266 RADIATION BIOLOGY 



to nearly the same extent, whereas histidine is more radiosensitive. This 

 larger value may be due either to a contribution from the glyoxaline part 

 of the molecule, since glyoxaline itself yields a certain amount of ammonia 

 on irradiation, or to a weakening of the strength of the C — N bond in the 

 a-amino group through the vicinity of the glyoxaline, or to both these 



causes. 



If the amino group is shifted from the a to the /3 position, a change in 

 radiosensitivity occurs as is shown by the difference between a-alanine 

 and jS-alanine. The occurrence of two imino groups in the combination 

 of two glycine molecules in ring form (glycine anhydride), 



CH2— NH— CO 



1 1 



CO— NH— CH2 



considerably reduces the ammonia yield, and similarly the peptide linkage 

 in leucylglycine and diglycylglycine does not contribute to deamination. 

 On the other hand, glycylglycine appears to be an exception, yielding 

 more ammonia than glycine. The tendency of an imino group to offer 

 resistance to attack by radicals is also shown in proline, 



CH2 CH2 



CH2 CH— COOH 



\ / 

 NH 



which does not yield any ammonia. The amino groups combined with 

 carbon dioxide in guanidine carbonate, 



(NH2— C— NH2)2H2C03 



NH 



or free in urea, 



NH2 



/ 

 C=0 



\ 

 NH, 



and thiourea, 



NHo 



/ 

 C=S 



\ 

 NH2 



are either completely inert or very nearly so. 



In addition to being influenced by structure, the ammonia yield is also 

 dependent on pH. When an equivalent amount of alkah is added to 

 glycine, deamination remains at the normal level, but an equivalent 

 amount of acid added to glycine or to the dipeptide glycylglycine 



