16 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



the chain shows no tendency to fold up or to crystallize, then it is known 

 that polymer chains tend to squirm about, assuming all possible shapes in 

 much the same manner that a piece of spaghetti does when it is tossed 

 into a pot of boiling water, or a string of beads does when it is tossed into 

 the air. This writhing chain is what is known as the random coil. Polymer 

 chains take on the multitude of shapes that characterize the random coil 

 for the same reason that gas molecules tend to fill up a room — the random 

 coil is a state of high entropy. 



Now it is important to note that the three principal hydrodynamic 

 properties of a random coil (that is, the viscosity increment, the rotatory 

 diffusion constant and the friction ratio) cannot be duplicated by any 

 single equivalent ellipsoid (see footnote 41 of Scheraga and Mandelkern, 

 30). It is probably also true that any other shape that differs markedly 

 from an ellipsoid — such as a hoop, a helix, a spiral or an elbow- — would 

 also not have hydrodynamic properties that are exactly the same as those 

 produced by any single ellipsoid. The preoccupation that protein chemists 

 have for rigid ellipsoids would therefore seem to be unwise when applied 

 to proteins under certain conditions. This is especially true because the 

 theory of the hydrodynamic properties of random coils has been thor- 

 oughly developed (see Flory, 6, chap. 14). A preliminary attempt to in- 

 terpret the hydrodynamic properties of some denatured proteins has been 

 given (15), but more experimental work ought to be done from this point 

 of view. A great deal could be learned about such details as the intra- 

 molecular cross-linking of protein molecules in this way. 



OPTICAL ROTATORY POWER AND PROTEIN DENATURATION 



The theory of the relationship between optical rotatory power and 

 molecular structure has not yet been worked out in detail, but certain 

 general principles are known which should be kept in mind by anyone 

 using this property to study proteins. Many people seem to have the mis- 

 conception that optical rotatory power somehow resides in the asymmetric 

 carbon atom. This is quite wrong. According to Pasteur, the sole require- 

 ment that must be fulfilled in order that a substance be able to rotate the 

 plane of polarization of light is that the molecules of which the substance 

 is composed must not be superimposible on their mirror images. All of 

 the modern theories of optical rotatory power are in complete harmony 

 with Pasteur's rule. This means, for instance, that the following two con- 

 formational isomers of the simple molecule, butane (which contains no 

 asymmetric carbon atom) , would show optical activity if they could be 

 separated from each other: 



