20 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



an optical activity that is small in an occasional compound containing 

 groups whose movements are restricted. On the other hand, the optical 

 activity should always be small in compounds containing groups that 

 orient freely about all of the single bonds that are attached to asymmetric 

 carbon atoms. Thus, a small optical activity is a necessary basis, but not 

 a sufficient one, for concluding that free orientation exists in a molecule 

 containing asymmetric carbon atoms. If [ajo is much greater than 10° 

 for organic molecules that do not have absorption bands in the visible 

 or near ultra-violet, then we can also conclude that the orientation about 

 single bonds is restricted. 



Most native proteins have values of [a]D that fall between —20° and 

 — 60°, and on denaturation [oJd invariably becomes more negative. In 

 strong urea solutions values as large as —120° may be reached at low tem- 

 peratures, but heat denaturation in the absence of urea usually leads to 

 values of —70° to —80°. (For detailed data see especially the papers of 

 Jirgensons, 11-13, and the following important paper by Schellman, 29.) 



The fact that native proteins have optical rotations of the same sign 

 and similar order of magnitude is an indication that some basic structural 

 resemblance between them must exist. One is tempted to believe that 

 this similarity may reside in the helical structure proposed by Pauling and 

 Corey (25), but recent work with synthetic polypeptides by Doty and 

 his co-workers indicate that the helix itself is probably dextrorotatory 

 (4). Yang and Doty (32) have made the very plausible suggestion that 

 the negative sign of the optical rotation of native proteins is a result of 

 the partial state of imperfect folding that exists in some degree in all 

 native proteins. That is, sections of the polypeptide chain may be folded 

 into helices, but intervening portions have the disorganized structure — and 

 also the greater levorotation — that characterize the denatured protein. 



The fact that denatured proteins have relatively large optical rotations 

 is an indication that although the molecule may have an over-all shape 

 resembling that of a random coil (which implies a certain amount of 

 flexibility in the chain), nevertheless a considerable amount of steric 

 hindrance must be operative, preventing completely free orientation about 

 the bonds attached to the asymmetric carbon atoms. Short sections of the 

 polypeptide chain must therefore be relatively stiff, even in the denatured 

 molecule. 



The magnitude of the optical rotation of denatured proteins in urea 

 solutions decreases markedly with increasing temperature, just as we 

 should expect if freedom of oi'ientation increased with increasing tempera- 

 ture. The temperature coefficients of the ojitical rotations of denatured 

 proteins in water are surprisingly small, however. 



There is an appreciable change in the optical rotation when chymo- 



