3] CONFIGURATION OF GLOBULAR PROTEINS 49 



sufficiently large to overcome the energy initially stabilizing a compact form 

 will transition to an extended randomly coiled configuration become pos- 

 sible. When a transition does occur, the configurational change will be much 

 more drastic than the shift in average configuration which takes place in 

 simple polyelectrolytes, with the result that the transition may be a slow 

 process and may not occur under reversible conditions. Another difference 

 between the anticipated behavior of globular proteins and the behavior 

 of simple polyelectrolytes is that the former may be prevented by the pre- 

 sence of disulfide bonds from acquiring configurations as extended as those 

 of simple polyelectrolytes. 



It has been noted already that the stabilization of a compact configuration 

 for isoelectric proteins depends markedly on the arrangement of charged 

 sites, and that disulfide bonds may have a stabiUzing effect in some proteins 

 and not in others. 'Hydrophobic' bonding is clearly subject to similar varia- 

 tion, and so is any stabilization energy which may result from hydrogen 

 bonding or intramolecular ion pairs. It is clear that some proteins may be 

 expected to withstand a much higher net charge without configurational 

 change than others, and this proves indeed to be the case. 



Ribonuclease^" is an example of a protein with unusual stability in this 

 regard. Its intrinsic viscosity (Fig. 7) remains essentially equal to that ob- 

 served at the isoelectric point from pH 2 to pH 11-5, and in this range it 

 is essentially independent of ionic strength. The small changes (<10%) in 

 [rj] which do occur are of the order of magnitude to be expected for the 

 effect of charge ('electroviscous effect') on the viscosity of rigid impenetrable 

 spheres.^ 



We shall now list some globular proteins which are known to undergo 

 a major change in configuration as a result of pH changes. Many of the 

 examples given are well-known instances of acid- or base-induced 'denatura- 

 tion'. The most thoroughly studied example (serum albumin), however, 

 represents a very rapid and reversible reaction which would not ordinarily 

 be classified as a 'denaturation'. We shall not, on the basis of the data 

 available, be able to shed much light on the forces maintaining the initially 

 compact configurations. We shall be able, however, to demonstrate that the 

 product of the pH-induced configurational change, in every instance which 

 has been subjected to experimental test, is, as anticipated, a flexible molecule 

 exhibiting behavior characteristic of simple linear polyelectrolytes. 



Two criteria of such behavior will be used. The first is the expansion 

 of the molecular domain with increasing net charge and/or the contraction, 

 at constant charge, with increasing ionic strength. This behavior is illus- 

 trated by the viscosity data for polymethacrylic acid shown in Fig. 3. Sedi- 

 mentation or diffusion coefficients provide the same information. 



The second criterion is based on the titration curve. In particular, we shall 

 examine the dependence on net charge of the quantity pH— log a/(l— a), 

 in which a represents the experimentally observed degree of dissociation of 



Dps 



