicals. At least one of the two precursors needed to form a dsb was a diamagnetic (molecular) product. The hypothesis is that EPR silent lesions are formed through a molecular pathway. For example, a two-electron oxidation of deoxyribose would result in a deoxyribose carbocation intermediate that ultimately leads to a strand break.

Low-dose gamma-ray irradiation induces translocation of Nrf2 into nuclear in mouse macrophage RAW264.7 cells

The transcription factor nuclear erythroid-derived 2-related factor 2 (Nrf2) regulates expression of genes encoding antioxidant proteins involved in cellular redox homeostasis, while gamma-ray irradiation is known to induce reactive oxygen species in vivo. Although activation of Nrf2 by various stresses has been studied, it has not yet been determined whether ionizing irradiation induces activation of Nrf2. Therefore, we investigated activation of Nrf2 in response to gamma-irradiation in mouse macrophage RAW264.7 cells. Irradiation of cells with gamma-rays induced an increase of Nrf2 expression. Even 0.1 Gy of gamma-irradiation induced a translocation of Nrf2 from cytoplasm to the nucleus, indicating the activation of Nrf2 by low-dose irradiation. Expression of heme oxygenase-1, which is regulated by Nrf2, was also increased at 24 h after irradiation with more than 0.1 Gy of gamma-rays. Furthermore, the activation of Nrf2 was suppressed by U0126, which is an inhibitor of the extracellular signal regulated protein kinase 1/2 (ERK1/2) pathway, suggesting involvement of ERK1/2-dependent pathway in the irradiation-induced activation of Nrf2. Our results indicate that low-dose gamma-irradiation induces activation of Nrf2 through ERK1/2-dependent pathways.

Frequency of background and radiation-induced apoptosis in leukocytes of individuals with alpha-thalassemia variants, assessed by the neutral comet assay

To study effects of ionizing radiation on apoptosis induction in leukocytes of alpha-thalassemia (alpha-thal) variants compared to normal controls, venous blood samples were obtained from 10 healthy volunteers and 30 alpha-thal patients. Different types alpha-thal were diagnosed by multiplex polymerase chain reaction (PCR). Blood samples were irradiated with three Gy gamma rays, used for comet assay, immediately or 48 h after irradiation. Results show that the frequency of background as well as apoptosis in silent alpha-thal carriers, alpha-thal carriers and controls was similar but there was a significant difference between Hb H patients and other groups in the study. The increased apoptosis in Hb H patients might suggest that accumulation of beta-globin and oxidative stresses are effective in causing apoptosis, and cells from these patients are more vulnerable to damage from radiation-induced toxic substances. Therefore, from alpha-thal patients, those with Hb H disease might be considered as radiosensitive in terms of apoptosis formation.

Mutations Caused by γ-radiation-induced Double-strand Breaks in a Shuttle Plasmid Replicated in Human Lymphoblasts

The mutagenicity of open-circular DNA (containing base damage and single-strand breaks) and linear DNA (containing base damage, single-strand breaks, and one double-strand break) produced in vitro by gamma-irradiation of shuttle vector pZ189, was analysed after the plasmid's repair and replication in the human lymphoblast line, GM606. By comparing the survival, mutation frequency, and types of mutations in descendants from the two DNA forms, the effects of the double-strand break were determined. The percentage of viable plasmids from linear DNA was two-fold lower than that from open-circular DNA, 7.8 versus 14.0 (compared with unirradiated, control DNA). The mutation frequency in progenies of the open-circular plasmid was 4.2 +/- 1.7 x 10(-3), compared with 7.8 +/- 0.1 x 10(-3) in progenies of the linear DNA, again, nearly a two-fold difference. Approximately 59% of the mutations from the linear DNA were deletions and 34% were base substitutions. In contrast, only 13% of mutations from open-circular DNA were deletions, but 87% were base substitutions. All recoverable deletions were small, ranging from 1 to 205 base pairs, and the majority contained direct repeats at the deletion junctions, indicating non-homologous recombinations. Thus, mutations found among descendants from the linear and open-circular DNAs were qualitatively similar but quantitatively different. The data suggests that producing one double-strand break in DNA by ionizing radiation causes a two-fold increase in both lethality and mutation frequency.

