242 BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



molecules by their attack on dissolved solute. Figure 9-5 illustrates these 

 two mechanisms. 



unshared 

 electron 



protein protein 



Indirect action Direct action 



Figure 9-5. Indirect versus Target Action of Ionizing Radiations. 



Effects on Some Biological Molecules 



In his recent book, Swallow" has collected the known effects of X irradia- 

 tion of hundreds of compounds of biological interest. For instance, the im- 

 portant generalization exists that reactive peroxides are formed from all the 

 biologically active amino acids in solution. In addition, the molecular prod- 

 ucts of irradiated water solutions are H 2 , H 2 2 and 2 , each of which, and 

 especially H 2 2 , can exert its chemistry on the solutes present. 



The results are easy to state in general, difficult to state in detail, in all 

 but the simplest cases. In general, new molecules can be produced from the 

 old ones (plus water), and these new ones may exert catalytic, toxic, or no 

 effect on the metabolic processes in the vicinity in which they are produced 

 or to which they are carried by blood and lymph. In particular, the ab- 

 sorbed radiation is known to reduce the catalytic activity of many enzymes, 

 and to alter their molecular weights and other physical properties. Large 

 molecules (Figure 9-6) can be broken into many parts, or can be cross-linked 

 through new hydrogen bonds or through the oxidation of two — SH groups 

 by H 2 2 , for example, to form an — S — S — bond, with distortion of the 

 molecule. 



One of the most intensely studied molecules from this point of view is the 

 nucleic acid, desoxyribonucleic acid (DNA). It is thought (the reasons were 

 given in Chapter 6) to be the main carrier of hereditary information in the 

 living system, and hence one that should not be tampered with in human 



