3] CONFIGURATION OF GLOBULAR PROTEINS 39 



have had poor success. ^^ (See, for instance, the criticisms^- ■^^•^^•^"* of a 

 recent attempt by Scheraga and Mandelkern.'^) 



More interesting than the small deviations from the ideal spherical model 

 is the question why the behavior of these proteins is as close as it is to the 

 behavior of rigid spheres. In other words, what are the forces which main- 

 tain so compact a structure? As was pointed out earlier, these can be attrac- 

 tive forces between solvent molecules or attractive forces between segments 

 of the polypeptide chain, or both. In addition, the fact that aqueous solu- 

 tions of these proteins are stable at all requires the existence, at the surface 

 of the molecular particle, of a few segments of the polypeptide chain which 

 have strong affinity for solvent. The following pages will list in turn each 

 of the forces which may make a contribution, and will try, on the basis of 

 general chemical principles, to assess the relative importance which each 

 might have. Most of the points which will be brought out have been made 

 before, notably in a recent review by Kauzmann, ^^ They bear repeating, 

 however, as a deterrent against the dangerous (though popular) practice of 

 ascribing the stability of compact proteins to one force alone. 



Attraction between solvent molecules. One of the relevant forces which with- 

 out doubt contributes to the stability of compact configurations is the hydro- 

 gen bonding between water molecules. This force repels non-polar groupings 

 since these would create holes in the hydrogen bonded structure of the 

 solvent. In aqueous detergent solutions this force acts intermolecularly, caus- 

 ing the non-polar portions of detergent molecules to aggregate, with the 

 formation of micelles. In proteins there are many non-polar side chains on 

 a single molecule. The same force is therefore expected to act intramolecularly 

 and to lead to a compact structure with the non-polar side chains in the 

 center. Kauzmann** has used the term 'hydrophobic' bonding to describe 

 the intramolecular attraction which results from this force, and we shall 

 employ the same term. 



Electrostatic forces between charged segments of the protein chain. Isoelectric 

 proteins have a net charge of zero. The molecules possess, however, a large 

 number of charges, both positive and negative, Coulombic forces between 

 these charges must exist, and they provide a second factor which inevitably 

 affects the stability of a compact configuration. The contribution of the 

 interaction between these charges to the free energy of a compact protein 

 molecule can be calculated by the method of Kirkwood,*' which takes into 

 account the fact that the dielectric constant within the molecule is different 

 from that of the solvent, Kirkwood's method has recently been adapted by 

 the author^^-^" to estimation of the electrostatic interaction energy of spheri- 

 cal protein molecules with specified locations of charges. In this method, 

 the molecule is treated as being impermeable to solvent and the charges 

 are taken to be at the surface, in contact with solvent, such that the distance 



