ENERGY OF ANTIBODY-ANTIGEN REACTIONS 



141 



3 4 5 



10 20 



0.05 0.1 0.2 03 0.4 0.5 1.0 2.0 



Equilibrium constant {K) 

 Fig. 10-3. Relation between standard free energy change (AF°) of a reaction 

 A-^B and the equilibrium constant. Also shown is relation between AF" and 

 the per cent composition of the equilibrium mixture with respect to B. (Slightly 

 modified from H. B. Bull, 1951, Physical Biochemistry, 2nd ed., Wiley, New 

 York, by permission). 



intracellular template which causes part of the new molecule to have 

 a configuration complementary to the antigenic determinant, a small 

 value for AF° is understandable. An antibody molecule that possessed 

 too strong an affinity for the fixed antigen molecule or intracellular 

 template would have difficulty leaving its place of formation and 

 getting into the circulation, as has been pointed out by Pauling 

 (1940) and Singer (1957). 



Another unexpected feature of Table 10-1 is that the values of 

 A^°, with two exceptions, are positive instead of negative. When 

 antibody molecules combine with a molecule of antigen, their freedom 

 of motion is restricted, and this loss of freedom constitutes a loss of 

 "configurational entropy." Therefore, one would expect antibody- 

 antigen reactions to be accompanied by a decrease in entropy. The 

 positive values reported therefore demand explanation. 



It has generally been supposed that the positive values for A5° 



