Model studies for the direct effect of high-energy irradiation on DNA. Mechanism of strand break formation induced by laser photoionization of poly U in aqueous solution

A comprehensive review of radioactive pollution treatment of uranium mill tailings

Natural uranium is a crucial resource for clean nuclear energy, which has brought significant economic and social benefits to humanity. However, the development and utilization of uranium resources have also resulted in the accumulation of vast amounts of uranium mill tailings (UMTs), which pose a potential threat to human health and the ecological environment. This paper reviews the research progress on UMTs treatment technologies, including cover disposal, solidification disposal, backfilling disposal, and bioremediation methods. It is found that cover disposal is a versatile method for the long-term management of UMTs, the engineering performance and durability of the cover system can be improved by choosing suitable stabilizers for the cover layer. Solidification disposal can convert UMTs into solid waste for permanent disposal, but it produces a large amount of waste and requires high operating costs; it is necessary to explore the effectiveness and efficiency of solidification disposal for UMTs, while minimizing the bad environmental impact. Backfilling disposal realizes the resource utilization of solid waste, but the high radon exhalation rate caused by the UMTs backfilling also needs to be considered. Bioremediation methods have low investment costs and are less likely to cause secondary pollution, but the remediation efficiency is low, it can be combined with other treatment technologies to remedy the defects of a single remediation method. The article concludes with key issues and corresponding suggestions for the current UMTs treatment methods, which can provide theoretical guidance and reference for further development and application of radioactive pollution treatment of UMTs.

Sterility of gamma-irradiated pathogens: a new mathematical formula to calculate sterilizing doses

In recent years there has been increasing advocacy for highly immunogenic gamma-irradiated vaccines, several of which are currently in clinical or pre-clinical trials. Importantly, various methods of mathematical modelling and sterility testing are employed to ensure sterility. However, these methods are designed for materials with a low bioburden, such as food and pharmaceuticals. Consequently, current methods may not be reliable or applicable to estimate the irradiation dose required to sterilize microbiological preparations for vaccine purposes, where bioburden is deliberately high. In this study we investigated the applicability of current methods to calculate the sterilizing doses for different microbes. We generated inactivation curves that demonstrate single-hit and multiple-hit kinetics under different irradiation temperatures for high-titre preparations of pathogens with different genomic structures. Our data demonstrate that inactivation of viruses such as Influenza A virus, Zika virus, Semliki Forest virus and Newcastle Disease virus show single-hit kinetics following exposure to gamma-irradiation. In contrast, rotavirus inactivation shows multiple-hit kinetics and the sterilizing dose could not be calculated using current mathematical methods. Similarly, Streptococcus pneumoniae demonstrates multiple-hit kinetics. These variations in killing curves reveal an important gap in current mathematical formulae to determine sterility assurance levels. Here we propose a simple method to calculate the irradiation dose required for a single log10 reduction in bioburden (D10) value and sterilizing doses, incorporating both single- and multiple-hit kinetics, and taking into account the possible existence of a resistance shoulder for some pathogens following exposure to gamma-irradiation.

Direct strand scission in double stranded RNA via a C5-pyrimidine radical

Nucleobase radicals are the major family of reactive intermediates produced when nucleic acids are exposed to γ-radiolysis. The 5,6-dihydrouridin-5-yl radical (1), the formal product of hydrogen atom addition and a model for hydroxyl radical addition, was independently generated from a ketone precursor via Norrish Type I photocleavage in single and double stranded RNA. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and only minor amounts of strand scission are observed at the initial site of radical generation. Strand scission occurs preferentially in double stranded RNA and in the absence of O(2). The dependence of strand scission efficiency from the 5,6-dihydrouridin-5-yl radical (1) on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. Internucleotidyl strand scission occurs via β-elimination of the 3'-phosphate following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical yields RNA fragments containing 3'-phosphate or 3'-deoxy-2'-ketonucleotide termini from competing deprotonation pathways. The ketonucleotide end group is favored in the presence of low concentrations of thiol, presumably by reducing the cation radical to the enol. Competition studies with thiol show that strand scission from the 5,6-dihydrouridin-5-yl radical (1) is significantly faster than from the 5,6-dihydrouridin-6-yl radical (2) and is consistent with computational studies using the G3B3 approach that predict the latter to be more stable than 1 by 2.8 kcal/mol.

