22 



If we use a sample of T-1 from a very clean solution, we find that loss 

 of ability to combine with the antibody follows exactly the pattern I have de- 

 scribed. You get a nice one-hit curve. Everything is straight-forward. Infact, 

 we can work out the molecular weight of the antigenic surfaces, and it fits very 

 nicely. It is a 22, 000 molecular weight for the unit represented. 



If, on the other hand, we use T-1 from a solution containing a lot of 

 broth (not pure T-1), then we are apparently unable to inactivate the surface at 

 all. Jane Setlow, who is working with me on this, found that when the bombard- 

 ment was hard enough, she could detect the stage where the activity had been 

 lost by looking at the changes in color occurring in the samples; if the color did 

 not change, there was no loss of ability to combine with antibodies. We found 

 that loss of activity was very slight until heavy bombardment was applied, after 

 which it increased rapidly. I do not know precisely what phenomenon occurs 

 here. But it is quite clear to me that it is possible for a virus to combine with 

 some of the protein and other molecules in broth in such a way that the surface 

 is now radiation-stable. Why and in what manner this happens I do not know. 



This is one experiment that I won't call disquieting, but it shows tliat 

 we have to think a little more than we have already. 



I might say that when we deal with commercial preparations and other 

 enzyme systems, we don't find such curious anomalies. Most of our work is 

 pretty straight-forward. But when we observe the hemagglutinins of the New- 

 castle virus, which constitute a number of units on the surface of the virus, and 

 if the virus has been dried in gelatin and then irradiated, we always get a single 

 hit type of inactivation for the process of losing ability to agglutinate red cells. 

 It is a single hit inactivation but its behavior is such that apparently 3 to 4 ioni- 

 zations are necessary (17). 



Some of you may wonder how we arrive at this conclusion. It is very 

 easy. Dr. Tobias and Dr. Zirkle will understand. In deference to their termin- 

 ology, we apply the linear energy transferred below and plot the process ob- 

 served in the reaction and we arrive at the sigmoid type of curve with points 

 something like those indicated in Fig. 3. We also measure the initial slope of 



electron bombardment like this. There 



isn't any question that such a sigmoid 

 curve cannot be explained by 1 ionization 

 event, but actually 3 or 4 ionization 

 events will give you this sort of relation. 



If the virus is dried-out-of-phos- 

 phate buffer, which can be done, then 

 the kind of curve that is obtained is of a 

 more usual type. It still gives nearly 

 the same maximum figure. We may not 

 be quite accurate enough to be able to 

 tell that for sure. In other words, in a 

 dried-out-of-phosphate buffer, 1 ioniza- 

 tion is adequate. 



So in the case of these two admit- 

 tedly complicated systems, which are, 

 nevertheless, more in keeping with what 

 radiobiological systems are like (after 

 all, biological material is not made up 



slope at origin 

 from electron data 



2 3 4 5 6 7 



PRIMARY IONIZATIONS PER 100 A 



Figure 3. The effect of bombarding 

 NDV with different energy deutrons as 

 measured by its hennagglutinating ability. 

 The cross-section changes follow an S- 

 shpaed curve. The slope at the origin 

 may be deduced from electron bombard- 

 ment. The line drawn is a theoretical 

 line based on an effective thickness of 

 50 A. and a sensitivity requirement of 

 four ionizations. 



