3] CONFIGURATION OF GLOBULAR PROTEINS 59 



Strength 0-15, the intrinsic viscosity is 4-5 cc./gram, in contrast to the value 

 of 3-5 cc./gram observed at the same pH at ionic strength 0-25.^" At pH 2 

 (ionic strength unspecified) there is also a change in absorption spectrum,'^ 

 the steepness of which with respect to pH suggests that a configurational 

 change is occurring. It is thus possible that ribonuclease tends to acquire 

 a polyelectrolyte-Uke configuration at the extreme acid end of its titration 

 curve if the ionic strength is sufiiciently low. 



The release of phenohc groups by ribonuclease, which occurs at or above 

 pH 11 '5, does not take place under reversible conditions. As was pointed 

 out earUer, this prevents us from deciding whether the stability of the original 

 compact configuration of this protein represents true thermodynamic sta- 

 bility, or whether it merely reflects a requirement for a larger free energy of 

 activation. One of the most fruitful approaches to an understanding of 

 ribonuclease and other proteins which change configuration under irrever- 

 sible conditions would be a search for conditions under which a reversal 

 to a compact configuration occurs. From a study of such a reversal, one 

 could obtain a measure of the true thermodynamic stabiHty of the compact 

 configuration. 



Ovalbumin. Ovalbumin, Hke ribonuclease, undergoes no major configura- 

 tional change between pH 2 and pH 11-5. This is shown by the titration 

 curve of Cannan and coworkers, ^^ which fits the compact sphere model 

 throughout this range of pH, and by the absence of change in the polariza- 

 tion of fluorescence, shown in Fig. 10.^°^ The viscosity data of Bull^ are 

 not entirely unequivocal, because tjs^/c was determined at each pH at a 

 single concentration and not extrapolated to give the intrinsic viscosity. At 

 the very low ionic strengths used in some of his experiments, this creates 

 considerable uncertainty. Within this uncertainty the results confirm the 

 conclusion reached from the other data cited. ^ 



Ovalbumin behaves Hke ribonuclease in another respect. Its phenofic 

 groups, initially inaccessible to titration, are slowly released near pH 12 or 

 higher. ^^ Whether this process is accompanied by expansion is not known. 



Insulin. In many ways, insufin is the best known of all globular proteins. 

 However, only scant information is available about the effect of pH on its 

 configuration. There is certainly no configurational change in acid solutions, 

 as has been shown by viscosity measurements of Yang and Foster.^^^ The 

 titration curve in acid solution is anomalous,^* but the anomaly is fully ex- 

 plained on the basis of molecular weight changes which this protein is known 

 to undergo (see below). 



Lysozyme. Lysozyme is another protein for which no expansion has yet 

 been observed. Steiner" has shown that the polarization of fluorescence of 

 dye molecules bound to lysozyme remains unchanged between pH 3-5 and 



