Publisher Summary The integrity of DNA is essential for the well-being of a cell. Damage to this molecule, whether caused by reaction with chemical mutagens, by irradiation with ultraviolet (UV) light, or by ionizing radiation, has dire consequences. Thus, much effort has been expended in attempting to understand the effects of these agents on DNA. This chapter describes some of the chemical mechanisms whereby DNA is damaged by ionizing radiation. The major advances in understanding of molecular mechanisms have come from the studies of simple model systems. The chapter explains the three basic model systems that have been designed to examine radiation-induced damage to DNA. Each type of system yields useful information, but the applicability of this information to the in vivo situation is limited in all cases. The chapter also discusses the indirect effects of the free radicals produced by the radiolysis of water, in terms of both initial species and final products.

Molecular mechanisms of radiation-induced damage to nucleic acids

Publisher Summary This chapter discusses the chemical consequences of radiation damage to DNA Irradiation of DNA causes ionizations in all parts of the DNA molecule, namely, the bases, the sugar-phosphate moiety, and any closely bound water; in addition, bound proteins are also ionized Even though ionizations occur throughout the DNA molecules, initial radicals have been observed only on the DNA bases in low-temperature ESR studies of moist DNA Radiation damage to DNA occurs in a number of ways, which, are classified as either direct or indirect The direct effect corresponds to direct ionization of the DNA resulting in the formation of radical cations and radical anions on the DNA itself The indirect effect corresponds to energy deposition in the surrounding phase followed by attack by radicals from this phase However, in complex systems such as the cell, damage may also be caused by irradiation of molecules bound to the DNA that transfer positive holes and/or electrons to the DNA strands or later react by cross-linking or hydrogen abstraction The quasi-direct effect refers to a third process, where ionizations from irradiation result in holes and dry electrons in biomolecular species and hydration water very near the DNA These then undergo fast transfer to the DNA to form ion radicals on the DNA itself The indirect effect produces many of the same radicals and diamagnetic products as the direct and quasi-direct effects

The chemical consequences of radiation damage to DNA

Publisher Summary The goal of the nucleic acid radiation chemist is to predict the quality and quantity of chemical alterations induced in DNA exposed to ionizing radiation. To accomplish this, problems of identifying a small number of alterations in a polymer of high molecular weight must be overcome. One approach has been to study model compounds, usually constituents of DNA. Model compounds have been studied primarily by end-product analysis, pulsed radiolysis of liquid systems, and trapping of unstable intermediates in the solid state. Most solid-state studies have centered on electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) techniques. This chapter reviews the information obtained by only ESR and ENDOR of solid samples, and discusses free-radical products and reactions. The chapter presents an update on the possible events in irradiated DNA and the schemes. It also discusses the ESR characteristics and reliability of free-radical assignments. The chapter further presents some equations and notations to provide some assistance to the non-ESR specialist and to point out some considerations that are sometimes overlooked but are necessary for a correct analysis of ESR data.

Solid-State Radiation Chemistry of DNA: The Bases1

Ionic base radicals in γ-irradiated DNA

SummaryThe free radicals induced by γ-irradiation at 77 K in non-deuterated and fully deuterated oriented samples of moist algal DNA have been studied by electron paramagnetic resonance (e.p.r.). The e.p.r. spectra from the non-deuterated sample were found to be similar to those from calf-thymus DNA studied previously. Numerical spectra simulations were performed based on the previously proposed interpretation of the DNA spectra at 77 K in terms of a mixture of two spectral components arising from anionic and cationic base radicals (probably on thymine and guanine, respectively). The simulations were found to account satisfactorily for the main characteristics of the e.p.r. spectra from both the non-deuterated and the fully deuterated algal DNA sample.

Ionic Base Radicals in γ-irradiated Oriented Non-deuterated and Fully Deuterated DNA

SummaryThe reactions of electrons with some pyrimidine derivatives were investigated. The electrons, produced photolytically in alkaline glasses, were reacted with solutes to form anions. It was found that anions of thymidine and thymidine-5′-phosphate easily undergo proton transfer from surrounding water molecules to the C6 position of the thymine ring. Anions of uracil and deoxycytidine do not undergo corresponding proton transfer reactions in the aqueous matrix at low temperatures. The role of electrons in the radiolysis of thymine-containing compounds including DNA is discussed. It is shown that electron reactions can account for the formation of the eight-lined E.S.R. spectrum formed in the radiolysis of these compounds.

Radicals Formed by Electron Transfer to Pyrimidine Derivatives

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Radicals Formed by Electron Transfer to Pyrimidine Derivatives