Reaction of Dithiothreitol and Para-nitroacetophenone with Different Radical Precursors of .OH Radical-induced Strand Break Formation of Single-stranded DNA in Anoxic Aqueous Solution

The yields of single-strand breakage (ssb) in single-stranded calf thymus DNA (ssDNA) have been determined after 60Co gamma-irradiation of aqueous anoxic solutions in the presence of different concentrations of dithiothreitol (DTT), ascorbate or trans-4,5-dihydroxy-1,2-dithiane, using low-angle laser light scattering. The influence of DTT on the kinetics of ssb formation has been determined by conductivity measurements in pulse radiolysis. The results suggest that strand breakage in ssDNA proceeds via two modes of about equal contribution and with half-lives of about 7 ms and 0.8s, respectively. Both modes reflect reactions of at least two DNA radicals, which react with DTT by hydrogen-atom transfer reactions with similar rate constants of about 5-9 x 10(5) dm3 mol-1 s-1. These hydrogen-atom transfer reactions inhibit strand break formation. The slow mode is shown to represent the decay of base-radicals to generate sugar radicals. The involvement of the oxidizing .OH adduct radical of guanine in the formation of strand breaks can be ruled out and there is no evidence for a contribution from the anion or radical anion of DTT to the inhibition of strand breaks via electron transfer reactions to DNA radicals.

Metal Ions Protect DNA Against Strand Breakage Induced by Fast Neutrons

Single and double strand breaks (SSB and DSB) are induced by fast neutrons in plasmid (pBR322) DNA in 1 mM potassium phosphate buffer (pH 7.25). Increasing the concentration of monovalent (Na+, Cs+, Li+), divalent (Mg2+, Ca2+) and trivalent (Al3+, Co3+ (NH3)6) metal cations strongly decreases the yield of DSB. The extent of the observed protection depends on the valence of the cation. The production of SSB is only slightly decreased, except for Al3+ and Co3+ (NH3)6, whose effects are particularly large (complete protection at 1 and 0.1 mM respectively). Circular dichroism spectra show that Al3+ induces an important structural change of DNA at the ion concentration where the protection becomes total. This change is probably a condensation (collapse), as in the well-known case of Co3+ (NH3)6. Our results suggest two mechanisms of protection by metal ions: (i) the induction of structural changes of DNA, that render less accessible the critical sites of attack by OH. radicals; and (ii) the stabilization of the double helical regions between two close-set nicks on opposite strands, that hinders the effective double strand breakage of DNA.

A Comparison of the Chemical Repair Rates of Free Radical Precursors of DNA Damage and Cell Killing in Chinese Hamster V79 Cells

One of the important temporal stages of radiation action in cellular systems is the chemical phase, where oxygen fixation reactions compete with chemical repair reactions involving reducing agents such as GSH. Using the gas explosion technique it is possible to follow the kinetics of these fast (greater than 1 ms) reactions in intact cells. We have compared the chemical repair kinetics of the oxygen-dependent free radical precursors leading to DNA single-strand and double-strand breaks, measured using filter elution techniques, with those leading to cell killing in V79 cells. The chemical repair rates for DNA dsb (670s-1 at pH 7.2 and 380s-1 at pH 9.6) and cell killing (530s-1) were similar. This is in agreement with the important role of DNA dsb in radiation induced cell lethality. The rate for DNA ssb precursors was significantly slower (210s-1). The difference in rate between DNA ssb and dsb precursors may be explained on the basis of a dsb free radical precursor consisting of a paired radical, one radical on each strand. The instantaneous probability of one or other of these radicals being chemically repaired and not proceeding to form a dsb will be twice that of ssb radical precursor. This agrees well with the concept of locally multiply damaged sites (LMDS) produced from clusters of ionizations in DNA (Ward 1985).

DNA DSB induced in human cells by charged particles and gamma rays: Experimental results and theoretical approaches

PURPOSE

To quantify the role played by radiation track structure and background fragments in modulating DNA fragmentation in human cells exposed to gamma-rays and light ions.

