RADIOHIOI.OGICAI. MFXlHWISM AT TlIK CELLULAR I.IAIJ. 



by sulphur groups accords with FUdjarn's interpretation of some aspects of 

 cysteamine protection*^. 



As a rcsuh of the systematic study of irradiated amino acids and proteins 

 by the group at Duke University, it is beheved that the observed spectra can 

 now be interpreted \\ iili much greater assurance*^ hut the subject of electron 

 spin resonance spectroscopy is still at a stage when quite divergent interpre- 

 tations are possible. Blumenfeld and Kalmanson*^ for example, make the 

 interesting suggestion that the very small signal which they observed in 

 irradiated native protein, as compared with that from irradiated denatured 

 protein, is due to the existence of conduction bands in the native protein. 

 As in other branches of spectroscopy, confident interpretation of spectra is a 

 goal which will be approached asymptotically. 



In the meantime much can be learnt from the study of the influence of 

 physical factors on the strength of signals of unknown origin. Some very 

 beautiful examples are provided in the field of photosynthesis through the 

 study of the paramagnetism of chloroplasts as a function of time and intensity 

 of illumination*". In radiobiology an interesting set of correlations has been 

 established between the sensitivity of seeds to radiation and factors such as 

 their water content at the time of irradiation, storage time, and the oxygen 

 content of the water in which they are germinated. When the electron spin 

 resonance signal from irradiated seed is examined, the signal strength is 

 found to depend on water content over the same range as that which in- 

 fluences radio-sensitivity. The signal decays with time, and an inter-relation 

 exists between water and oxygen as in their effects on radio-sensitivity** '^^ 

 Remembering that some of the criteria of radiobiological damage almost 

 certainly reflect chromosomal lesions, whereas the broad signal obtained 

 from the irradiated seed may well be a superposition of signals from a great 

 variety of organic radicals, we should not necessarily expect complete 

 correspondence, and it would not have been surprising if no correspondence 

 had been found at all. The results, therefore, suggest that irradiated seeds— 

 and irradiated spores which Powers et al.^^ have found to exhibit a most 

 interesting temperature-oxygen interdependence — merit further study by 

 electron spin resonance spectroscopy. 



Seeds are the materials of choice for current investigations because of the 

 slow rate of change of electron spin resonance signal with time in these 'near 

 dry' systems, and the same may be true of spores. Even in barley seed, 

 however, some important resonances may have decayed during the period of 

 irradiation. Conger and Randolph** estimate a decay in wheat germ of 

 15 per cent in the first 45 min. Zimmer et al.^* estimate a decay of 50 per 

 cent in the first hour in grass seed. Higher dose rates than those employed 

 in these investigations would have been advantageous. 



Present indications are that although the radio-sensitivity of systems of 

 low water content is very interesting, it is essentially different in a number of 

 important respects from that of cells which compose most mammalian and 

 plant tissues. The goal as far as the radiobiological application of electron 

 spin resonance spectroscopy is concerned, must remain the examination of the 

 common cell types. For this purpose it will almost certainly be necessary to 

 deliver the total dose in a single pulse of very short duration. As mentioned 

 earlier, this facility is already available for pulse lengths down to 2 ;j.sec. 



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