38 CHARLES TANFORD [3 



collagen and myosin, obviously do not resemble compact spheres in aqueous 

 solution. The configuration of these proteins thus presents a problem dif- 

 ferent from that of the other proteins listed. We shall not discuss these 

 proteins in this paper. It is worth noting, however, that their different con- 

 figuration is not primarily a result of their high molecular weight. Other 

 proteins of high molecular weight, such as fumarase^^ and catalase,^^-^^ 

 appear from sedimentation and diffusion studies to have a compact con- 

 figuration near their isoelectric points. 



Table 1 



INTRINSIC VISCOSITIES OF ISOELECTRIC GLOBULAR PROTEINS 

 IN AQUEOUS SOLUTION 



{a) Unsolvated sphere of density 1 -33, by Einstein's equation.22 



{b) Polyisobutylene in cyclohexane.^^ 



(c) Poly-y-benzyl glutamate in dimethyl formamide.^^ 



Two questions arise in connection with those proteins which do possess 

 a compact, close to spherical shape. The first (and the less interesting) con- 

 cerns the meaning of the relatively small deviations of the observed viscosity 

 (or of other pertinent properties) from the theoretical values for perfect 

 rigid, solvent-free spheres. Part of the deviation undoubtedly results from 

 incorporation of some solvent in the dissolved particle.*-^''* Deviations from 

 spherical shape^ and non-rigidity presumably account for the remainder. 

 Attempts to obtain quantitatively the contribution of each of these factors 



* It is important to note, however, that the water so incorporated must be largely bound 

 or trapped water. That the protein particle is not permeable to the solvent as a whole is 

 shown unequivocally by the effect of ionic strength on titration curves. s 7 For compact 

 proteins this effect is much smaller than it would be if salt ions could enter the space 

 between charges, where they would most effectively shield interaction between charges. 