Efficiency and pattern of UV pulse laser-induced RNA-RNA cross-linking in the ribosome

Escherichia coli ribosomes were irradiated with a KrF excimer laser (248 nm, 22 ns pulse) with incident pulse energies in the range of 10-40 mJ for a 1 cm2 area, corresponding to fluences of 4.5 to 18 x 10(9) W m(-2), to determine strand breakage yields and the frequency and pattern of RNA-RNA cross- linking in the 16S rRNA. Samples were irradiated in a cuvette with one laser pulse or in a flow cell with an average of 4.6 pulses per sample. The yield of strand breaks per photon was intensity dependent, with values of 0.7 to 1.3 x 10(-3) over the incident intensity range studied. The yield for RNA-RNA cross-linking was 3 x 10(-4) cross-links/photon at the intensity of 4.5 x 10(9) W m(-2), an approximately 4-fold higher yield per photon than obtained with a transilluminator. The cross-link yield/photon decreased at higher light intensities, probably due to intensity-dependent photoreversal. The pattern of cross-linking was similar to that observed with low intensity irradiation but with four additional long-range cross-links not previously seen in E.coli ribosomes. Cross- linking frequencies obtained with one laser pulse are more correlated to internucleotide distances than are frequencies obtained with transilluminator irradiation.

The reaction of triplet 2-methyl-1,4-naphthoquinone (menadione) with DNA and polynucleotides

The photoreaction of 2-methyl-1,4-naphthoquinone (MQ, menadione) with DNA and polynucleotides in argon-saturated aqueous solution (pH 7) was studied. Results from laser flash photolysis experiments indicate that triplet quinone reacts with DNA and polyA but not detectably with polyU by one-electron oxidation of the bases of the nucleic acid with formation of the radical anion of the quinone. Irradiation of argon-saturated solutions containing MQ and DNA or polynucleotides (polyU, polyA, polyG or polyC) with 334 nm light leads to an increase in molecular weight for single-stranded DNA, polyA and to a much less extent for polyU. This finding indicates crosslink formation with quantum yields in the range of 10(-5)-10(-3).

Photochemistry of DNA and related biomolecules: quantum yields and consequences of photoionization

The reactions of nucleic acids and constituents, which can be induced by laser UV irradiation, are described. Emphasis is placed on the quantum yields of various stable photoproducts of DNA and model compounds upon irradiation at 193, 248, 254 or 266 nm. In particular, those quantum yields and processes are discussed which involve photoionization as the initial step and occur in aqueous solution under well defined conditions, e.g. type of atmosphere. The efficiencies of some photoproducts, with respect to photoionization using irradiation at 193 or 248 nm, are presented. Radical cations of nucleobases are important sources of damage of biological substrates since they can cause lesions other than dimers and adducts, e.g. strand breakage, abasic sites, crosslinks or inactivation of plasmid and chromosomal DNA. While competing photoreactions, such as hydration, dimerization or adduct formation, diminish the selectivity of the photoionization method, a combination with model studies on pyrimidine- and purine-containing constituents of DNA has brought about an enhanced insight into the reaction mechanisms. The knowledge concerning the lethal events in plasmid and cellular DNA has been greatly improved by correlation with the chemical effects obtained by gamma-radiolysis, vacuum-UV (< 190 nm) and low-intensity irradiation at 254 nm.

