Clustered DNA damages are the most detrimental modifications induced by ionizing radiation in cells and several mechanisms have been proposed for their formation. We report measurements of such damages induced by a single low energy electron via the formation of the two major core-excited resonances of DNA located at 4.6 and 9.6 eV. Cross-links and single and double strand breaks (SSBs and DSBs) are analyzed by gel electrophoresis. Treatment of irradiated samples with Esherichia coli base excision repair endonucleases reveals base damages (BDs). DSBs resulting from such treatments arise from clustered damages consisting of at least two BDs or one BD accompanied by a SSB. The total DNA damages induced by 4.6 and 9.6 eV electrons are 132 ± 32 and 201 ± 36 × 10–15 electron–1 molecule–1, comprising 43% and 52% BDs, respectively. We propose a unifying mechanism to account for these clustered damages, DSBs, and single BDs, as well as all previously measured isolated lesions.

Vacuum experiments with monoenergetic low-energy electrons (LEEs) impacting on biomolecules have revealed basic mechanisms of radiobiological damage. To move closer to cellular conditions, we explore the possibility of damaging DNA at standard atmospheric pressure with electrons of 0–1.5 eV. These electrons are produced by UV light incident on a tantalum or n-Si substrate, onto which a five-monolayer film of DNA is deposited. Total conformational damage, cross-links, and double- and single-strand breaks (SSBs) are measured as a function of LEE fluence. The photoelectron energy distribution can be modified by changing the work function of the substrate or the photon energy. In a N2 atmosphere with a tantalum substrate, the yields of SSBs (5 ± 2 × 10–14 electron–1 molecule–1) and their number per energy deposited (46 ± 36 × 10–4/eV) are in good agreement with those obtained, under similar conditions, by monoenergetic 0.5–1.0 eV electron impact in vacuo and 1.5 KeV X-ray photoelectrons at atmospheric pressure, respectively. SSB is the only conformational change detected; it results from electron capture on a DNA base followed by dissociative electron attachment at the phosphate unit.

Platinum chemotherapeutic agents, such as cisplatin (cis-diamminedichloroplatinum(II)), can act as radiosensitizers when bound covalently to nuclear DNA in cancer cells. This radiosensitization is largely due to an increase in DNA damage induced by low-energy secondary electrons, produced in large quantities by high-energy radiation. We report the yields of single- and double-strand breaks (SSB and DSB) and interduplex cross-links (CL) induced by electrons of 1.6–19.6 eV (i.e., the yield functions) incident on 5 monolayer (ML) films of cisplatin–DNA complexes. These yield functions are compared with those previously recorded with 5 ML films of unmodified plasmid DNA. Binding of five cisplatin molecules to plasmid DNA (3197 base pairs) enhances SSB, DSB, and CL by factors varying, from 1.2 to 2.8, 1.4 to 3.5, and 1.2 to 2.7, respectively, depending on electron energy. All yield functions exhibit structures around 5 and 10 eV that can be attributed to enhancement of bond scission, via the initial formation of core-excited resonances associated with π → π* transitions of the bases. This increase in damage is interpreted as arising from a modification of the parameters of the corresponding transient anions already present in nonmodified DNA, particularly those influencing molecular dissociation. Two additional resonances, specific to cisplatin-modified DNA, are formed at 13.6 and 17.6 eV in the yield function of SSB. Furthermore, cisplatin binding causes the induction of DSB by electrons of 1.6–3.6 eV, i.e., in an energy region where a DSB cannot be produced by a single electron in pure DNA. Breaking two bonds with a subexcitation-energy electron is tentatively explained by a charge delocalization mechanism, where a single electron occupies simultaneously two σ* bonds linking the Pt atom to guanine bases on opposite strands.

X-ray photoelectron spectroscopy (XPS) is harnessed as an in situ efficient characterization technique for monitoring chemical bond transformation in DNA and cisplatin–DNA complexes under synergic X-ray irradiation. By analyzing the variation of relative peak area of core elements of DNA as a function of irradiation time, we find that the most vulnerable scission sites in DNA are those containing phosphate and glycosidic bonds. Compared to DNA, the effective rate constants of the corresponding phosphodiester and glycosidic bond cleavages for cisplatin–DNA complexes are 1.8 and 1.9 folds larger. These damages and their enhancements are similar to those induced by low energy electrons (LEE). Consistently, the magnitude of the secondary electron distribution produced by the X-rays on the cisplatin–DNA complexes is considerably increased compared to that of pristine DNA. The data suggest that DNA radiosensization by cisplatin results not only from the sensitization of DNA to the action of LEE, but also from an increase the production of LEE at the site of binding of the cisplatin. The results provide new insights into the mechanisms of cisplatin-induced sensitization of DNA under X-ray irradiation, which could be helpful in the design of new cisplatin-based antitumor drugs.

