THE SYNTHESIS OF PROTEINS 2$ 



9. Some viruses at least (bacteriophage, Northrop, 1939; tobacco 

 mosaic, Loring, 1940) can form saturated solutions. This result 

 shows that the virus can exist in solution or, more precisely, that 

 the liquid containing the virus is a single phase. 



Such solubility experiments afford a decisive test of the question 

 whether the virus molecules are in "solution" or not. From the kinetic 

 point of view any particle is a molecule and there is no distinction 

 experimentally or theoretically between particles and molecules (cf. 

 Taylor, 1925, p. 1279). For this reason the word molecule seems appro- 

 priate until evidence is found contradicting its use. 



From the point of view of thermodynamic equilibria as stated in 

 Gibb's phase rule, however, there is a sharp and definite distinction be- 

 tween suspensions and solutions. A suspension is a two-phase system and 

 a solution is a single-phase system. It is perfectly possible to decide, 

 therefore, whether a virus "solution" consists of one or two phases.^ 



Quantitative measurements have not been made on other viruses, but 

 any virus which crystallizes must exist as a saturated solution, in equi- 

 librium with the crystals.® 



Such solubility experiments have not been carried out with larger 

 viruses, such as vaccinia, but it appears very unlikely that any indication 

 of a saturated solution would be obtained in these cases. 



5 There is some uncertainty as to the application of the phase rule to colloidal solu- 

 tions, but the results obtained with proteins (Northrop, 1939) show clearly that the 

 rule may be applied to these compounds. The following experimental results are possible 

 in a study of the solubility of viruses : 



/. The concentration of virus in the liquid is zero or variable and no equilibrium can 

 be shown to exist between the solid and liquid. Such systems are suspensions, but no 

 further conclusion can be drawn since the phase rule cannot be applied to systems 

 which are not in equilibrium. 



2. The concentration of virus in the liquid is constant and in equilibrium with the 

 solid virus. 



a. The concentration of virus in the solution is independent of the quantity of solid 

 virus present. The system consists of two phases and two components, and the virus 

 therefore is a single component. The virus is in solution in the liquid phase. No such 

 results have been reported. 



b. The concentration of virus in solution varies with the amount of solid. There are 

 two phases and three or more components. The virus is in solution in the liquid phase. 

 The virus preparation consists of two or more components. 



This is the result obtained with bacteriophage. (Northrop, 1938) and tobacco mosaic 

 preparations (Loring, 1940), although the bacteriophage curve is as good as that ob- 

 tained with many normal proteins. 



^ The fact that the solutions may be concentrated in the centrifuge does not affect this 

 conclusion, since this simply amounts to increasing the force of gravity and any solu- 

 tion may theoretically be concentrated in this way. The gravitational field simply 

 represents another degree of freedom. 



