INACTIVATION OF VIRUSES 397 



inactivation; hydrolysis or any type of breakdown of the protein might 

 produce the opposite effect. 



The inaccuracy of the biological methods of assay is a severe handicap in 

 studies of the kinetics of inactivation. It can be overcome only by systematic 

 repetition of experiments under rigidly standardized conditions and a careful 

 statistical analysis of the results. Even so, observations often have to be 

 extended into extreme ranges in order to reveal significant trends. In the 

 past this has not been clearly realized. Most of the kinetic studies, limited in 

 number in the first place, were designed on the more or less conscious assump- 

 tion of the molecular nature of the virus particle and aimed at identification 

 of '"essential" sites in the molecule. Time is actually overdue for a revision 

 of this basic philosophy and a reappraisal of the data already collected. 



Presumably, each and every link in the complex chain molecules that carry 

 the specific biological activity is equally essential. The activity rests, not 

 upon a number of mdividual active groupings, but upon a specific pattern of 

 forces. For the maintenance of the pattern, secondary bonds, particularly H 

 bonds, no doubt are of great importance. Thus, as a hypothetical example, a 

 reaction involving an amino group may lead to permanent inactivation even 

 if it is readHy reversible and the amino group can be reconstituted. What 

 may not be reconstituted, however, is the original pattern. If by the primary 

 reaction an H bond is broken, the delicate balance of intramolecular forces 

 is disturbed and a new equilibrium of tensions will be established, i.e., a 

 distortion of the configuration will ensue. A reconstitution of the amino group 

 will restore the net charge and the elementary composition, but hardly the 

 specific pattern. In this example the amino group may or may not be essen- 

 tial; if so, only as part of the pattern. For such reasons, attempts at identifica- 

 tion of essential sites will presumably tell less about the mechanisms behind 

 the specific activity of the virus than about the mode of action of the chemical 

 agent used. 



From a practical pomt of view a study of the relative importance of 

 avidity and the "membrane" effect of the protein cover seems to be of definite 

 importance. It was once thought that viruses could be "attenuated" by 

 chemical means. Later this concept was revoked and the effect interpreted 

 merely as a reduction in titer. However, thermal mactivation has been found, 

 under certain conditions, to exert a selective pressure upon a genetically 

 heterogeneous virus population — thus producing a shift in the average bio- 

 logical properties of the surviving fraction. A similar phenomenon has not 

 yet been described in connection with chemical inactivation but has to be 

 considered as a possibility. What seems to be definitely established, however, 

 is the occurrence of avidity changes in the course of chemical treatment. 

 These are demonstrable primarily as a reduction of the rate of adsorption of 

 the virus onto the host cell, i.e., as a lower collision efficiency, and they may 



