DISINTEGRATION OF VIRUSBS 



39 



U, representB a molecule of urea, and K' auid K* • are equlllbrivon conetante. 



By analogy with the reasoning involved in deriving equation (7)> one can 

 deduce that the over-all rate constant k is dependent upon the constant k' , 

 the two equilibrium constants, the urea concentration to the power x, and the 

 appropriate activity coefficient ratio. If the reaction between virus and urea 

 is exothermic, K* will decrease as temperature is increased. K'», on the other 

 hand representing an endothermic reaction, will increase as temperature increase*. 

 If the rate of decrease of K' is greater than the rate of increase of K'*, the 

 product of the two will decrease as temperature is increased. In other words, 

 this particular urea denaturation process will have a negative differential tem- 

 perature coefficient. This is only half of the story. It is also necessary to 

 postulate a parallel process with a positive temperature coefficient. Such a 

 process could result from a mechanism similar to that postulated above, except- 

 ing that in this case the decrease of K* with temperature must be less than the 

 increase of K*' with temperature. If these considerations are right, then the 

 specific reaction velocity of urea denaturation should be proportional to the 

 sum of two terms, each of which governs a separate process. The assumption that 

 there are at least two mechanisms for the urea denaturation of tobacco mosaic 

 virus, that they both depend upon a reaction between urea and virus to give a 

 complex which is then denatured, and that at least one of these reactions has a 

 positive differential rate temperature coefficient can account for the way in 

 which the overall urea denaturation of tobacco mosaic virus varied in rate with 

 temperature. Both of the postulated mechanisms involve the reaction of urea with 

 virus. One can deduce from this that the reaction rate ought to be proportional 

 to some power of the urea concentration, that is, the logaurithm of the reaction 

 rate ought to be proportional to the logarithm of the urea concentration. In 

 figure 31 sre shown results of an experiment in which the reaction rate was meas- 

 ured in urea solutions of different concentrations. 



LOG|o K 



FIGURE 31 - RELATIONSHIP BETWEEN THE SPECIFIC REACTION VELOCITY 

 IN MIN.'l at 0° AND 4?° C. OF THE UREA DENATURATION OK TOBACCO 

 MOSAIC VIRUS PROTEIN AND THE MOLAR CONCENTRATION OF UREA. (M. A. 

 lauffer, J. Amer. Chem. Soc. 6?, 1793 (1943) )• 



