46 CHARLES TANFORD [3 



happen: (1) the pH of the solution is changed, (2) protons are removed from 

 or added to appropriate acidic or basic groups, (3) the net charge of the pro- 

 tein molecule is increased. Of these changes, the last-named is the one most 

 likely to affect the configuration, as was pointed out as early as 1931 by 

 Wu.^°^ For the increase in net charge necessarily results in repulsion be- 

 tween charges and a net positive contribution of electrostatic interaction 

 to the free energy of a compact configuration. Some calculations for the 

 models of Fig. 2 are shown in Table 2. They were made using the same 

 equations and assumptions discussed earlier. Table 2 shows changes in 

 electrostatic interaction energy between 10,000 and 20,000 calories for the 

 models used, which contain 12 or 20 charged sites. The corresponding 

 changes for real proteins can easily amount to 100,000 calories/mole. 



Table 2 



THE CONTRIBUTION OF ELECTROSTATIC INTERACTION TO 

 THE FREE ENERGY OF COMPACT SPHERICAL PROTEINS 



* The model structures to which these calculations apply are shown in Fig. 2. Where 

 two values are shown for the same net charge they refer to alternate forms which are 

 likely to be present with that net charge. For model C, for example, with a charge of 

 —4, the first figure is for the form with protons removed from two phenoHc groups, the 

 second for the form with protons removed from one phenoUc and one amino group. 

 11 stands for ionic strength. 



