62 CHARLES TANFORD [3 



will also operate against polymerization.* An interesting situation arises, 

 however, when an expanded protein is suddenly returned to near its iso- 

 electric point. The randomly coiled configuration is now no longer the 

 stable one, but return to a compact configuration may require specific coil- 

 ing of the polypeptide chains, and, as a result, may be a slow reaction. Under 

 these conditions polymerization may compete with return to a compact 

 shape, even if the latter is thermodynamically stable. Insolubility at the iso- 

 electric point, which is so often a consequence of denaturation, may be 

 explained in this way. The configurational changes described earher for 

 serum albumin and hemoglobin are reversible, so that return to a compact 

 shape is the favoured reaction for these proteins. In view of Kauzmann's 

 suggestion** that the number of disulfide bonds may determine the rate of 

 return to a compact configuration, it is interesting to note that, of these two 

 proteins, serum albumin has a large number of disulfide bonds (about 16), 

 but hemoglobin has none. 



CONCLUSION 



The most important conclusion which can be drawn from this paper is that 

 our ignorance of the configuration of proteins in aqueous solution greatly 

 exceeds the few facts which have been established. It appears possible that 

 the final answer to the problems which have been posed may be different 

 for each of the proteins which have been discussed. Moreover, a complete 

 theory will have to account not only for the properties of these globular 

 proteins, but it must also explain why proteins such as collagen appear to 

 have no stable compact configuration at all in aqueous solution. 



ACKNOWLEDGMENTS 



The work of the author and coworkers described in this paper has been supported by 

 research grants from the National Science Foundation and from the National Institutes 

 of Health, Public Health Service. This paper was written while the author was a John 

 Simon Guggenheim Memorial Fellow at Yale University. 



REFERENCES 



1. J. W. ANDEREGG, W. W. BEEMAN, S. SHULMAN and P. KAESBERG, /. Am. 



Chem. Soc, 77, 2927 (1955). 



2. K. AOKi and j. f. foster,/. Am. Chem. Soc, 78, 3538 (1956). 



3. R. ARNOLD and J. TH. G. OVERBEEK, Rec. trav. chim. Pays-Bas, 69, 192 (1950). 



4. s. BJORNHOLM, E. BARBU and M. MACHEBOEUF, Bull. SOC. chim. biol., 34, 1083 



(1952). 



* The slow polymerization of serum albumin in acid solutions"»^!-*^^ appears to be 

 due to a sulfhydryl-disulfide exchange reaction.^^ j^g association of serum albumin-® 

 and ovalbumin-^ after expansion by urea has been explained on a similar basis. These 

 reactions therefore involve changes in primary bonding. 



