A PHYSICAL MECHANISM FOR THE INACTIVATION 

 OF PROTEINS BY IONIZING RADIATION 



Robert Platzman and James Franck 



Department of Physics, Purdue University, Lafayette, Indiana and 



Research Institutes {Pels Fund), The University of Chicago, 



Chicago, Illinois 



Abstract — The extraordinary sensitivity of living systems and certain of their components to 

 ionizing radiation must stem, at least in part, from a great sensitivity of individual molecular 

 or macromolecular species. Analysis of the interaction of such species and swiftly moving 

 charged atomic particles shows that the initial events of energy transfer cannot be responsible 

 for this sensitivity, but that events immediately subsequent to ionization acts definitely can be. 

 This is because the time scale for the production of new electric charges is so short as to evoke 

 a violent reaction of the medium, a reaction which is related intimately to the dielectric be- 

 havior at very great frequencies. Such behavior is not as yet fully explored experimentally, 

 for many of the frequencies concerned lie between the readily accessible infrared and the 

 microwave regions, but the known dielectric properties of highly polar systems like protein 

 and nucleic-acid macromolecules do disclose the existence of regions of strong dielectric 

 absorption, which are to be identified with dipole oscillations and rotations of polar atoms 

 and molecular groupings. The wave of polarization to which a sudden production of electric 

 charge in the interior of the macromolecule gives rise must cause profound degradation of the 

 molecular organization. This may be viewed as resulting from rupture of many weak polar 

 bonds (such as hydrogen bonds) which maintain the intricate organization and which are 

 involved in the above-mentioned dielectric absorption, the ruptures being essentially simul- 

 taneous. The dynamic effect on the molecular structure therefore is without parallel in any 

 other variety of action presently accessible to experimental study — for example, thermal, 

 chemical, or electrochemical action, all of which are in the present context essentially adiabatic 

 in character. The mechanism clearly explains at once the striking difference in sensitivity of 

 the media to ionizing radiation and to ultraviolet light. An approximate quantitative analysis 

 suggests that inactivation of common proteins by a single ionization act is unlikely, but 

 rather that several may be required. Since the effects of the ionizations in a particle track or 

 electron spur are additive (these events being virtually simultaneous), the familiar influence of 

 spatial correlations of ionization is qualitatively explained. The greater radiation sensitivity 

 at elevated temperatures is another obvious consequence. Other predictions of the theory are 

 a dependence of radiation sensitivity upon molecular anisotropy, and a wide variation in the 

 injury to identical molecules exposed to ionizing radiation. 



I. INTRODUCTION 



Living systems embody two distinct varieties of intricate organization. One is 

 the complex static structure of the macromolecules which are essential com- 

 ponents of cells; the other is dynamic and is manifested in the delicate organiza- 

 tion whereby the functions of the cell and of the organism are achieved. 



Living systems are extraordinarily sensitive to ionizing radiation. This is 

 perhaps the most striking single result of experiments in radiobiology and has 

 been emphasized repeatedly (1,2, 3, 4). It is customary, and plausible, to identify 



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