3] CONFIGURATION OF GLOBULAR PROTEINS 53 



sedimentation^^ and diffusion coefficients^*-'*'* lead to the same conclusion,* 

 as shown by Fig. 8. The effect of ionic strength on the sedimentation coeffi- 

 cient has been measured at one pH,^^ and it also supports this conclusion. 

 Essentially similar data have been obtained by several other workers. *-'-^°'^* 

 The titration of carboxyl groups below pH 4 is also in accord with poly- 

 electrolyte-like behavior.^** 



Serum albumin also undergoes a reversible change in configuration on 

 the alkaline side of the isoelectric point, above pH 11. The polyelectrolyte- 

 behavior of the product is indicated strikingly by the titration data of 

 Fig. 9.^^ There is a very pronounced increase in viscosity near the same 



11.5 



t 



11.0 



10.5 



- 40 



-100 



-60 -80 



NET CHARGE 

 Fig. 9. Logarithmic plot for titration of the phenolic groups (upper curve) and amino 

 groups (lower curve) of bovine serum albumin (Tanford, Swanson and Shore^^). 



pH.^* Both the acid and the alkaline reaction are accompanied by marked 

 changes in fluorescence polarization (Fig. 10)^°^ and in optical rotatory 

 power.^12-^^ 



An interesting aspect of these configurational changes is that they are not 

 symmetrical about the isoelectric point. The acid change occurs when the 

 net charge is about +10, the alkahne change occurs at a net charge of about 

 — 50. It seems improbable that the electrostatic interaction energy of a 

 unique compact configuration could be sufficiently asymmetric about the 

 isoelectric point to account for this observation. These data therefore pro- 

 vide one of the strongest arguments in favor of the idea that intramolecular 



* Needless to say, a change in sedimentation coefficient alone or in diffusion coefficient 

 alone need not signify a change in configuration, but could reflect instead, a change in 

 molecular weight. Combination of the sedimentation and diffusion data of Fig. 8 leads 

 to the conclusion that no change in molecular weight occurs. 



