THE SURFACE OF VIRUSES 135 



tant, whereas antibody receptors are not. When any one bacterial 

 receptor is covered, the virus is inactive. 



The number of receptors can now be estimated. If it is 

 assumed that the antibody molecules are of molecular weight 

 160,000, with a length of 235 A and a diameter of 44 A, and that 

 they attach end on, we have: 



Antibody attachment area = tt X 22^ '^ 1,500 A^ 

 Radius of T-2 phage = 350 A 



Surface area, ignoring tail, = 47r X 350^ '~ 1.5 X 10^ A^ 

 So total area is equivalent to 1,000 molecules. 



Now when 90 antibody molecules cover the appropriate spots, 

 one bacterial receptor is just covered. This is one-eleventh of the 

 whole surface and it can, therefore, be concluded that when one 

 of 11 bacterial receptors are covered, the phage is inactive. 



This fact of there being eleven receptors is of importance 

 as it is another datum in the structure of viruses. 



Serological Inactivation of Viruses 



The techniques indicated in the two previous chapters should 

 be applicable to the destruction of serological affinity. In fact, 

 the technique of bombardment by ionizing radiation should be 

 very suitable because we have just seen that something like a 

 thousand antibody receptors must exist on a virus surface and 

 so the size of each of these must be close to that of an enzyme 

 molecule, which is inactivated by one primary ionization. The 

 rather doubtful character of inactivation of infectivity, where it 

 is questioned whether one primary ionization is sufficient, should 

 not apply to the smaller molecules of the antibody receptors. 

 We therefore describe some experiments in which the serological 

 affinity of TMV, southern bean mosaic virus, and T-1 phage 

 has been studied (Pollard and Dimond, 1952; Pollard and Jane 

 Setlow, unpublished). 



In the case of the two plant viruses, the technique used was 

 to bombard virus preparations by deuterons and to assay for 

 the amount of precipitate formed by centrifugation, pellet 

 solution, and spectrophotometry. The results for TMV and 



