12 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



conditions, however, it is found that serum albumin undergoes an instan- 

 taneous change to a much less compactly folded state as soon as it is ex- 

 posed to acid or urea, whereas ovalbumin undergoes similar changes much 

 less rapidly. If we define denaturation in terms of changes in the way the 

 polypeptide chain is folded, we must conclude that ovalbumin is more re- 

 sistant than serum albumin to denaturation by acids and by urea. The 

 reluctance of serum albumin to lose its solubility in water at the isoelectric 

 point is merely the result of the ability of its polypeptide chain to refold 

 into a state closely similar to (if not identical with) that of the native 

 protein as soon as the denaturing agent (acid or urea) is removed. It is 

 obvious that if we were to use solubility in water at the isoelectric point 

 as the sole criterion of denaturation, we should overlook some important 

 instances in which proteins temporarily lose the structures that charac- 

 terize the native form. It is clearly desirable that in setting up experimental 

 criteria for denaturation, these criteria should be applicable while the 

 protein is exposed to the denaturing agent. If this is not done, and if the 

 protein can change its structure reversibly, it is possible to gain a com- 

 pletely false impression of the lability of the native structure in the pres- 

 ence of denaturing agents. 



METHODS AVAILABLE FOR OBSERVING STRUCTURAL CHANGES 

 IN PROTEINS DURING DENATURATION 



If we adopt the more fundamental approach to the phenomenon of de- 

 naturation and choose as the criterion of denaturation a change in the 

 state of folding of the polypeptide chain rather than loss of solubility or 

 biological activity, then suitable experimental methods of detecting these 

 changes in folding must be available. In order to avoid the uncertainties 

 caused by reversible denaturation, these methods should be usable when 

 the protein is in the presence of the denaturing agent. 



The properties of proteins that change when the polypeptide chain 

 changes its state of folding fall into two general classes. One class of 

 property depends on the over-all geometrical shape of the molecule and 

 only indirectly, if at all, on the chemical composition and the precise 

 spatial relationships of the different parts of the polypeptide chain to their 

 immediate neighboring parts. The other class of property depends on the 

 short range interactions of each part of the chain with other parts that 

 are close to it. For brevity the two classes may be called shape properties 

 and short-range properties. The distinction between the two classes may 

 best be clarified by listing the more important members of each class. 



Shape Properties (Many of the following properties depend also on 

 the molecular size, so that an independent measurement of the molecular 

 weight may be necessary in judging the structural significance of a change 



