214 THE PHYSICS OF VIRUSES 



similar to the rate of change of energy for the Van der Waals 

 forces between large molecules. 



A force of this nature is dependent on the average charge 

 fluctuations in the molecules, and these will, in turn, depend on 

 conditions in the solution which attach or detach protons from 

 ionic groups. Therefore, the condition of the molecule can have a 

 considerable eflfect on this variety of force. 



This kind of force is not inherently specific; that is, it does 

 not depend on the presence of similar groups or complementary 

 structures. However, the interaction energy may well be in- 

 creased in value if certain kinds of complementary grou])ings 

 exist in the two molecules. Kirkwood and Shumaker point out 

 this fact, but make no definite calculation of interprotein forces 

 numerically. 



We now have three classes of force to consider, one rejiulsive 

 and under metabolic control by changing the ionic strength and 

 one attractive and also dependent on whatever cellular con- 

 ditions determine the proportion of charge subject to fluctua- 

 tion. The third force, the Van der Waals force, is not under con- 

 trol by the nature of the solvent in the cell at all. 



Potential Energy Diagrams for Two 

 Macromolecules 



There is not a great deal of value in trying to plot accurate 

 potential diagrams for the interaction between two biological 

 molecules since neither the size nor the structure of these mole- 

 cules is accurately known. Nevertheless, it is worth while to get 

 some idea of the order of magnitude of the potential energies 

 involved so that suggestions for the physical processes in virus 

 multiplication can be made. 



To make a definite case, in Fig. 8.9 we i)lot the potential- 

 energy curve for two southern bean mosaic virus ])articles ac- 

 cording to the attractive-force formula of Hamaker and the 

 repulsive-force formula given by Verwey and Overbeek. The 

 value of the surface potential, ypo, has been chosen quite arbi- 

 trarily as kT/e for a temperature of 300° K. It can be seen that 

 the potential energy, U, is slightly negative at large distances. 



