406 S. GARD AND O. MAAL0E 



of the proteins, at higher concentrations, denaturation — stretching of the 

 molecules, unmasking of sulfhydryl groups, solubility changes, and often a 

 dissociation into molecular subunits. The effect is supposed to be mediated 

 by attachment of these agents to the sites of hydrogen bonds which are 

 thereby broken. 



Urea is the most extensively studied member of this group of substances. 

 It has been found to have little or no effect in concentrations below 3 M, but 

 seems to inactivate most viruses if used in sufficiently high concentrations. 

 A review of the earlier literature is found in an article by Bawden and Pirie 

 (1940a). The most comprehensive and systematic studies concern plant- 

 viruses. The rod-shaped viruses (TMV, potato X) are broken down into smaller 

 fragments with liberation of nucleic acid (Bawden and Pirie, 1937, 1940a; 

 Mehl, 1938; Frampton and Saum, 1939; Frampton, 1939; Martin, 1939; 

 Stanley and Lauffer, 1939; Lauffer and Stanley, 1943; Lauffer, 1943). In 

 spherical viruses (tomato bushy stunt, tobacco necrosis) neither fragmenta- 

 tion nor release of nucleic acid was observed (Bawden and Pirie, 1940a). 

 Denaturation demonstrable by chemical or physicochemical methods is, 

 however, to be regarded as the last step in a chain of reactions. Inactivation 

 is not a result of denaturation but precedes it, and the nature of the reactions 

 directly responsible for the loss of infectivity is as yet unknown. The kinetics 

 of denaturation of TMV was studied by Lauffer (1943). In this connection it 

 may suffice to mention that the reaction rate increased with pH above the 

 isoelectric point and that it showed a minimum at a temperature of about 

 20°C., rapidly increasing with decreasing as well as increasmg temperatures. 

 Unfortunately, inactivation has been less extensively studied. It shows, how- 

 ever, the same dependence upon pH and temperature as denaturation 

 (Bawden and Pirie, 1940a). Only one experiment has been reported which 

 gives an idea of the shape of the inactivation-time curve (Lauffer and Stanley, 

 1943). In this case infectivity in the first 6 minutes disappeared at a rate 

 corresponding to about 4 log miits per hour. Subsequently, the inactivation 

 rate decreased gradually, reaclung about 0.016 logs/hour in the interval of 

 30 to 96 hours, with a survival at the latter time of about lO-^-s. The con- 

 tinuous decrease in inactivation rate suggests the occurrence of side reactions 

 leading to enhanced resistance, i.e., a mechanism of principally the same type 

 as that discussed in the section on formaldehyde. 



The effect of 2 to 3 M urea on a cofactor-dependent T4 bacteriophage was 

 studied by Sato (1956). He observed a rapid loss of infectivity, but at the 

 same time an increasmg fraction of the surviving phage became cofactor- 

 independent. Isotope studies showed the adsorbability of the treated phage 

 and its activity to run strictly parallel. Both "activation" and inactivation 

 displayed the pattern of temperature and pH dependence characteristic of 

 protein denaturation. Apparently, area acts primarily on the receptor 



