DEPENDENCE OF INTERACTION ENERGY UPON FIT 



303 



which may be called the " equation of fit " since it represents the depend- 

 ence of interaction energy on distance at any chosen separation. It may be 

 convenient to call d<p!dd the " index of fit." The indices of fit have been 

 plotted for two types of interaction in Fig. 6-20 from which it may be 

 noted that the importance of accurate fit varies with the separation be- 

 tween the groups, naturally rising very rapidly as the groups approach clos- 



FiG. 6-20. Variation of the index of fit with distance. Curve A is 



for an ion-ion interaction (same characteristics as in Fig. 6-13) and 



curve B is for the interaction of a methyl group within a protein 



cavity (as in Fig. 6-19). 



er tlian the equilibrium distance, due to repulsion forces, and showing 

 a maximum at a separation distance of approximately 0.5 A greater 

 than the equilibrium distance. Near the equilibrium distance the fit is 

 not so acute because of the relative flatness of the energy-distance curves. 



