36 



VIRUSES 



a 



'J 



17.5 

 15.5 

 13.5 

 11.5 

 9.5 

 7.5 

 5.5 

 3.5 



6 



o 

 si 



28i 



-H f! 



S 24 



S 



■S20 

 16 



o 



a 



I 12 

 .2 



T) 



2 3 4 5 6 

 Time in hours 



FIGURE 28 - DECREASE IIT SOLUBILITY, STREAM DOUBLE REFRACTION, 

 TURBIDITY Ain> INFECTIVITY OF TOBACCO kOSAIC VIRUS, PLOTTED AS 

 A FUITCTION OF TIKE OF EXPOSURE AT ROOM TEMPERATURE TO 6li UREA. 

 (W.M. Stanley and M.A. Lauffer, Science 89, 34^ (1939) ). 



Accompanying this breakdown is a gradual appearance of free sulfhydryl groups 

 as indicated by the nitroprusside test, 'i'hese chemical changes are paralleled 

 by certain physical changes: decrease in the ability to show stream double 

 refraction characteristic of tobacco mosaic virus solutions, and decrease in 

 the turbidity or opalescence of the material. The course of the reaction can 

 be followed by measuring the decrease in the amount of protein soluble in di- 

 lute electrolyte solutions using standard chemical procedures, ihia ia cert- 

 ainly the most foolproof method, but also the most • tedious . The reaction can 

 be followed much more easily by measuring the decrease in turbidity of the 

 virus, using a photoelectric colorimeter. Normal active virus in urea solu- 

 tion as in v/ater, is opalescent. The completely denatured material is water- 

 clear in urea. Some sacrifice in reliability might result from the adoption 

 of this technique, but the advantage gained from the much greater convenience 

 more than compensates . 



As in the other denaturation processes, particular attention was paid the 

 kinetics of the reactions involved. In Figure 29 it can be seen tliat the log 

 of the amount of protein rem.aining undenatured decreases linearly with time 

 when tobacco mosaic virus is dissolved in 6M urea. 



