104 THE PHYSICS OF VIRUSES 



the functioning configuration corresponds to one minimum of 

 potential energy, if not the lowest there is. The denaturation of 

 such a molecule can be thought of as the changing of its con- 

 figuration from the inherited form to one of perhaps greater 

 ultimate stability but no longer of the proper form for its function. 



Now a virus, as we have seen, is not a molecule but an aggre- 

 gate of molecules. Each of these has a function, or plays a part 

 in some function. The loss of biological form on the part of one of 

 these molecules may not necessarily influence the behavior of 

 the virus — it will depend on what feature of its activity we are 

 studying. If, of course, there exists some function which requires 

 the entire virus to be intact, then the loss of this function would 

 take place if one molecule were denatured. In practice, it 

 seems as though infectivity is a function which requires at least 

 a great part of the virus to be intact, and probably loss of 

 infectivity follows when one key molecule is inactivated. This 

 will certainly vary from virus to virus. 



Therefore, as a start, we consider the process of inactivating 

 one large biological molecule and see whether the description 

 of the process is apt for the description of inactivating viruses. 



In Fig. 4.1 is a token representation of part of a protein 

 molecule. We have taken the Pauling-Corey-Branson (1951) 

 helical structure as a basis for the figure, but this is not essential. 

 There are three classes of bond : The backbone bond, which binds 

 the polypeptide chain, consists of covalent bonds of energy 

 between 3 and 5 ev per bond. These appear in the figure as the 

 bonds forming the helix. There are then the intrachain bonds 

 which make the helix tight and firm, these appear as dashed lines; 

 and the interchain bonds which hold the helices together. In the 

 figure, these are shown as thick dashed lines. These last two types 

 of bond are probably varied in character and include sulfur 

 bridges, hydrogen bonds, and ionic bonds, but they are certainly 

 weaker per bond than the covalent backbone bonds. 



Such a molecule, possessing, say, eight chains bound together, 

 although nowhere bound with great strength outside of the poly- 

 peptide chain, has a considerable stability. It is instructive 

 to construct a model of even one chain and notice that the 