Role of protein thiols in intrinsic radiation protection of DNA and cells

Previous data have been consistent with the hypothesis that the thiol depleter dimethyl fumarate (DMF) increases radiation sensitivity of hypoxic mammalian cells by a combination of two actions: depletion of glutathione (GSH) which interferes with the chemical competition between damage fixation and repair and depletion of protein thiol (PSH) which causes inhibition of enzymatic repair processes. However, one cannot rule out the possibility that PSH also acts in the chemical competition to restitute damaged DNA. The studies presented here have addressed this question by studying the effects of ionizing radiation on isolated nuclei which contain very low levels of GSH, but substantial amounts of PSH, compared to intact cells. The results show that DNA damage, measured using the non-denaturing filter elution assay, is increased about 1.6-fold in isolated nuclei irradiated in air, compared to whole cells, and about 4.3-fold in nuclei irradiated in N2. Thus, the OER for DNA damage decreases from about 3 in whole cells to 1 in isolated nuclei. Also, although DMF increases radiation-induced DNA damage in whole cells irradiated in hypoxia about 2-fold, it does not increase DNA damage in isolated nuclei irradiated in hypoxia. These data do not support the idea that PSH can act in the chemical competition reaction to chemically repair radiation-induced DNA radicals. The data are discussed in relation to the effect that various procedures for nuclei isolation can have on radiation sensitivity of DNA and on the OER. We also address the question of whether radiation-induced DNA damage measured by non-denaturing elution correlates with cell killing.

Strand breakage in poly(C), poly(A), single- and double-stranded DNA induced by nanosecond laser excitation at 193 nm

Single- and double-stranded calf thymus DNA and two polynucleotides (0.4 mM) were studied in aqueous solution at pH approximately 7 using pulsed, 20 ns laser excitation at 193 nm. Monophotonic ionization of the nucleic acids is suggested from the linear dependences of the concentration of ejected electrons and the number of single- and double-strand breaks (ssb, dsb, respectively) on laser intensity (IL) in the range (0.2-3) x 10(6) W cm-2. The quantum yields of formation of hydrated electrons (phi e-) and ssb and dsb (phi ssb and phi dsb) are therefore independent of IL. In contrast, under 248 nm excitation these quantum yields increase linearly with IL under otherwise comparable conditions. Nevertheless, several effects and mechanistic implications are analogous using lambda exc = 193 and 248 nm. For polycytidylic acid, poly(C), in Ar-saturated solution for example, the efficiency of ssb per radical cation (eta RC = phi ssb/phi e-) is similar to the efficiency of ssb per OH radical (eta OH). For polyadenylic acid, poly(A), and single- and double-stranded DNA eta RC (lambda exc = 193 nm) is significantly smaller than eta OH. The ratio phi ssb (N2O)/phi ssb (Ar) is approximately 2 for poly(C), approximately 4 for poly(A) approximately 10 for DNA; the conversion of hydrated electrons into OH radicals in N2O-saturated solution and smaller eta RC than eta OH values in the case of DNA account for these results. For double-stranded DNA phi dsb does not depend on IL but increases linearly with the dose, indicating an accumulative effect of two ssb to generate one dsb. The critical distance for this event is 60-85 phosphoric acid diester bonds.

The chemistry of free-radical-mediated DNA damage

In the living cell, ionizing radiation can cause DNA damage by the direct effect (ionization of DNA) and the indirect effect (reaction of radicals formed in the neighborhood of DNA with DNA, e.g., OH, eaq-, H, protein- and glutathione-derived radicals). Properties of the base radical cations have been studied in model systems using SO4- radical to oxidize the nucleobases in aqueous solution. The pKa values of some nucleobase radical cations are reported, so are the ensuing reactions of the thymidine radical cation with water. The products of reactions are compared with those formed by OH radical attack. The reaction of eaq- with the nucleobases yields radical anions. Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported. A brief account is given on the reaction of carbon-centered radicals with the nucleobases. These reactions may mimic the reactions of protein-derived radicals with DNA. Glutathione is present in cells at rather high concentrations and is expected to act as an H- or electron-donor in repairing radiation-induced DNA damage (chemical repair). As thiyl radicals are known to also undergo the reverse reaction, i.e., H-abstraction from suitable solutes, some experiments are reported which probe this type of reaction with dilute DNA solutions. In some polynucleotides radical transfer from the base radical to the sugar moiety occurs with the consequence of strand breakage and base release. Some currently held mechanistic concepts are discussed. Attention is drawn to some important open questions which should be addressed in the near future.

