Pierre Cloutier

Cross Sections for Low-Energy (10 – 50 eV) Electron Damage to DNA

Abstract Boudaïffa, B., Cloutier, P., Hunting, D., Huels, M. A. and Sanche, L. Cross Sections for Low-Energy (10 – 50 eV) Electron Damage to DNA. Radiat. Res. 157, 227 – 234 (2002). We report direct measurements of the formation of single-, double- and multiple strand breaks in pure plasmid DNA as a function of exposure to 10 – 50 eV electrons. The effective cross sections to produce these different types of DNA strand breaks were determined and were found to range from approximately 10−17 to 3  × 10−15 cm2. The total effective cross section and the effective range for destruction of supercoiled DNA extend from 3.4 to 4.4  × 10−15 cm2 and 12 to 14 nm, respectively, over the range 10 – 50 eV. The variation of the effective cross sections with electron energy is discussed in terms of the electrons inelastic mean free path, penetration depth, and dissociation mechanisms, including resonant electron capture; the latter is found to dominate the effective cross sections for single- and double-strand breaks at 10 eV. The most striking observations are that (1) supercoiled DNA is approximately one order of magnitude more sensitive to the formation of double-strand breaks by low-energy electrons than is relaxed circular DNA, and (2) the dependence of the effective cross sections on the incident electron energy is unrelated to the corresponding ionization cross sections. This finding suggests that the traditional notion that radiobiological damage is related to the number of ionization events would not apply at very low energies.

Low-energy (5-40 eV) electron-stimulated desorption of anions from physisorbed DNA bases.

Abstract Abdoul-Carime, H., Cloutier P. and Sanche, L. Low-Energy (5–40 eV) Electron-Stimulated Desorption of Anions from Physisorbed DNA Bases. We present the results of experiments on anion desorption from the physisorbed DNA bases adenine, thymine, guanine and cytosine induced by the impact of low-energy (5–40 eV) electrons. Electron bombardment of DNA base films induces ring fragmentation and desorption of H–, O–, OH–, CN–, OCN– and CH2– anions through either single or complex multibond dissociation. We designate the variation of the yield of an anion with electron energy as the yield function. Below 15 eV incident electron energy, bond cleavage is controlled mainly by dissociative electron attachment. Above 15 eV, the portion of a yield function that increases linearly is attributed to nonresonant processes, such as dipolar dissociation. A resonant structure is superimposed on this signal around 20 eV in the anion yield functions. This structure implicates dissociative electron attachment and/or resonant decay of the transient anion into the dipolar dissociation channel, with a minimal contribution from multiple inelastic electron scattering. The yields of all desorbing anions clearly show that electron resonances contribute to the damage of all DNA bases bombarded with 5–40 eV electrons. Comparison of the ion yields indicates that adenine is the least sensitive base to slow electron attack. Electron-irradiated guanine films exhibit the largest yields of desorbed anions.

Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons

Abstract Panajotovic, R., Martin, F., Cloutier, P., Hunting, D. and Sanche, L. Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons. Radiat. Res. 165, 452–459 (2006). We determined effective cross sections for production of single-strand breaks (SSBs) in plasmid DNA [pGEM 3Zf(-)] by electrons of 10 eV and energies between 0.1 and 4.7 eV. After purification and lyophilization on a chemically clean tantalum foil, dry plasmid DNA samples were transferred into a high-vacuum chamber and bombarded by a monoenergetic electron beam. The amount of the circular relaxed DNA in the samples was separated from undamaged molecules and quantified using agarose gel electrophoresis. The effective cross sections were derived from the slope of the yield as a function of exposure and had values in the range of 10−15– 10−14 cm2, giving an effective cross section of the order of 10−18 cm2 per nucleotide. Their strong variation with incident electron energy and the resonant enhancement at 1 eV suggest that considerable damage is inflicted by very low-energy electrons to DNA, and it indicates the important role of π* shape resonances in the bond-breaking process. Furthermore, the fact that the energy threshold for SSB production is practically zero implies that the sensitivity of DNA to electron impact is universal and is not limited to any particular energy range.

Low energy electron induced DNA damage: effects of terminal phosphate and base moieties on the distribution of damage.

