CHAPTER 10 

 Energy of Antibody-Antigen Reactions 



Direct Calorimetry 



In the early days of immunochemistry, methods were not available 

 for measuring the amounts of free antigen or antibody, or both, re- 

 maining after antibody and antigen have reacted. Therefore calcula- 

 tion of the free energy change from direct measurements of the equi- 

 librium constant was not possible. The earlier estimates of the strength 

 of the antibody-antigen bond were based on attempts to measure the 

 heat of reaction AH. It will be seen from equation (14) in the previous 

 chapter, which we can rewrite as follows, 



AF° = AH° - TAS° T = const. (1) 



that if the entropy change were zero, AH° would equal AF° , and 

 such a measurement would be an adequate measure of the strength 

 of the antibody-antigen bond. In fact, we may regard equation (14) 

 as a statement that in order to make AH° a reliable index of the 

 tendency of the reaction to take place, we have to correct it by al- 

 lowing for the entropy change AS° . A positive entropy change will 

 make a negative AF° still more negative, a negative entropy change 

 will make it less negative or even positive. Somewhat unexpectedly, 

 recent work suggests that in serological reactions AS° is. in fact, not 

 large, though usually not zero. 



Nevertheless, not too much has been learned about antibody- 

 antigen reactions by direct calorimetry. The first attempt, by Bayne- 

 Jones (1925), gave results that we now know were nearly a million 

 times the correct value. Two later determinations, the first by 



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