60 CHARLES TANFORD [3 



pH 10. Yang and Foster^^^ have demonstrated that the intrinsic viscosity 

 is constant between pH 2-3 and pH 7-5. The titration curve of lysozyme^"**'^" 

 shows no feature suggestive of expansion at any pH, 



Apart from the major configurational changes discussed so far, there is 

 hkeUhood of the occurrence of minor changes in configuration. There may, 

 for example, be several different arrangements of the polypeptide chain (or 

 chains) which have 'hydrophobic' regions of comparable stability. A change 

 in pH and the resultant change in the number and positions of charged sites 

 may stabilize one such form at the expense of another, A similar situation 

 can arise if hydrogen bonds contribute to the stability of the original com- 

 pact form. The rupture of some of these bonds by titration could lead, if 

 the net charge is still low, to a new compact configuration with a new set 

 of hydrogen bonds. 



There are small changes in the sedimentation coeflücient of jS-lactoglobulin 

 near pH 5 and pH 8,^^ which might reflect such minor changes in configura- 

 tion. A similar small change in sedimentation coefficient occurs for oval- 

 bumin, near pH 4.^^ It would be desirable to obtain confirmatory evidence 

 of these changes by other methods. 



Minor configurational changes need not necessarily be accompanied by 

 changes in hydrodynamic properties. Klotz and coworkers, for example, 

 have observed changes in the ability of serum albumin to bind calcium^^ 

 and certain anions^" and uncharged molecules,^^ which they have ascribed 

 to a configurational change. The change is observed near pH 7, where no 

 change in hydrodynamic properties can be detected.^^ Karush^^ jj^s sug- 

 gested that the unusual ability of serum albumin to bind ions of all kinds is 

 evidence for quite general configurational adaptability. That these observa- 

 tions based on binding properties reflect configurational changes is not an 

 unassailable conclusion, for alternative interpretations are possible. 



POLYMERIZATION OF PROTEINS 



This paper has dealt with the various forces which, in water solution, attract 

 one portion of a protein molecule to another portion of the same molecule. 

 It is obvious that the same forces must be able to act intermolecularly and 

 thus bring about polymerization. Whether polymerization will actually take 

 place, depends on the extent to which these forces have been able to be 

 utilized intramolecularly. 



Association between protein molecules in their compact configurations 

 occurs in some proteins and not in others. It does not occur in ribonuclease,^^ 

 the molecular weight of which is independent of pH or ionic strength be- 

 tween pH 2 and pH 11. It does not occur in serum albumin. ^^ The very slow 

 association' '^^-^^ which this protein undergoes is observed only on stand- 

 ing over a period of days, and then only subsequent to loss of the compact 

 configuration.^^ Insulin, on the other hand, is polymerized at all pH values 