Low energy electrons (LEE) induce DNA damage by dissociative electron attachment, which involves base release (N-glycosidic bond (N-C) cleavage) and the formation of strand breaks (phosphodiester-sugar bond (C-O) cleavage). The effect of terminal phosphate and base moieties was assessed by exposing DNA model compounds to LEE in the condensed phase followed by HPLC-UV analysis of products remaining on the surface. First, we report that the presence of terminal phosphate groups in monomers (pT, Tp, pTp) and dimers (pTpT, TpTp, pTpTp) increases overall damage by 2-3-fold while it decreases N-C and C-O bond cleavage by 2-10-fold. This suggests that the capture of LEE directly by the terminal phosphate does not contribute to N-C and C-O bond cleavage. Second, we report that terminal bases appear to shield the internal base from damage, resulting in a bias of damage toward the termini. In summary, the presence of terminal phosphate base moieties greatly affects the distribution of LEE induced damage in DNA model compounds.

Phosphodiester and N-glycosidic bond cleavage in DNA induced by 4-15 eV electrons

Thin molecular films of the short single strand of DNA, GCAT, were bombarded under vacuum by electrons with energies between 4 and 15 eV. Ex vacuo analysis by high-pressure liquid chromatography of the samples exposed to the electron beam revealed the formation of a multitude of products. Among these, 12 fragments of GCAT were identified by comparison with reference compounds and their yields were measured as a function of electron energy. For all energies, scission of the backbone gave nonmodified fragments containing a terminal phosphate, with negligible amounts of fragments without the phosphate group. This indicates that phosphodiester bond cleavage by 4-15 eV electrons involves cleavage of the C-O bond rather than the P-O bond. The yield functions exhibit maxima at 6 and 10-12 eV, which are interpreted as due to the formation of transient anions leading to fragmentation. Below 15 eV, these resonances dominate bond dissociation processes. All four nonmodified bases are released from the tetramer, by cleavage of the N-glycosidic bond, which occurs principally via the formation of core-excited resonances located around 6 and 10 eV. The formation of the other nonmodified products leading to cleavage of the phosphodiester bond is suggested to occur principally via two different mechanisms: (1) the formation of a core-excited resonance on the phosphate unit followed by dissociation of the transient anion and (2) dissociation of the CO bond of the phosphate group formed by resonance electron transfer from the bases. In each case, phosphodiester bond cleavage leads chiefly to the formation of stable phosphate anions and sugar radicals with minimal amounts of alkoxyl anions and phosphoryl radicals.

Damage induced to DNA by low-energy (0-30 eV) electrons under vacuum and atmospheric conditions.

In this study, we show that it is possible to obtain data on DNA damage induced by low-energy (0-30 eV) electrons under atmospheric conditions. Five monolayer films of plasmid DNA (3197 base pairs) deposited on glass and gold substrates are irradiated with 1.5 keV X-rays in ultrahigh vacuum and under atmospheric conditions. The total damage is analyzed by agarose gel electrophoresis. The damage produced on the glass substrate is attributed to energy absorption from X-rays, whereas that produced on the gold substrate arises from energy absorption from both the X-ray beam and secondary electrons emitted from the gold surface. By analysis of the energy of these secondary electrons, 96% are found to have energies below 30 eV with a distribution peaking at 1.4 eV. The differences in damage yields recorded with the gold and glass substrates is therefore essentially attributed to the interaction of low-energy electrons with DNA under vacuum and hydrated conditions. From these results, the G values for low-energy electrons are determined to be four and six strand breaks per 100 eV, respectively.

On the role of low-energy electrons in the radiosensitization of DNA by gold nanoparticles

