3] CONFIGURATION OF GLOBULAR PROTEINS 51 



the distance between charges and to the presence of more shielding electro- 

 lyte between them. The typical result, illustrated by Fig. 6, is that pH — log 

 a/(l — a) becomes virtually independent of net charge.^ 



Two additional criteria will be used as circumstantial evidence for the 

 absence of configurational change. These are optical rotatory power and 

 the rotational relaxation time of bound fluorescent dye molecules. These 

 properties are quite sensitive to configuration, although they do not lend 



4.0 



ÎS ' 3.5 



X 

 Q. 



3.0 



14 10 



NET CHARGE 



Fig. 5. Logarithmic plot for titration of the carboxyl groups of ribonuclease (Tanford 

 and Hauenstein^^). Ionic strength is 0-15 for the top curve, 0-03 for the middle curve, and 

 01 for the lowest one. The dashed lines show the slopes of Fig. 4 for corresponding ionic 

 strengths. The differences (20 to 40%) presumably reflect the relative positions of various 

 kinds of charged sites.®** 



1^ 

 «I 



0.2 



0.4 0.6 



DEGREE OF DISSOCIATION 



0.8 



Fig. 6. Logarithmic plot for titration of the carboxyl groups of polymethacrylic acid 

 (Arnold and Overbeek^). Ionic strength is 0010 to 001 7 for the top curve, 0-10 for the 

 middle curve, and 1 -0 for the lowest one. 



