ELECTRON SPIN RESONANCE INVESTIGATIONS 65 



radiation might recombine with "holes" and this is shown by the in- 

 creasing concentration of unpaired electrons in the sample, snch un- 

 paired electrons being already localized. 



In 1958-1959 the ESll spectra of irradiated nucleic acid compounds 

 was investigated. This work was carried out in collaboration with 

 Passynsky and Shen-Pei-Gen (Shen-Pei-Gen et al., 1959). 



Nucleic acids and their components were irradiated under the same 

 conditions as before. We also irradiated nucleoproteins and some 

 artificial complexes of nucleic acids with proteins and other compounds. 



As in the investigations of amino acids and proteins we found a dis- 

 tinct dependence of free radical yield on the complexity of the struc- 

 tures. For instance irradiation of nucleic acid bases and ribosides with 

 a dose of 10"^ r gave a free radical yield of lO^^ to 10^9 particles 

 per gi'am. The free radical yield is one to three orders less for high 

 molecular weight preparation of nucleic acids. 



The most important result, from the point of view of radiobiology, 

 IS that although the radical yield per gram for low molecular weight 

 compounds is much higher than for those of high molecular weight, the 

 number of damaged molecules per ionization event is, however, much 

 higher (50 to 70 times) because of the great dimensions of the nucleic 

 acids. 



This conclusion is in excellent agreement with other purely biological 

 experiments in radiation cytology and genetics, which showed that a 

 disturbance in even one section of a high polymeric nucleic acid, which 

 is a part of the chromosomes, can lead to non-reversible and even lethal 

 damage to the cells. We can explain the small radical yield in irradiated 

 high-polymer nucleic acids on the basis of our hypothesis about the 

 existence of molecular "conduction channels". 



It is interesting that in one of their last papers on the irradiation of 

 nucleic acid compounds, Shields and Gordy (1959) also conclude that 

 nucleic acids have semi-conductive properties. 



In conclusion it is necessary to point out that all the experimental 

 data given above were obtained on lyophilized, solid specimens. Be- 

 cause of this, direct comparison of these data with those for biological 

 species containing much water is not quite correct. In aqueous media 

 additional difficulties, due to the secondary action of short-lived active 

 free radicals created by water radiolysis, can occur. However, we are 

 sure that these special featui'es of the ionizing action of radiation on 

 biological polymers are due not to aggregation, but to properties of 

 their molecular structure. 



From this ]3oint of view molecules of i:)rotein and nucleic acid may be 

 considered in aqueous medium as particles of a solid body. So we think 