Transients of uracil and thymine derivatives and the quantum yields of electron ejection and intersystem crossing upon 20 ns photolysis at 248 nm

Transients of uracil and a series of 17 correlated pyrimidines, e.g. methylated bases, nucleosides, nucleotides, and polyuridylic acid [poly(U)] were studied after 248 nm excitation by 20 ns laser pulses. The transient absorption spectra in aqueous solution at room temperature reveal the triplet state and the hydrated electron (e-aq), while the corresponding radical cation could not be observed at pH 6-7. Fast loss of the chromophore in the 260-290 nm range within 0.1 microsecond was observed in aqueous solution in some cases [e.g. poly(U), 5'-UMP, uridine, uracil] and in others (thymine, thymidine) virtually not. This photobleaching is assigned to formation of the photohydrate. The concentration of e-aq shows a quadratic dependence on the laser pulse intensity (IL) in the range (0.2-2) x 10(7) W cm-2 and the quantum yield of electron ejection (phi c-) thus depends linearly on IL. This behaviour, suggesting that the photoionization involves a two-step absorption process, was found for poly(U) and all pyrimidine monomers examined. At a constant IL value of 2 x 10(7) W cm-2, phi c- ranges from 3 x 10(-3) for 1,3-dimethylthymine to 4 x 10(-2) for poly(U). The triplet state shows a much larger transient absorbance (delta A, typically in a broad range, e.g. 290-500 nm) than that of the neutral radical resulting from the radical cation. The triplet state in organic solvents (acetonitrile and ethanol) shows generally a significantly larger delta A value than in aqueous solution. The estimated quantum yields of intersystem crossing at room temperature are compared with those of phosphorescence at -196 degrees C.

Single- and double-strand break formation in double-stranded DNA upon nanosecond laser-induced photoionization

Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA.

Photochemistry of DNA using 193 nm excimer laser radiation

Photoproducts in double-stranded DNA induced by 193 nm radiation have been investigated. Double-stranded, supercoiled pBR322 DNA in buffered aqueous solution was exposed to varying fluences of 193 nm radiation from an ArF excimer laser. The quantum yields for formation of cyclobutylpyrimidine dimers, frank strand breaks and alkali labile sites were calculated from the conversion of supercoiled (Form I) DNA to relaxed (Form II) DNA after treatment with Micrococcus luteus dimer-specific endonuclease, no treatment, or treatment with alkali and heat, respectively. The quantum yields were 1.65 (+/- 0.03) X 10(-3) for pyrimidine dimers, 9.4 (+/- 3.2) X 10(-5) for frank strand breaks and 9.6 (+/- 3.6) X 10(-5) for alkali labile sites. The quantum yields for pyrimidine dimers and strand breaks and alkali labile sites were not affected by 10 nM mannitol. The relative quantum yields for these DNA photoproducts induced by 193 nm radiation differed markedly from those produced by 254 nm radiation.