MATERIALS AND METHODS

Human fibroblasts were exposed in vitro to different doses (in the range from 40 - 200 Gy) of (60)Co gamma-rays and 0.84 MeV protons (Linear Energy Transfer, LET, in tissue 28.5 keV/microm). The resulting DNA fragments were scored under two electrophoretic conditions, in order to optimize separation in the size ranges 0.023 - 1.0 Mbp and 1.0 - 5.7 Mbp. In parallel, DNA fragmentation was simulated both with a phenomenological approach based on the "generalized broken-stick" model, and with a mechanistic approach based on the PARTRAC (acronym of PARticle TRACk) Monte Carlo code (1.32 MeV photons were used for the simulation of (60)Co gamma-rays).

RESULTS

For both gamma-rays and protons, the experimental dose response in the range 0.023 - 5.7 Mbp could be approximated as a straight line, the slope of which provided a yield of (5.3 +/- 0.4) x 10(-9) Gy(-1) bp(-1) for gamma-rays and (7.1 +/- 0.6) x 10(-9) Gy(-1) bp(-1) for protons, leading to a Relative Biological Effectiveness (RBE) of 1.3 +/- 0.2. From both theoretical analyses it appeared that, while gamma-ray data were consistent with double-strand breaks (DSB) random induction, protons at low doses showed significant deviation from randomness, implying enhanced production of small fragments in the low molecular weight part of the experimental range. The theoretical analysis of fragment production was then extended to ranges where data were not available, i.e. to fragments larger than 5.7 Mbp and smaller than 23 kbp. The main outcome was that small fragments (<23 kbp) are produced almost exclusively via non-random processes, since their number is considerably higher than that produced by a random insertion of DSB. Furthermore, for protons the number of these small fragments is a significant fraction (about 20%) of the total number of fragments; these fragments remain undetected in these experiments. Calculations for 3.3 MeV alpha particle irradiation (for which no experimental data were available) were performed to further investigate the role of fragments smaller than 23 kbp; in this case, besides the non-random character of their production, their number resulted to be at least as much as half of the total number of fragments.

CONCLUSION

Comparison between experimental data and two different theoretical approaches provided further support to the hypothesis of an important role of track structure in modulating DNA damage. According to the theoretical approaches, non-randomness of fragment production was found for proton irradiation for the smaller fragments in the experimental size range and, in a significantly larger extent, for fragments of size less than 23 kbp, both for protons and alpha particles.

Nanoscale detection of ionizing radiation damage to DNA by atomic force microscopy

The detection and quantification of ionizing radiation damage to DNA at a single-molecule level by atomic force microscopy (AFM) is reported. The DNA damage-detection technique combining supercoiled plasmid relaxation assay with AFM imaging is a direct and quantitative approach to detect gamma-ray-induced single- and double-strand breaks in DNA, and its accuracy and reliability are validated through a comparison with traditional agarose gel electrophoresis. In addition, the dependence of radiation-induced single-strand breaks on plasmid size and concentration at a single-molecule level in a low-dose (1 Gy) and low-concentration range (0.01 ng microL(-1)-10 ng microL(-1)) is investigated using the AFM-based damage-detection assay. The results clearly show that the number of single-strand breaks per DNA molecule is linearly proportional to the plasmid size and inversely correlated to the DNA concentration. This assay can also efficiently detect DNA damage in highly dilute samples (0.01 ng microL(-1)), which is beyond the capability of traditional techniques. AFM imaging can uniquely supplement traditional techniques for sensitive measurements of damage to DNA by ionizing radiation.

Cluster Effects within the Local Effect Model

The local effect model predicts the relative biological effectiveness (RBE) for different ions and cell lines starting from the corresponding experimental photon data and an amorphous track structure model. Here we present an extension of the model that takes cluster effects of single-strand breaks (SSBs) at the nanometer scale into account. In line with the main idea of the local effect model, we take the yields of SSBs and double-strand breaks (DSBs) from experimental photon data and use a Monte Carlo method to distribute them onto the DNA. We score clusters of SSBs where individual SSBs are separated by less than 25 bp as additional DSBs. Assuming that the number of DSBs is a measure of cell lethality, we derive a modified cell survival curve for photons that takes these cluster effects into account. In combination with an improved radial dose distribution, we find that the extended local effect model including cluster effects reproduces most experimental data better than the original local effect model and thus enhances the accuracy of the local effect model.

