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On the Localization of Radiation Effects 

 in Molecules of Biological Importance 



MARTIN D. KAMEN 



Mallinckrodt Institute of Radiology 



Washington University Medical School 



St. Louis, Missouri 



A biological system is a theorist's nightmare. Even in its simplest 

 manifestation it consists of a semiliquid, heterogeneous aggregation of 

 molecules of all sizes and complexities, interacting in a precisely ordered 

 manner by means of various mechanisms largely unknown. Study of 

 these systems underlines the embarrassing gaps in our knowledge of the 

 liquid state and of polymer chemistry at the presumably more amenable 

 level of inorganic physical chemistry. 



The abstractions derived from the quantum description of molecules 

 like methane or benzene in the gaseous state, or from the radiochemistry 

 of simple molecules in aqueous solutions, are necessary but not sufficient 

 for an understanding of the interaction of radiation with biological 

 systems. At present the only recourse is to grope empirically — at least 

 until we come to grasp an adequate understanding of protein structure, 

 enzyme synthesis, and the mechanism of enzyme action. However, it 

 does not follow that studies of the effect of radiations in complex systems 

 cannot be made and their results correlated with what is already known 

 from the study of simpler systems. 



Among the questions which can be posed in such a study is whether, 

 in large molecules of biological importance like protein or nucleoprotein, 

 there is varying radiosensitivity in any particular bonds or regions. Ex- 

 periments in which external radiation is used may provide data like 

 ionic and excitation yields and quantum efficiencies. But they give no 

 information on this very fundamental point. 



It is possible to make an experimental approach to this problem using 

 chemical complexing agents specific for particular atomic groupings, 

 like S — H and free amino groups. A more direct attack, however, would 

 seem to result from placement of unstable isotopes of elements involved 

 in definite bond sites. An example which comes to mind readily is sub- 



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