ABSTRACT
Attachment .........................................................................202 6.3 Large DNA Molecular Structures ................................................................203
6.3.1 Concluding Remarks on Large DNA Molecular Structures .............220 6.4 Appendix: Abbreviations ............................................................................. 221 Acknowledgment ................................................................................................... 223 References .............................................................................................................. 223
Biomolecules are quite sensitive to damage caused by high-energy radiation of natural or man-made origin. There is a sequence of three major groups of initial events that can lead to damage: primary, secondary, and reactive (Sanche 2009). The “rst two are considered in Figure 6.1 for the case of a two-component biomolecules AB. Multiply charged products are omitted from this “gure. The direct, primary processes are ionization and excitation which can lead to dissociation. In cells, single and multiple ionization leads to the production of copious amounts of low-energy secondary electrons of the order 3 × 104/MeV of deposited energy. These electrons thus carry a large fraction of the energy of the impinging radiation. Their most probable energy lies between 9 and 10 eV (Pimblott and LaVerne 2007). These low-energy electrons (LEEs) can interact resonantly or directly with the irradiated biomolecules; the latter interaction is depicted by the elastic channel and left vertical arrow in Figure 6.1, which represents inelastic channels. These latter channels, along with electron recombination with ions, can produce excited neutral species. The resonance processes involve the formation of a transient negative ion (TNI) having a lifetime lying between 10−15 and 10−3 s. The extra electron in the TNI can occupy a usually empty orbital of the ground state molecule (i.e., a shape resonance) or of an electronically singly excited state of the molecule (i.e., a core-excited resonance). If the energy of the core-excited resonance is less than that of the parent electronically excited state of the neutral molecule, the incoming electron is captured essentially by the electron af“nity of that state; in this case, it falls in the category of Feshbach resonances which usually have a fairly long
FIGURE 6.1 Reactions induced by primary ionizing radiation and secondary electrons on a molecule AB. (Adapted from Sanche, L. 2009. Low-Energy Electron Interaction with DNA: Bond Dissociation and Formation of Transient Anions, Radicals, and Radical Anions, pp. 239-293. John Wiley & Sons, Inc.; Sanche, L./Greenberg, M. (Ed.), Radical and Radical Ion Reactivity in Nucleic Acid Chemistry, pp. 239-293, 2010. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reprinted with permission.)
lifetime, of the order of 10−12 to 10−14 s. When the TNI state lies above the parent electronically excited state, it is called a core-excited shape resonance. The TNI can decay via resonance stabilization, dissociation, or inelastic scattering leaving behind an excited molecule. The excited molecules that are produced by secondary processes can also ionize or dissociate as shown in the upper part of Figure 6.1. Finally, the radicals and ions produced by primary and secondary processes proceed through reactive scattering and chemical reactions with nearby biomolecules.