In vivo gamma-rays induced initial DNA damage and the effect of famotidine in mouse leukocytes as assayed by the alkaline comet assay

Ionizing radiation induces a variety of lesions in DNA, each of which can be used as a bio-indicator for biological dosimetry or the study of the radioprotective effects of substances. To assess gamma ray-induced DNA damage in vivo in mouse leukocytes at various doses and the effect of famotidine, blood was collected from Balb/c male mice after irradiation with 4 Gy gamma-rays at different time intervals post-irradiation. To assess the response, mice were irradiated with doses of gamma-rays at 1 to 4 Grays. Famotidine was injected intra-peritoneally (i.p) at a dose of 5 mg/kg at various time intervals before irradiation. Four slides were prepared from each sample and alkaline comet assay was performed using standard protocols. Results obtained show that radiation significantly increases DNA damage in leukocytes in a dose dependent manner (p < 0.01) when using appropriate sampling time after irradiation, because increasing sampling time after irradiation resulted in a time dependent disappearance of DNA damage. Treatment with only 5 mg/kg famotidine before 4 Gy irradiation led to almost 50% reduction in DNA damage when compared with those animals which received radiation alone. The radioprotective capability of famotidine might be attributed to radical scavenging properties and an anti-oxidation mechanism.

Conformational properties of DNA after exposure to gamma rays and neutrons

DNA aqueous solutions were irradiated with 0-40 Gy of 60Co gamma rays and 0-1.5 Gy of (Pu-Be) neutrons. Thermal transition spectrophotometry (TTS) was used to trace the changes in the DNA conformation at the above doses. Previous results using the perturbed angular correlation (PAC) method were used to complement to the current analysis. The TTS and PAC methods are two different approaches to the study of the effects of radiation on DNA. Both showed that neutrons are more effective than gamma rays in inducing DNA damage. The TTS method showed that neutrons are 11 +/- 5 times more efficient than gamma rays, while the PAC method had shown this value to be 34 +/- 4. From the current study we deduced that the radiation damage to DNA is not a spontaneous effect but rather is an ensemble of damaging events that occur asynchronously. Any single method selected for the study of such damages can concentrate on only a part of the damage, leading to over- or underestimation of the relative effectiveness of the neutrons.

Are endogenous clustered DNA damages induced in human cells?

Although clustered DNA damages are induced in cells by ionizing radiation and can be induced artifactually during DNA isolation, it was not known if they are formed in unirradiated cells by normal oxidative metabolism. Using high-sensitivity methods of quantitative gel electrophoresis, electronic imaging, and number average length analysis, we found that two radiosensitive human cell lines (TK6 and WI-L2-NS) accumulated Fpg-oxidized purine clusters and Nth-oxidized pyrimidine clusters but not Nfo-abasic clusters. However, four repair-proficient human lines (MOLT 4, HL-60, WTK1, and 28SC) did not contain significant levels (<5/Gbp) of any cluster type. Cluster levels were independent of p53 status. Measurement of glycosylase levels in 28SC, TK6, and WI-L2-NS cells suggested that depressed hOGG1 and hNth activities in TK6 and WI-L2-NS could be related to oxybase cluster accumulation. Thus, individuals with DNA repair enzyme deficiencies could accumulate potentially cytotoxic and mutagenic clustered DNA damages. The absence of Nfo-detected endogenous clusters in any cells examined suggests that abasic clusters could be a signature of cellular ionizing radiation exposure.

Modelling study on the protective role of OH radical scavengers and DNA higher-order structures in induction of single- and double-strand break by gamma-radiation

PURPOSE

To quantify the protective effects of (non-histonic) OH-radical scavengers and DNA higher-order structures in induction of single- (ssbs) and double-strand breaks (dsbs) by gamma-rays.

