60 L. A. BLUMENFELD AND A. E. KALMANSON 



behaviour of unpaired electrons; the magnitude of exchange inter- 

 actions and the degree of delocalization of such electrons, and other 

 important parameters ; 



4. The analysis of hyperfine ESR structure (resulting from the inter- 

 action of unpaired electrons and nuclear magnetic moments of surround- 

 ing atoms) allows us to make the structure of paramagnetic particles 

 clear. 



Free radicals formed by ionizing radiations have unpaired electrons. 

 For this reason the ESR method has now become one of the most wide- 

 spread and effective methods of investigating initial radiation-induced 

 chemical effects. The investigation of the kinetics of free radical for- 

 mation, while the dose rate, the energy of radiation and the intensity of 

 emitting sources are varied ; the study of the temperature effect and 

 the effect of oxygen ; the study of the effects of humidity ; the investi- 

 gation of anti-radiation compounds ; the determination of the quantum 

 yields of radiation -induced chemical reactions; the evaluation of the 

 recombination energy of radicals by use of kinetic curves; all these 

 l^roblems and others can be solved by means of ESR spectroscopy. 



Hutchison (1949) was the first to observe the ESR spectra of irradia- 

 ted organic compounds. He irradiated alkyl halides with neutrons 

 and found them to form r-centres, which give ESR signals. 



ESR investigations of irradiated compounds, important biologically, 

 were begun in three countries independently: by Combrisson and 

 tJbersfeld (1954) in France, by Gordy and his collaborators (1955, 1958) 

 in the U.S.A. and by us in the U.S.S.R. (Blumenfeld, 1958; Blumenfeld 

 and Kalmanson, 1957a, b,c; 1958a, b; Kalmanson and Blumenfeld, 

 1958). 



Combrisson and tJbersfeld irradiated amino acids in an atomic pile 

 directly. The wide energy spectrum of the source, the high total dose 

 rate and the high intensity of the irradiation lead to extensive destru- 

 tion of the amino acids. In sucli conditions the ESR spectra of the free 

 radicals were rather similar with only a slight hyperfine structure. 



Since 1955 moi*e complete and interesting investigations of the ESR 

 sjjectra of iiTadiated biological materials have been carried out in the 

 U.S.A. by the group headed by Gordy, who investigated such important 

 irradiated biological compounds as amino acids, proteins, hormones, 

 vitamins, fats and nucleic acids. 



The most interesting and unexpected result of this work proved to 

 be that although the amino acids, of which proteins are constituted 

 gave ESR spectra after irradiation that were characteristic for each 

 amino acid, irradiated ])roteins always gave rise to only two types of 

 signal: either to a doublet with a splitting of 15 to 16 oersted, or to a 



