36 CHARLES TANFORD [3 



coiled macromolecules are ordinarily described in terms of an average 

 configuration. Fig. \c may be taken as a schematic representation of this 

 average configuration. Exactly random coiling can occur only if the reac- 

 tion, 2 segment-solvent neighboring pairs -> 1 segment — segment neighbor- 

 ing pair + 1 solvent — solvent neighboring pair, has exactly zero free energy 

 change, a situation which can arise only fortuitously ('ideal' solvent*). The 

 class of randomly coiled polymers in solution is therefore ordinarily defined 

 so as to include polymer-solvent systems in which there are slightly more 

 ('good' solvent) or slightly fewer ('poor' solvent) segment-solvent contacts than 

 would be predicted on the basis of statistics alone. The corresponding aver- 

 age configurations can be described schematically as moving from Fig. \c 

 towards Fig. \d or Fig. \b, respectively. Most of the common non-polar 

 polymers, dissolved in non-polar solvents, are randomly coiled if this more 

 flexible definition of the term is used. 



(2) Helical rods. If the predominant attractive force in a polymer-solvent 

 system is a specific attraction between a particular atom of each segment 

 of a chain molecule for another particular atom of another segment, then 

 the probable result is a helical configuration. ^'-^^ The resulting overall shape 

 is that of a long, thin rod, as illustrated by Fig. \e. The most familiar helical 

 configuration is the Pauling a-helix.^^ This configuration describes certain 

 polypeptides in some solvents, e.g. poly-y-benzyl glutamate in dimethyl- 

 formamide.^^ 



(3) Compact, rigid, impenetrable spheres. If the only important force in a 

 polymer-solvent system is either solvent-solvent attraction or non-specific 

 segment-segment attraction (or both) then segment-solvent contacts will be 

 minimized, a result best achieved by insolubility of the polymer in the sol- 

 vent. If, however, the polymer contains more than one kind of segment, 

 and if some of the segments have strong affinity for solvent, segment-segment 

 or solvent-solvent attraction being otherwise the predominant force, then 

 the polymer will be soluble and its optimum configuration will be one which 

 minimizes segment-solvent contacts except for those segments which have 

 strong affinity for solvent. An ideaUzation of the resulting configuration is 

 a compact, rigid sphere, impermeable to solvent (Fig. \a). The segments 

 attracted to solvent are on the outside. The remainder fill the interior space, 

 which is devoid of solvent. 



The three extremes just discussed were chosen because they may be un- 

 equivocally distinguished by almost any physical measurement. For instance, 

 compact rigid spheres have much smaller radii of gyration and much larger 



* The definition here given in terms of the stability of individual segment-solvent pairs 

 is, of course, an oversimplification. Flory24 should be consulted for a full discussion of 

 this subject. 



