Direct Effects of Ionizing Radiation on Macromolecules.

There is considerable literature concerning the effects of ionizing radiation on matter. In general, the irreversible effects of radiation are due to the ionization of valence electrons. In all cases, irradiation of macromolecules in liquid (usually aqueous) solution resulted in much greater effects than irradiation of the same material when dry or frozen. This is due to the diffusible radiation products, primarily from the solvent, in which the specific radiation effect depends on the particular radiolytic product as well as the particular reactive ligand on the macromolecule. Dry samples obviously have no solvent radiation products. In frozen samples, the same initial radiation products are created as in the liquid state. However, the diffusion constant of these products differs by a factor greater than 105. In the frozen state these products still react (albeit more slowly because of the lower temperature), but are restricted to only nearby molecules. All the neighboring molecules essentially are other solvent molecules. Therefore, in both the frozen and dry states, virtually all the radiation damage to biopolymers and to synthetic polymers is that due to the interaction of the radiation directly on that molecule – this is the direct effect of radiation. Studies of these direct effects of ionizing radiations on macromolecules have been widely reported and many remarkable findings were published many years ago, most notably by Charlesby, Chapiro and Dole (1-3) in synthetic macromolecules and Pollard (4) in biological macromolecules. Many similar observations were reported, such as the fact that radiation sensitivity was dependent on temperature (5). Subsequently a vast literature has appeared in both fields, mostly without awareness of the other field. There has evolved a detailed picture of direct radiation effects identical in both kinds of molecules. Properties described in molecules of one species can therefore be studied further in the other species. High energy electrons interact randomly with all electrons in their trajectory (6). In each interaction there is a transfer of energy from the radiation to the exposed material. The transfer of energy from radiations to target materials occurs over a wide range of energy. Starting in 1929 (7), an extensive literature concerns the loss of energy (‘straggling’) of high energy electrons. The spectrum of energy losses has been widely described, especially for high energy electrons (6, 8,9). There are several putative mechanisms for these interactions with different amounts of energy transferred, such as the ‘glancing’ and ‘head-on’ described by Fano (9). Less attention was focused on the fate of this energy in irradiated matter. If the amount of energy transferred is small, the orbital electrons are excited to a higher energy level in the atom. After a period of time, they drop down to a lower energy level and emit excess energy, often as ultraviolet light. The atom and its’ associated molecule are returned to their original state. However, if the energy transferred is greater than the ionization potential of an electron, the electron is ejected from that atom (called a ‘primary ionization’; the emitted electron is called a ‘secondary electron’). Energy depositions larger than the ionization potential would leave ‘excess’ energy which could be donated as kinetic energy to the ejected electron or absorbed by the parent atom or molecule. The energy spectrum of emitted electrons has been determined in many different materials. The ‘excess’ (kinetic) energy of these secondary electrons is very small (9), “a few eV” (10). According to Fano (9) “… the great majority of the secondary electrons have a rather low energy. …. can transfer energy only to the external electrons of atoms, and this only when passing right through or very close to an atom. … Low-energy secondaries dissipate most of their energy within a short distance from their point of origin. This distance is of the order of 10 A in solid or liquid materials ….”. At 25 C. the range of 0.1 eV electrons in water is close to 1 nm, while for 10 eV electrons it is ~10 nm (11). This indicates that (a) the parent molecule is the principal recipient of the additional energy and (b) little further damage is caused by the secondary electrons. The minimum energy to cause an ionization in a molecule is therefore dependent on the minimum ionization potential of the constituent atoms. Ionization potentials of inner electrons are considerably higher. However, if any of these are ejected, outer electrons will fall into that hole resulting in ionization of the valence electron. Protein molecules are composed of five major elements, H C O N and S, and their lowest ionization potentials range from 10.357 to 14.53 eV. In a typical 50 000 molecular weight protein, the relative abundance of each of these atoms is known. Thus the energy deposition for the minimum ionization can easily be calculated. Similar calculations can also be done for polyethylene and for a typical cell. The latter uses the elemental composition of a dry cell, otherwise oxygen and hydrogen would overwhelm the analysis. The minimum transfer that causes an ionization is described above. From these data, the low energy spectrum can be determined [Fig 1]. This shows why the spectrum has a maximum (the most probable energy deposition) near 20 eV. Above 20 eV there are contributions due both to energy transfers in excess of the ionization potential and also interactions with inner electrons. At the high end of the energy depositions, the distribution losses follow the inverse square law (as E−2) (8,12). In these spectra, the “most probable” energy loss is 20 eV and the average energy deposition is near 60 ev - which has been experimentally verified (13) and theoretically predicted (14). Figure 1 Fraction of ionizations due to each element versus low energy depositions for a typical protein (squares), polyethylene (triangles) and a typical dry cell (diamonds).