MATERIALS AND METHODS

Spatial distributions of energy depositions by gamma-rays in liquid water were modelled with the track structure modules of the biophysical simulation code PARTRAC. Such distributions were superimposed on different DNA structure models (e.g. linear DNA, SV40 'minichromosomes' and compact chromatin), and direct energy depositions in the sugar-phosphate were considered as potential (direct) ssbs. The diffusion and interaction of the main chemical species produced in liquid water radiolysis were explicitly simulated, and reactions of *OH with the sugar-phosphate were considered as potential (indirect) ssbs. Two ssb on opposite DNA strands within 10 base pairs were considered as one dsb. Yields of ssb and dsb Gy(-1) Dalton(-1) in different DNA target structures were calculated as a function of the *OH mean lifetime, whose inverse value was taken as representative of the scavenging capacity of the DNA environment.

RESULTS AND CONCLUSIONS

A further validation of the models implemented in the PARTRAC code has been provided, thus allowing a better understanding of the mechanisms underlying DNA damage. More specifically, the protection due to *OH scavengers was separately quantified with respect to that due to histones and chromatin folding, which could be 'switched off' in the simulations. As expected, for a given value of the environment scavenging capacity, linear DNA was more susceptible to strand breakage than SV40 minichromosomes, which in turn showed higher damage yields with respect to cellular DNA due to the larger accessibility offered to *OH. Furthermore, by increasing the scavenging capacity, the break yields decreased in all structures and tended to coincide with direct damage yields. Very good agreement was found with available experimental data. Comparisons with data on 'nucleoid' DNA (i.e. unfolded and histone-depleted DNA) also suggested that the experimental procedures used to obtain such structures might lower the environment scavenging capacity owing to the loss of cellular scavengers.

Radiation-activated nuclease activity of o,o'-Diphenyleneiodonium cations (DPI): a reductively initiated chain reaction involving the C1' chemistry

o,o'-Diphenyleneiodonium cations (DPI) convert relatively harmless radiation-produced electrons into efficient DNA cleaving agents. The cleavage products are unaltered DNA bases, 5-methylenefuranone (5-MF), and a complete set of 3' and 5'-phosphorylated DNA fragments. The production of alkali-labile sites is a minor factor in the process. Based on the production of 5-MF, it is concluded that DNA cleavage by DPI cations involves (but may not be limited to) the C1' chemistry. The loss of 3-aminoDPI (ADPI) cations bound to highly polymerized calf thymus DNA appears to be due to a short-chain reaction with an apparent length of up to 2.1 ADPI cations consumed for each radiation-produced electron. The suggested chain reaction mechanism includes the one-electron oxidation of DNA radicals (including the C1' sugar radical) by ADPI cations bound to the same duplex. The yields of DNA loss in complexes formed by ADPI with short synthetic duplexes indicate that there is more than a 60% probability of DNA damage after one-electron reduction of ADPI.

Simulation of DNA damage after proton irradiation

The biophysical radiation track simulation model PARTRAC was improved by implementing new interaction cross sections for protons in water. Computer-simulated tracks of energy deposition events from protons and their secondary electrons were superimposed on a higher-order DNA target model describing the spatial coordinates of the whole genome inside a human cell. Induction of DNA double-strand breaks was simulated for proton irradiation with LET values between 1.6 and 70 keV/microm and various reference radiation qualities. The yield of DSBs after proton irradiation was found to rise continuously with increasing LET up to about 20 DSBs per Gbp and Gy, corresponding to an RBE up to 2.2. About half of this increase resulted from a higher yield of DSB clusters associated with small fragments below 10 kbp. Exclusion of experimentally unresolved multiple DSBs reduced the maximum DSB yield by 30% and shifted it to an LET of about 40 keV/microm. Simulated fragment size distributions deviated significantly from random breakage distributions over the whole size range after irradiation with protons with an LET above 10 keV/microm. Determination of DSB yields using equations derived for random breakage resulted in an underestimation by up to 20%. The inclusion of background fragments had only a minor influence on the distribution of the DNA fragments induced by radiation. Despite limited numerical agreement, the simulations reproduced the trends in proton-induced DNA DSBs and fragment induction found in recent experiments.

