30 



YOS, BADE AND JEHLE 



One can also consider a molecule II arbitrarily different from I. (Bade, 1954). 

 In either case one evaluates 



(A^ii- A.4i„)/A.4ii 



The answer to this question covers one part of the problem of specificity. 

 Next, one has to realize that the specifically interacting molecules are always 

 suspended in a medium which is also subject to London forces. One needs 

 therefore to evaluate differential effects in the attraction, i.e., one has to 

 consider ''buoyancy". 



One might illustrate this with a slightly oversimplified scheme, which takes 

 account of buoyancy effects, by considering only two globular macromolecules 

 suspended in an otherwise homogeneous isotropic medium made up from 

 smaller molecules. One compares an arrangement in which the two macro- 

 molecules I, I are closest neighbors (left side of Fig. 1) with one in which they 

 are not (right side). A short consideration, well-known from the theory of 

 mixtures, shows that one can take care of the buoyancy effect by grouping part 

 of the medium molecules into globular regions each occupying the same volume 

 as does a molecule I. Those aggregates are then named II (open circles in Fig. 1) 

 and may be called conceptual aggregates. Under certain general assumptions 

 one finds that the difference in free energy of the two arrangements in Fig. 1 

 is equal to the corresponding difference in Fig. 2 and one calls it the rearrange- 

 ment free energy for the quadruplet situation represented in Fig. 2. 



AiAi II = A^i I + A^ii „ - 2A.4i „ 



A more complete analysis may be given with the following assumptions which 

 are only made to delimit the present calculations: 



(a) For the sake of simplicity consider a mixture of two kinds of molecules, 

 I and II, where the distances of closest intermolecular approach R have the 



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Fig. 1 



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Fig. 2 



