384 S. GARD AND O. MAAL0E 



2. Theoretical Aspects 



Heat inactivation of viruses may also be studied from theoretical view- 

 points. The aim has frequently been to characterize the process in terms of 

 the energy changes involved; e.g., the activation energy, AH, as defined l)y 

 Arrhenius, or the heat and entropy of activation, AH* and AS*, derived 

 from the theory of absolute reaction rates (Stearn, 1949). 



It has been pointed out (Pollard, 1953; Woese, 1956) that the precise 

 meaning of these parameters is not too clear when dealing with large and 

 complex units like virus particles. However, the fact remains that data 

 obtained by studying thermal inactivation of a virus at different temperatures 

 frequently lend themselves to calculations of internally consistent values for 

 AH or for AH* and AS*. This shows (1) that, at least to a first approximation, 

 the virus particles are inactivated accordmg to first-order kinetics; and (2) 

 that the rate constant is frequently found to depend on temperature in a 

 simple manner.^ 



We shall now review some of the experimental evidence upon which such 

 calculations have been based and, in particular, we shall consider the depen- 

 dence of the inactivation rate on factors such as ionic environment and 

 hydration. 



Nanavutty (1930) exposed coliphages to temperatures around 50°C. and 

 observed that the loss of infectivity was strictly exponential except for a 

 "tailing-off" effectat the level of 10 ^ survivors. The rate constant character- 

 izing such an inactivating process naturally depends on environmental con- 

 ditions. A striking example was furnished by Burnet and McKie (1930), who 

 studied the effect of heating different dysentery phages to 60°C. for one 

 hour in solutions containing NaCl and CaClg in varying concentrations. The 

 contour maps constructed from the data obtained in this way are very reveal- 

 ing and show that, for each phage, maximum survival was obtained when the 

 sodium and calcium ion concentrations w^ere balanced in a characteristic 

 manner. Departure in either direction from the balance greatly reduced the 

 survival (from a maximum of 40 % to less than 0-1 %). The protecting effect 

 observed when broth was added to suspensions of phage in NaCl solutions 

 was ascribed partly to the calcium and magnesium ions and partly to reducing 

 components and colloidal material contained in the broth. The authors 



^ The rate constant at the absolute temperature T, k{T), is estimated from the familiar 

 first-order equation, N/N^ = e exp ( — k{T)t), where N/N^ is the fraction of virus remain- 

 ing active after exposure to the temperature T for t seconds. According to Arrhenius, 

 log k(T) — Ci — AH/RT, where Cj is a constant and E the gas constant; log k{T) plotted 

 against l/T should therefore give a straight line with slope — AH JR. The theory of 

 absolute reaction rates requires that log k{T) ~ Cj +A»S*/i2 — A.H*/RT, where Cj is 

 approximately constant in the temperature intervals usually employed; as before, log k{T) 

 plotted against IjT should therefore give a straight line from which estimates of AH* 

 and A«S'* can be obtained. 



