IONIZING RADIATIONS AND PROTEINS 



235 



It was possible to prevent formation of this fast-moving component by 

 addition of cysteine. 



The protein changes on irradiation are extremely temperature-depend- 

 ent, especially those which result in denaturation and precipitation. 

 In fact, protein solutions that become cloudy after 2 hr, when irradiated 



40 60 



Time, min 



Fig. 16. Protein denaturation and precipitation by x-ray irradiation as measured by 



light absorption at 2510 A at different times after irradiation. The E values are the 



difference between the irradiated and the control. X-ray dose, 71,900 r. 



at 10° C with 71,900 r, remain clear for hours when kept at 3°. However, 

 as soon as the irradiated clear protein solution is brought to 27° pre- 

 cipitation occurs. This process can be seen clearly on measuring the 

 absorption of light at 2510 A. The log {Iq/Ix) (E) values rose steadily 

 with time (Fig. 16). Denaturation and subsequent precipitation not 

 only are temperature-dependent but also are processes requiring a rather 

 long time for completion. Similar protection produced by keeping 

 x-irradiated frogs at a low temperature must be due to this delay in 

 protein denaturation. 



Ionizing radiations produce, according to Bacq et al. (17), a contracture 