Phosphorescence of nucleic acids and DNA components at 77 K

The emission spectra of nucleic acids, pyrimidine and purine nucleotides, nucleosides and bases and a series of pyrimidine derivatives were obtained using UV light excitation in glasses (ethanol and 2:1 mixtures of ethylene glycol and water (EG-H2O); also partly in butyronitrile and 2-methyltetrahydrofuran) at 77 K. The quantum yields of fluorescence phi f and phosphorescence phi p of some 30 compounds are presented; for several substituted uracils they are reported for the first time. The values cover a range from phi f = 0.0002 and phi p = 0.001 for uracil in ethanol to phi f = 0.50 for guanosine in acidic ethanol and phi p = 0.095 for guanosine-5'-monophosphate in EG-H2O (pH 6-7). The phosphorescence lifetime tau p at 77 K ranges from about 0.3 s (uracil moiety) to 3 s (adenine moiety). The measured tau p, phi f and phi p values are compared with those available in the literature.

Photosensitized reactions of poly(U) with tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate

The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio.

Binding of Ru(bpy)3(2+) and Ru(phen)3(2+) to polynucleotides and DNA: effect to added salts on the absorption and luminescence properties

The interactions of tris(2,2'-bipyridyl)ruthenium(II) chloride and tris-(1,10-phenanthroline)ruthenium(II) chloride, Ru(bpy)3Cl2 and Ru(phen)3Cl2 respectively, with nucleic acids were studied by means of absorption spectroscopy and time-resolved and steady state luminescence techniques in unbuffered aqueous solution at room temperature as a function of added salt, oxygen and the [nucleotide]/[sensitizer] ratio (N/S). The hypochromicity of the visible absorption band of Ru(ligand)3(2+) and the changes in the luminescence intensity and luminescence decay kinetics are considerably larger in the presence of double-stranded calf thymus DNA (dsDNA) than in the presence of single-stranded DNA and polynucleotides. This is suggested to be the result of partial intercalation of the ruthenium complex into the dsDNA rather than just its higher charge density with respect to ssDNA. Spectral changes in the presence of dsDNA increase with increasing N/S ratio (maximum changes reached at N/S = 10-12, half-value 3-4). This is postulated to be due to a transition from mainly electrostatic binding to a binding in which partial intercalation plays an increased role. Addition of alkali or alkaline earth salts at very low concentrations stabilizes partial intercalation whereas higher salt concentrations lead to a release of the ruthenium complex from the strand. This effect of the salt cation increases in the order Cs less than Rb less than K less than Na less than Li less than Ba less than Sr less than Ca less than Mg less than Be. For Ru(bpy)3(2+) the presence of 0.5 mM Mg(ClO4)2 or 6 mM NaClO4 are sufficient to release 50% of the ruthenium complexes which are bound to the dsDNA (N/S = 10); the corresponding half-concentrations for Ru(phen)3(2+) are 0.8 mM and 40 mM respectively. The half-concentrations for release increase with increasing N/S ratio and decrease with the ionic radius of the added salt.

Methylene blue photosensitised strand cleavage of DNA: effects of dye binding and oxygen

It is shown that methylene blue (MB+) photosensitises DNA in either aerated or deaerated solutions, causing direct cleavage of phosphodiester bonds and rendering additional bonds labile to alkali. Evidence from unwinding and fluorimetric studies indicates that MB+ binds to DNA in at least 2 ways. Intercalation, which optimally induces helical unwinding of 24 degrees +/- 2 degrees per MB+, is markedly reduced upon neutralisation by Mg2+ of the DNA phosphates, while significant non-intercalative binding persists as shown by substantial fluorescence quenching at Mg2+ concentrations where there is little unwinding. MB+ induces photolysis at both low and high Mg2+ concentration - intercalation is apparently not required for photolysis. The quantum yield for strand breakage varies from 1-3 X 10(-7) under different conditions and is oxygen enhanced. The DNA cleavage is guanine specific. The 3' termini of the primary MB+-induced DNA photoproducts, unlike those generated by chemical sequencing retain an alkali labile adduct on the terminal phosphate.