Four different gold nanoparticle (GNP) preparations, including naked GNPs and GNPs coated either with thiolated undecane (S-C(11)H(23)), or with dithiolated diethylenetriaminepentaacetic (DTDTPA) or gadolinium (Gd) DTDTPA chelating agents, were synthesized. The average diameters, for each type of nanoparticle, are 5 nm, 10 and 13 nm, respectively. Dry films of plasmid DNA pGEM-3Zf(-), DNA with bound GNPs and DNA with coated GNPs were bombarded with 60 keV electrons. The yields of single and double strand breaks were measured as a function of exposure by electrophoresis. The binding of just one GNP without coating to DNA containing 3197 base pairs increases single and double strand breaks by a factor of 2.3 while for GNPs coated with S-C(11)H(23) this factor is reduced to 1.6. The GNPs coated with DTDTPA and DTDTPA:Gd in the same ratio with the DNA, produce essentially no increment in damage. These results could be explained by the attenuation by the coatings of the intensity of the low-energy photoelectrons emitted from the GNPs. Thus, coatings of GNPs may considerably attenuate the short-range low-energy electrons emitted from gold, leading to a considerable decrease of radiosensitization. According to our results, the highest radiosensitization should be obtained with GNPs having the shortest possible ligand, directed to the DNA of cancer cells.

Low-energy electron-induced DNA damage: effect of base sequence in oligonucleotide trimers.

DNA damage induced by low-energy electrons (LEEs) has attracted considerable attention in recent years because LEEs represent a large percentage of the total energy deposited by ionizing radiation and because LEEs have been shown to damage DNA components. In this article, we have studied the effect of base sequences in a series of oligonucleotide trimers by the analysis of damage remaining within the nonvolatile condensed phase after LEE irradiation. The model compounds include TXT, where X represents one of the four normal bases of DNA (thymine (T), cytosine (C), adenine (A), and guanine (G)). Using HPLC-UV analysis, several known fragments were quantified from the release of nonmodified nucleobases (T and X) as well as from phosphodiester C-O bond cleavage (pT, pXT, Tp, and TXp). The total damage was estimated by the disappearance of the parent peaks in the chromatogram of nonirradiated and irradiated samples. When trimers were irradiated with LEE (10 eV), the total damage decreased 2-fold in the following order: TTT > TCT > TAT > TGT. The release of nonmodified nuclobases (giving from 17 to 24% of the total products) mainly occurred from the terminal sites of trimers (i.e., T) whereas the release of central nucleobases was minor (C) or not at all detected (A and G). In comparison, the formation of products arising from phosphodiester bond cleavage accounted for 9 to 20% of the total damage and it partitioned to the four possible sites of cleavage present in trimers. This study indicates that the initial LEE capture and subsequent bond breaking within the intermediate anion depend on the sequence and electron affinity of the bases, with the most damage attributed to the most electronegative base, T.

Dissociative attachment reactions in electron stimulated desorption from condensed O2 and O2‐doped rare‐gas matrices

Desorption of the ions O−, O−2,O−3 (and/or O2⋅O−) induced by electron impact on pure O2 multilayer films and Ar, Kr, and Xe matrix films containing O2 is reported. In addition to these anions, the ionic complexes M⋅O− (M=Ar and Kr) are also observed to desorb from Ar and Kr matrices, respectively. In the range 4–16 eV, the incident electron energy (Ei) dependence of the yields (i.e., the yield functions) of all the diatomic and triatomic anions exhibit features which can be correlated with the O− yield function; indicating that, these anions are produced by dissociative attachment reactions whose first step involves the formation of O−2 quasibound states. From analysis of all yield functions and variations of the anion yields as a function of O2 concentration in the matrices, we find that the simplest dissociative transient state, which can propel in vacuum an M⋅O− or O2⋅O− ion, must have the configuration M⋅O2⋅O−*2. To explain the formation of O−2 and O−3 ions below Ei≂6 eV, the existence of an electroni...

Measurements of charge accumulation induced by monochromatic low-energy electrons at the surface of insulating samples

We investigate charging of insulators with an apparatus that allows measurements of trapped charges resulting from the impact of monoenergetic electrons of 0.1–28 eV. Details are given on the construction and operation of this instrument. A high-resolution electron monochromator provides a pulsed electron beam of variable energy and current. Accumulated surface charge is monitored using a Kelvin probe and a high-sensitivity electrometer. An ultraviolet source of adjustable maximum frequency allows the sample to be discharged for multiple measurements on the same sample. We illustrate the use of the instrument with preliminary measurements for ∼100 μm thick samples cut from an industrial polyethylene cable. The incident electron-energy dependence of the trapping probability exhibits large variation and indicates that electrons with energies <5 eV are the most efficiently trapped; charging near 10 eV is attributed to dissociative electron attachment to polyethylene molecules.