THERMAL INACTIVATION OF VIRUSES 109 



In the inactivation of enzymes and the denaturatlon of 

 proteins, it is common to find values of A»S^ of the order of 

 10-100 calories/mole/degree. These produce a drastic change 

 in A'l, and it is in order to examine what processes can operate 

 to give a high entropy of activation. Examining Eq. 4.4 for 

 entropy, we see that the first term corresponds to the effect 

 of changing the energy per state of vibration. This can take 

 place if there is considerable expansion (and some recent work 

 by the author indicates that, for proteins, this may be so), 

 but, since the range of temperatures covered in biological 

 inactivations is quite small, the predominant cause of an entropy 

 increase is the second term. This means there is an increase 

 in the number of available forms of vibration. Two major 

 reasons for this exist: the first is the opening up of chains, 

 which permits a whole class of pendulum-like oscillations to 

 take place; and the second is the release of bound water which is 

 now free to rotate in two degrees of freedom per molecule where 

 before these possible modes were held down. So when figures 

 are seen later regarding entropies of activation, they are to 

 be associated with these two possibilities. It will be seen, as is 

 reasonable, that in the dry state activation entropies are quite 

 small, so that probably a very large part of the entropy of 

 activation is concerned with hydration. 



Thermal Inactivation of Viruses 



Many measurements of the thermal inactivation of viruses 

 have been made since this is a primary piece of knowledge in 

 the handling of pathogens. Not very many are suitable for 

 measuring rate constants and so are not suitable for determining 

 the activation — heat change and entropy change just discussed. 

 The usual procedure is to describe the 10-min thermal inactiva- 

 tion point, which is obviously a very practical piece of informa- 

 tion but is, alone, not too informative from our point of view. 



The fact that the thermal inactivation of a bacterial virus 

 follows first-order kinetics was shown by Krueger (1931), who 

 studied a staphylococcus phage. Careful studies on four plant 

 viruses were made by Price (1940). Some of his results are 



