VIRUS GENETICS, MULTIPLICATION, AND PHYSICS 



209 



Verwey and Overbeek give the approximate value for the repul- 

 sive potential, Ur, as 



Un = Ka^^M In (1 + e 



— t(s— 2) 



)] 



where K is the dielectric constant; \1/q is the surface potential; 

 which is of the order of 30-150 mv, corresponding to 4.8 X 10~^^ 

 erg; r is j)a or [^iTiiin'^e^/KkTY'a; and 5 is the ratio of the 

 separation of the centers of the spheres to the radius of each. 



Fig. 8.8. Variation of intervirus distance with pH for TMV. x\ minimum 

 occurs at the isoelectric point, which indicates that forces exist wiiich are 

 dependent on ionic atmospheres. Data due to Bernal and Fankuchen (1941). 



The fact that the force between virus particles is dependent 

 on the ionic atmosphere is shown by the way in which inter- 

 particle distance for TMV was found by Bernal and Fankuchen 

 to depend on pH. This is shown in Fig. 8.8. It can be seen that 

 there is a minimum distance at the isoelectric point, 79H 3.4, 

 with a rise on each side. 



It is quite clear that repulsion alone cannot explain the 

 assembly of macromolecules to form a virus unit. Some kind of 