Gold nanoparticles (GNP) passivated by dithiolated diethylenetriaminepentaacetic (DTDTPA) linkers, GNP@DTDTPA, have been synthesized. The well-defined ZnO nanocomposite (GNP@DTDTPA/ZnO) functionalized by GNP@DTDTPA was prepared via a facile and green self-assembly approach. The specific interaction mechanism responsible for the self-assembly motif was elucidated by XPS, zeta potential and FTIR analysis. The self-assembly process was established primarily by a large amount of polar functional groups such as carboxyl (COOH), carbonyl (CO), and amide (NH–CO) groups in the DTDTPA profile, which impels GNP@DTDTPA to bind intrinsically with hydroxyl groups on the ZnO surface through hydrogen bonding interactions. On the other hand, the attractive electrostatic force between the negatively charged GNP@DTDTPA and the positively charged ZnO surface also contributes to the monodispersivity of GNP@DTDTPA on the ZnO support. The GNP/ZnO obtained after calcination of GNP@DTDTPA/ZnO retains the mono-distribution of GNP and exhibits more enhanced photocatalytic and photoelectrochemical performances compared to pure ZnO. We propose a possible mechanism that the well-distributed GNP could serve as an “electron reservoir” and improve the separation efficiency of photogenerated electron–hole pairs. This method could provide a simple and straightforward approach for achieving a uniform distribution of noble-metal nanoparticles on the surface of semiconductors for versatile photocatalytic and photoelectrochemical applications.

Absolute cross sections (CSs) for the interaction of low energy electrons with condensed macromolecules are essential parameters to accurately model ionizing radiation induced reactions. To determine CSs for various conformational DNA damage induced by 2–20 eV electrons, we investigated the influence of the attenuation length (AL) and penetration factor (f) using a mathematical model. Solid films of supercoiled plasmid DNA with thicknesses of 10, 15 and 20 nm were irradiated with 4.6, 5.6, 9.6 and 14.6 eV electrons. DNA conformational changes were quantified by gel electrophoresis, and the respective yields were extrapolated from exposure–response curves. The absolute CS, AL and f values were generated by applying the model developed by Rezaee et al. The values of AL were found to lie between 11 and 16 nm with the maximum at 14.6 eV. The absolute CSs for the loss of the supercoiled (LS) configuration and production of crosslinks (CL), single strand breaks (SSB) and double strand breaks (DSB) induced by 4.6, 5.6, 9.6 and 14.6 eV electrons are obtained. The CSs for SSB are smaller, but similar to those for LS, indicating that SSB are the main conformational damage. The CSs for DSB and CL are about one order of magnitude smaller than those of LS and SSB. The value of f is found to be independent of electron energy, which allows extending the absolute CSs for these types of damage within the range 2–20 eV, from previous measurements of effective CSs. When comparison is possible, the absolute CSs are found to be in good agreement with those obtained from previous similar studies with double-stranded DNA. The high values of the absolute CSs of 4.6 and 9.6 eV provide quantitative evidence for the high efficiency of low energy electrons to induce DNA damage via the formation of transient anions.

Gold nanoparticles (GNP) passivated by dithiolated diethylenetriaminepentaacetic (DTDTPA) linkers, GNP@DTDTPA, have been synthesized. The well-defined ZnO nanocomposite (GNP@DTDTPA/ZnO) functionalized by GNP@DTDTPA was prepared via a facile and green self-assembly approach. The specific interaction mechanism responsible for the self-assembly motif was elucidated by XPS, zeta potential and FTIR analysis. The self-assembly process was established primarily by a large amount of polar functional groups such as carboxyl (COOH), carbonyl (CO), and amide (NH–CO) groups in the DTDTPA profile, which impels GNP@DTDTPA to bind intrinsically with hydroxyl groups on the ZnO surface through hydrogen bonding interactions. On the other hand, the attractive electrostatic force between the negatively charged GNP@DTDTPA and the positively charged ZnO surface also contributes to the monodispersivity of GNP@DTDTPA on the ZnO support. The GNP/ZnO obtained after calcination of GNP@DTDTPA/ZnO retains the mono-distribution of GNP and exhibits more enhanced photocatalytic and photoelectrochemical performances compared to pure ZnO. We propose a possible mechanism that the well-distributed GNP could serve as an “electron reservoir” and improve the separation efficiency of photogenerated electron–hole pairs. This method could provide a simple and straightforward approach for achieving a uniform distribution of noble-metal nanoparticles on the surface of semiconductors for versatile photocatalytic and photoelectrochemical applications.