140 THE PHYSICS OP^ VIRUSES 



of about 45,000. This is somewhere near equivalent to the large 

 antigen for SBMV. 



This type of bombardment is probably a nearly equal mixture 

 of indirect action due to free radicals and direct action due to 

 primary ionization. The radicals act on the surface and many 

 of them are needed for one inactivation. So indirect radiation 

 action is a poor way to reduce infectivity and retain serological 

 potency. For this reason, it is likely that the estimates made 

 above give too large a size for the antigenic units. 



There should be great practical application for dry, or well 

 protected, electron bombardment of viruses. Deuteron bom- 

 bardment should be even better. 



Thermal Inactivation of Serological Affinity 



The question as to whether infectivity and serological affinity 

 inactivate thermally in the same way is clearly interesting. 

 Bawden (1950, p. 253) gives figures for the loss of infectivity 

 and of serological titer for potato virus X and bushy stunt 

 virus after 10-min heating at pH G and different temperatures. 

 The two viruses behave quite differently. Bushy stunt loses all its 

 serological affinity between 80° C and 85° C, whereas the infec- 

 tivity gradually diminishes over the range 50° C to 85° C. On the 

 other hand, potato virus X loses its infectivity over a range from 

 59° C to 68° C and loses its serological affinity at about the same 

 rate. Bawden points out that at 50° C, potato virus X loses its 

 infectivity slowly without much serological loss. Probably the 

 denaturation point is reached rapidly for virus X but not for 

 bushy stunt. 



Further studies, in which the logarithmic character of the loss 

 of serological affinity is approximately demonstrated, have been 

 made by the author and Dimond (1953) for southern bean 

 mosaic virus and by the author and Jane Setlow (1953) for T-1 

 phage. Several curves for T-1 serological inactivation are shown 

 in Fig. 5.6. These data were obtained by the technique of 

 neutralization of infectivity. Inactivations were carried out, 

 both wet and drv, with reasonable fit to a first-order reaction 



