108 



THE PHYSICS OF VIRUSES 



Consider now the three predominant classes of bond. The 

 covalent bond has widely spaced energy levels, so that, for 

 thermal purposes, only two or three values of the energy need be 

 considered. The helix bonds and the interhelix bonds have more 

 closely spaced levels, and so more energy values need to be 

 considered. 



Eyring's theory of the activated state supposes that if F 

 is plotted against a general reaction coordinate, which is, 

 broadly speaking, a bond length, then there exists a second 

 configuration separated by an intervening hump as shown in 



Fig. 4. '2. Representation of a free-energy potential barrier which must be 

 crossed to permit inactivation of a biological molecule. 



Fig. 4.2. For a large molecule to attain the energy AF^ cer- 

 tainly involves many other bonds as well, so that AF^ includes 

 changes in covalent, helix, and interhelix bonds. 



A measured value of AF^ therefore, can be analyzed into 

 AH^ — TAS^ and corresponds to a total energy requirement of 

 A/?^ balanced by an entropy term which expresses the effect of a 

 tendency to reach the most probable state. 



If this expression, AF^ = AZ/^ - TAS^ is substituted into 

 the Eyring formula for absolute reaction rates, it can be seen 

 to yield 



kT 



ASt 



k, 



h 



AHt 

 ' RT 



(4.6) 



where, now, AS^ appears as a positive exponent which aids the 

 speed of the reaction. 



