Biological Effects of Ionising Radiations 123 



a sharp fall in the proportion of molecules decomposed to ions 

 formed by the radiation as the ion density increases from 50 to 

 3,500 ions per micron. Dale and Meredith, in collaboration with 

 the writer, have recently examined carefully the inactivation of 

 dilute solutions of the enzyme carboxypeptidase by X-rays and 

 alpha rays. The alpha-yield was found to be only about one- 

 twentieth of the X-ray yield, indicating a sharp fall in efficiency 

 of the radiation with increasing ion density. It would appear 

 that, in the case of the densely-ionizing alpha particles, a high 

 proportion of the products resulting from the ionization of the 

 water becomes ineffective before they reach the enzyme mole- 

 cules awaiting inactivation. More experiments of this kind are 

 urgently needed to throw light on the mechanism by which such 

 inactivations are brought about in dilute aqueous solutions, par- 

 ticularly in view of their relevance to the biological studies. The 

 influence of ion density on the inactivation of enzyme systems 

 under in vivo conditions also awaits investigation. 



Ion Density in Relation to the Inactivation of 

 Elementary Biological Units 



Perhaps the best understood examples of ion-density de- 

 pendence are in connection with the direct inactivation of 

 elementary biological units, such as viruses and genes, by the 

 ionization of their constituent atoms. As separate articles of 

 this series are devoted to viruses and genes, a brief reference 

 will suffice. 



The distinctive feature of the .effects under consideration is 

 that they are produced whenever an ionizing particle leaves two 

 or three ion-pairs anywhere within the unit. It is possible that 

 a single ion-pair suffices, but ion-pairs are, in fact, formed in 

 clusters of 1, 2 or more pairs, the average number being 3 pairs, 

 and rather accurate experiments would be necessary to be certain 

 that the effect is invariably produced by a single ion-pair. 

 Whether this is so or not, it is clear that since each cluster 

 contains an average of 3 ion-pairs, the distance apart of the 



