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Precursors of Solvated Electrons in Radiobiological Physics and Chemistry Elahe Alizadeh and Léon Sanche* Groupe en Sciences des Radiations, Département de Médecine Nucléaire et Radiobiologie, Faculté de médecine et des sciences de la santé, Université de Sherbrooke, J1H 5N4 Sherbrooke, Canada usually not considerably perturbed. Under this condition, the interaction can be described within the first Born approximation.3−5 The validity of this approximation leads to the concept of generalized oscillator strength in the limit of small momentum transfer.4,5 This means that a fast primary charged particle can be seen as emitting electromagnetic radiation of all wavelengths with equal amplitudes. It follows that the quanta of energy absorbed by the molecules along the path of the fast charged particles depends on the probability of these molecules (i.e., the optical oscillator strength) to absorb photons of different wavelength. Since the optical oscillator strength for small (e.g., H2O) and large (e.g., DNA) biomolecules is greatest at an energy of about 22 eV,6 when this primary interaction leads to ionization, the CONTENTS distribution of electrons has a maximum below 15 eV. Highenergy photons (i.e., X- or γ-rays), usually produce a primary 1. Introduction A fast electron by Compton scattering or the photoelectric effect.7 2. Secondary Electrons in Water C Thus, the above reasoning also applies to high-energy 2.1. Primary Processes in Water C electromagnetic radiation. 2.2. Presolvated Electrons in Water E From these fundamental considerations, it can be realized 2.3. Monte Carlo Track Structure in Water E that damage to biomolecules is mainly created via the action of 3. LEEs Damage to Cellular Constituents H ions and secondary electrons (SEs) ranging from near zero eV 3.1. LEE Interactions with DNA I 3.1.1. Single DNA Building Blocks I up to half of the energy of the primary charged particle. These 3.1.2. DNA Backbone J SEs are precursors to solvated electrons. This review article 3.1.3. Short DNA and Plasmid DNA K concerns the interactions of such electrons in the time before 3.2. LEE Damage to Protein Subunits: Amino they become solvated and, more specifically, their interactions Acids and Peptides L with biomolecules and the damage they induce. Although the 3.3. Approaching LEE Interactions under Cellular energy distribution of SEs has a most probable energy ∼9−10 Conditions L eV,8 those of initial higher energy undergo successive energy 3.4. LEEs in Radiobiology: Radiosensitization and losses via, for example, electronic excitation and ionization. The Radiation Therapy P latter creates further generations of electrons of increasingly 4. Prehydrated Electrons Interacting with Biomolelower energies. Thus, as all electrons necessarily reach the low cules Q energy range (E < 30 eV), it becomes crucial to determine the 5. Summary and Conclusions S interaction of and damage created by low-energy electrons Author Information T (LEEs) in biological media. As SEs slow down, they form Corresponding Author T reactive species, i.e., ions, excited molecules, and free radicals. Notes T Electrons, with energy below the electronic excitation threshold Biographies T of biomolecules, slow down by losing energy to intramolecular Acknowledgments U and intermolecular vibrations of the medium, until they become Abbreviations U less mobile as the molecules orient toward the charge. The References U action of all SEs generated at different energies and the reactions they induce are, however, complex and are usually best described with probabilistic models in Monte Carlo 1. INTRODUCTION calculations. When a high-energy charged particle interacts with a biomolecule in living tissues or cells, about 80% of the energy Special Issue: 2012 The Solvated Electron absorbed ionizes the medium, whereas the rest produces 1,2 electronic and vibrational excitation. As a fast charged particle Received: February 15, 2012 moves in the biological medium, its energy and momentum is © XXXX American Chemical Society
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dx.doi.org/10.1021/cr300063r | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
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Figure 1. Schematic diagram showing the approximate time scale of events in water radiolysis. The time it takes for an electron in condensed water to localize in the trapping states is indicated on the right, and the inset shows the SE solvation process in more detail.
molecules, electron or ions are solvated with molecules to form solvated electrons and ions, respectively. The recombination of positive ions with electrons, particularly in the condensed phase, is classified as geminate and bulk recombination. The relative importance of these two processes is closely related to the transport mechanism for electrons, i.e., to the magnitude of the electron drift mobility on the electron conduction band in condensed medium. Phenomenologically, it is more dependent on the properties of the medium molecules. In some cases, particularly in admixed systems, in both the gas and condensed phases, energy or charge transfer processes occur from molecules of the major component in the system to the added solute or dopant molecules.12 A large portion of this article is devoted to the interaction of the precursors of the solvated electron with DNA. The most significant effect of ionizing radiation in biology arises from radiation-induced genomic DNA damage. The interaction of a high-energy ionizing radiation with a living cell results in the deposition of energy into target molecules that can ultimately cause collapse of cellular functions, resulting in programmed cell death (apoptosis) within hours or days.13,14 Thus, while the biological effects of ionizing radiation are evident as slow
The interactions occurring during slowing-down processes of electrons in matter take place during the physical and physicochemical stages, that is, within a time scale of 10−12 s. During these stages, radical species are produced.9 By further energy exchange with the medium, LEEs finally become trapped by electrostatic interaction with the induced and permanent dipoles (and higher multipoles) of the surrounding biomolecules. At this stage, electrons can be considered as being thermalized, and later they become solvated. If they exist in an excited state of the solvation cage, they are considered as presolvated. For example, interaction of LEEs in water results in most of them being solvated (more specifically referred to as hydrated in water) within 10−12 s. In this state, they are trapped in a deep potential well of ∼3.2 eV. Electrons in the thermal and epithermal ranges disappear predominantly by recombination, attachment, or diffusion. Electrons with characteristic energies are selectively captured by some molecules to form stable negative ions. It is generally accepted that molecular clusters, large aggregates of molecules, and molecules in the condensed phase can capture electrons with large cross sections due to electron attachment dynamics different from those in the gas phase (i.e., isolated single molecules).10,11 For polar B
dx.doi.org/10.1021/cr300063r | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
Review
characteristic temporal stages, which begins with the physical (