New aspects in the free-radical chemistry of pyrimidine nucleobases

i) It has been known for some time that OH radicals and H atoms react with the pyrimidines by adding to the C(5)-C(6) double bond, but only the u.v.-spectra of the sum of these radicals have been reported so far. It will be shown how to arrive at the individual spectra of the C(5) and the C(6) adduct radicals. ii) alpha-Hydroxyalkyl radicals are known to inactivate biologically active DNA. In contrast to the electrophilic radicals H and OH they are nucleophilic and the hydroxymethyl radicals add exclusively at the C(6) position of 1,3-dimethyluracil (k approximately 10(4) dm3 mol-1 s-1). In the corresponding thymine derivative this reaction also occurs, but one third of the hydroxymethyl radicals abstract an H-atom from the C(5)-methyl group thereby forming an allylic radical. In the course of these reactions pyrimidines with an exocyclic double bond are formed. These products react much more rapidly with hydroxymethyl radicals than the starting material leading to highly hydroxymethylated material at very low doses. iii) The direct effect of ionizing radiation which would produce a pyrimidine base radical cation can be mimicked by reacting the pyrimidine with SO4-, a very good electron acceptor. In water, the radical cation of 1,3-dimethyluracil is rapidly (t1/2 less than 2 microseconds) converted into the C(5) OH adduct radical. In the presence of peroxodisulphate a chain reaction sets in which leads to the cis-glycol.

Specific deprotonation reactions of the pyrimidine radical cation resulting from the menadione mediated photosensitization of 2'-deoxycytidine

Menadione(2-methyl-1,4-naphthoquinone) was shown to sensitize 2'-deoxycytidine to near ultraviolet light according to two main mechanisms. Reaction of a water molecule with the initially photo-induced pyrimidine radical cation and subsequent addition of molecular oxygen leads to the preponderant formation of the four cis and trans diastereoisomers of 5,6-dihydroxy-5,6-dihydro-2'-deoxyuridine. Pyrimidine ring opening and rearrangement products are also generated through the intermediate 6-hydroxy-5,6-dihydro-2'-deoxyurid-5-yl radical. The competitive deprotonation reaction of the radical cation is likely to involve two sites. Loss of an amino group proton is the likely initial event to explain the formation of 2'-deoxyuridine which is resistant to further photooxidation. The second deprotonation reaction involves the osidic carbon C(1'). The resulting radical will further react with oxygen leading to the release of free cytosine with concomitant formation of 2-deoxy-D-ribono-1,4-lactone. This reaction which is not prevented by hydroxyl radical scavengers constitutes to our knowledge the first example of a pyrimidine radical which is able to initiate selective intramolecular reaction at position 1 within the sugar moiety.

Mechanism of radiation-induced strand break formation in DNA and polynucleotides

The present state of our knowledge of the OH radical-induced strand break (sb) formation in presence and absence of oxygen in aqueous solution is reviewed for poly(U), poly(C), poly(A) and single- and double-stranded DNA as substrates. It was shown earlier that a single OH radical can induce a double-strand break in DNA. As a key step in the mechanism an interstrand radical transfer is postulated. The OH radical reaction is part of the indirect effect of gamma-irradiation. In addition recent results are presented concerning sb formation by the direct effect of high-energy irradiation using laser-induced photoionization for the formation of radical cations.

Ultrafast chemical repair of DNA single and double strand break precursors in irradiated V79 cells

The fast kinetics of reactions of free radical precursors of DNA single strand breaks (ssb) and double strand breaks (dsb) have been determined in Chinese hamster V79 cells by fast mixing and irradiation methods using the alkaline unwinding technique to assay breaks. Fast chemical repair of oxygen-dependent ssb and dsb precursors was observed and approached completion within 10-20 ms of irradiation. Treatment of cells with the glutathione synthesis blocking agent, buthionine sulphoximine, showed that approximately half of the chemical repair was attributable to intracellular non-protein thiols. The nature of the residual repair is obscure, but it is apparently not attributable to non-protein thiols. Similar repair rates and thiol dependences were also found for cell kill. With all three endopoints, oxygen competes with and blocks the chemical repair.