560 



12. RATES OF INHIBITION 



from which: 



log ki = log A 



E. 



2.3RT 



(12-44) 



where E.^^^ is the apparent energy of activation and A is interpreted as 

 the collision frequency of the reacting molecules. The slope of the plot of 

 log ki against IjT is used to determine the activation energy: 



LOG k, 



E,,, = - 2.37?(8lope) 



0.00310 



l/T- 



0.0032 



0.0033 



0.0034 



0.0035 



(12-45) 



0.0036 



Fig. 12-19. Plot of log l\ against \/T for an inhibition in which JH* is 15 kcal/mole 

 and AS* is 5 cal/mole/deg (Eq. 12-41). 



The enthalpy of activation from Eq. 12-42 is: 



AH* = - 2.3i?(slope) - RT 



and therefore: 



E,,, = AH* + RT 



(12-46) 

 (12-47) 



The value of RT is around 0.62-0.64 kcal/mole for temperatures in the 

 biological range and since most enthalpies of activation would be between 

 10-30 kcal/mole, the difference between E^^^ and JH* is usually not signif- 

 icant. 