Changes in DNA flexibility after irradiation with gamma rays and neutrons studied with the perturbed angular correlation method

Neutron and gamma irradiation of buffered solutions of calf thymus DNA resulted in changes in the dynamics of the macromolecule. In the low-dose region (0.8-10 cGy of 239Pu-Be neutrons and 0.34-3 Gy of 60Co gamma rays), the flexibility of DNA decreased as indicated by slower rotation of the molecules. Neutrons appeared to be approximately 35 times more effective than 60Co gamma rays. The rotational correlation time, tau C, was measured using the perturbed angular correlation (PAC) method. Its variation appears to follow a linear-exponential behavior. An attempt is made to formulate this behavior as a function of the energy deposited on the macromolecule (radiation dose), the average threshold energy (dose) required to form new lesions, and the available population of intact DNA sites.

DNA damage and apoptosis in the mussel Mytilus galloprovincialis

The effects of known genotoxic substances (4-nitroquinoline-N-oxide, benzo[a]pyrene, teniposide, etoposide, cycloheximide, tributyltin) on human cells (FLC, HL-60) and on mussels were investigated. The correlations between formation of DNA strand breaks and DNA fragmentation characteristic for the process of apoptosis were estimated. Strand breaks induced by 4-nitroquinoline-N-oxide and benzo[a]pyrene did not correlate with DNA fragmentation detected in the process of apoptosis. Induction of internucleosomal DNA fragmentation in HL-60 cells was initiated by teniposide, etoposide and tributyltin, while in the gills of mussels this was detected only with tributyltin. Levels of DNA strand breaks in natural mussel populations, living at locations under the influence of urban and industrial wastes, do not mirror the apoptotic processes.

Computer simulation of initial events in the biochemical mechanisms of DNA damage

Understanding the systematic and quantitative correlation between the physical events of energy deposition by ionizing radiation and the ensuing chemical and biochemical processes leading to DNA damage is one of the goals in radiation research. Significant progress has been made toward achieving the stated goal by using theoretical modeling techniques. These techniques are strongly dependent on computer simulation procedures. A review of such techniques with details of various stages of simulation development, including a comparison with available experimental data, is presented in this article.

Securing genome stability by orchestrating DNA repair: removal of radiation-induced clustered lesions in DNA

In addition to double- and single-strand DNA breaks and isolated base modifications, ionizing radiation induces clustered DNA damage, which contains two or more lesions closely spaced within about two helical turns on opposite DNA strands. Post-irradiation repair of single-base lesions is routinely performed by base excision repair and a DNA strand break is involved as an intermediate. Simultaneous processing of lesions on opposite DNA strands may generate double-strand DNA breaks and enhance nonhomologous end joining, which frequently results in the formation of deletions. Recent studies support the possibility that the mechanism of base excision repair contributes to genome stability by diminishing the formation of double-strand DNA breaks during processing of clustered lesions.

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.

Oxygen-dependent DNA damage amplification involving 5,6-dihydrothymidin-5-yl in a structurally minimal system

5,6-Dihydrothymidin-5-yl (1) was independently generated in a dinucleotide from a phenyl selenide precursor (4). Under free radical chain propagation conditions, the products resulting from hydrogen atom donation and radical-pair reaction are the major observed products in the absence of O(2). The stereoselectivity of the trapping process is dependent on the structure of the hydrogen atom donor. No evidence for internucleotidyl hydrogen atom abstraction by 1 was detected. The tandem lesion (17) resulting from hydrogen atom abstraction from the C1' position of the adjacent 2'-deoxyuridine by the peroxyl radical derived from 1 (3) is observed under aerobic conditions. The structure of this product is confirmed by independent synthesis and its transformation into a second independently synthesized product (24). Internucleotidyl hydrogen atom abstraction is effected selectively by the 5S-diastereomer of the peroxyl radical. The formation of dinucleotide 17 provides further support for the novel O(2)-dependent DNA damage amplification mechanism involving 1 reported previously (Greenberg, M. M.; et al. J. Am. Chem. Soc. 1997, 119, 1828).