**8.1. Transient anions in halogen compounds**

After such extensive studies on LEE‐induced damage under "near"‐cellular conditions, it was only very recently that the lethal effects of LEEs in cells have been demonstrated by Sahbani et al. [154], who investigated the biological functionality of DNA, via a simple model system comprising *E. coli* bacteria and plasmid DNA bombarded by LEEs. In these experiments, highly ordered DNA films were arranged on pyrolytic graphite surface by molecular self‐assembly technique using 1,3‐diaminopropane ions to bind together the plasmid DNAs [155]. This assembly technique mimics somewhat the action of amino groups of the lysine and arginine amino acids within the histone proteins. These authors measured the transformation efficiency

**Figure 5.** (a) Variation of transformation efficiency of *E coli* by pGEM 3Zf(‐) plasmids irradiated by 0.5–18 eV electrons

. The vertical axis is inverted. Effective yield functions for (b) single‐strand breaks

at a fluence of 27 × 1013 electrons/cm2

200 Radiation Effects in Materials

(SSBs), (c) double‐strand breaks (DSBs), and (d) DNA cross‐links [183].

Bromouracil, which can replace thymine in DNA during cell replication, and bromouridine were the first radiosensitizing candidates to be investigated theoretically and experimentally with LEEs. The studies [157–167] confirmed the prediction of Zimbrick and coworkers [168] that the radiosensitizing properties of these compounds arose from DEA of solvated electrons, and further showed that DEA of higher energy (0–7eV) electrons was also involved in radiosensitization. Platinum bromide, aromatic compounds containing nitro group and other halogenated thymidine derivatives were found to play similar roles [58, 70, 71, 163–165, 169, 170]. Following early investigations with solvated electrons [168], a relatively large number of experiments have been performed both in the gas (see Section 4.4) and condensed phases [160, 161, 165] to study electron scattering from––and attachment to––halogenated pyrimidines. Several experiments were performed using SAMs of BrdU‐containing oligonucleotides [157, 158, 171]. These included the detection of the electron‐stimulated desorption of ion and neutral species and HPLC analysis of damaged films, as well as electronic and vibrational electron‐ energy loss spectra for gaseous bromouracil [159]. These studies revealed that the radiosensi‐ tization properties of halogen compounds are more complicated than previously anticipated [168]. Within the 0–7 eV energy range, resonant electron scattering mechanisms with halour‐ acils lead to more complex molecular fragmentation than that occurs with thymine, which produces a different range of anionic and neutral radical fragments. When formed within DNA, such fragments could react with local subunits, and thus lead to lethal clustered damage, further to that already occurring in unsensitized DNA. The most striking evidence of a huge enhancement of LEE damage obtained upon Br substitution in thymine is seen in the early results of Klyachko et al. [160], who found that, in the presence of water, DEA to bromouracil could be enhanced by orders of magnitude compared to the dry compound. Differences between wet and dry TMA states of halogenated pyrimidines have recently been investigated by Cheng et al. [172]. They applied Koopman's theorem in the framework of long‐range corrected density functional theory for calculation of the TMA states and self‐consistent reaction field methods in a polarized continuum to account for the solvent. Their results indicate that the TMAs of these molecules are more stable in water, but to differing degrees.

The radiosensitization properties of halouracils depend not only on hydrated electrons, but also on LEEs and on DEA. However, the high propensity of LEEs of very low energies (i.e., <1 eV) to fragment bromouracil and deoxybromouridine (BrUdR) may, according to the theory, exist only in single‐stranded DNA [165]. This important prediction was confirmed by Cecchini et al. [173] for the case of solvated electrons and was commented upon by Sevilla [174]. Solutions of single‐ and double‐stranded oligonucleotides, and of double‐stranded oligos containing mismatched bubble regions, were irradiated with *γ*‐rays, and the concentrations of various reactive species produced, including solvated electrons, were controlled with scav‐ engers. When in the absence of oxygen, OH radicals were scavenged, BrUdR was shown to sensitize single‐stranded DNA, but could not sensitize complementary double‐stranded DNA. However, when BrUdR was incorporated in one strand within a mismatch bubble, the nonbase‐paired nucleotides adjacent to the BrUdR, as well as several unpaired sites on the opposite unsubstituted strand, were highly sensitive to *γ*‐irradiation. Since LEEs and solvated electrons fragment BrUdR by the same DEA mechanism [162–165, 168], these results imply that the strong sensitizing action of BrUdR to electron‐induced damage is limited to single‐ stranded DNA, which can be found in transcription bubbles, replication forks, DNA bulges, and the loop region of telomeres. These results are clinically relevant since they suggest that BrUdR sensitization should be greatest for rapidly proliferating cells [173, 174]. When injected into a patient being treated for cancer, BrUdR quickly replaces a portion of the thymidine in the DNA of the fast‐growing malignant cells, but radiosensitization occurs only when DNA is in a single‐stranded configuration (e.g., at the replication forks during irradiation). From this conclusion, it appears advantageous to administer to patients receiving BrUdR, another approved drug, such as hydroxyurea, to increase the duration of the S‐phase of cancer cells (i.e., the replication cycle). This addition would increase the probability that SEs would interact with bromouracil while bound to DNA in its single‐strand form. Such a modality provides an example of how our understanding the mechanisms of LEE‐induced damage can help to improve radiotherapy [174].
