3] CONFIGURATION OF GLOBULAR PROTEINS 41 



The fact that most protein molecules in solution have very low permanent 

 dipole moments,^^ strongly suggests that positive and negative charges are 

 in fact more or less evenly distributed. Such an arrangement always leads 

 to a net negative electrostatic interaction energy for the isoelectric state, 

 with the result that electrostatic forces contribute materially to the stability 

 of compact configurations.* 



Further calculations for these same models will be reported in Table 2. 

 It should be noted that the isoelectric point is not necessarily the point at 

 which the electrostatic interaction energy is a minimum. For model C, for 

 instance, the minimum occurs when the net charge is —4. 



Intramolecular hydrogen bonds. A third force is provided by intramolecular 

 hydrogen bonds. ^^-^^'^^-^^ It is convenient to distinguish between two kinds 

 of intramolecular hydrogen bonds. The first is the familiar peptide hydrogen 

 bond ( — C = 0. . .HN=).^'"^^ The second kind is the hydrogen bond be- 

 tween side-chain groups. ^^ Examples of the latter are the bond between 

 undissociated carboxyl groups, well known in the dimeric form of carboxylic 

 acids in some nonaqueous solvents, and the frequently postulated phenol- 

 carboxylate bond. Such hydrogen bonds are probably the principal factor 

 determining the configuration of proteins in the solid state. ^^ In aqueous 

 solutions, however, there is a competition between formation of such intra- 

 molecular bonds and formation of hydrogen bonds between the same groups 

 and water molecules, so that their effect on configuration is likely to be 

 reduced. When appropriate small molecules are studied, to which the same 

 kind of competition applies, equilibrium is found to lie on the side of hydro- 

 gen bonds to water. Thus Schellman''" has calculated, from the thermo- 

 dynamic behavior of urea in water solution, that in such solutions the 

 association of two urea molecules by hydrogen bonds of the peptide type 

 is accompanied by 3. positive free energy change (zlF° = + 1990 calories/mole). 

 Similarly, for the association of tyrosine with acetate^"'' by formation of 

 phenol-carboxylate hydrogen bonds, AV°> 1400 calories/mole. 



Of course, these model reactions are between small molecules which are 

 free in solution, and are therefore accompanied by a loss of translational 

 entropy. The formation of the corresponding bonds between segments of 

 a polypeptide chain involves in place of this factor a loss of rotational 

 entropy. The difiference between these quantities could well be sufficiently 

 large to change the sign of the free energy of formation of such hydrogen 

 bonds in aqueous protein solutions. In fact, Schellman'^ made a calcula- 

 tion to show that this is just what would happen in the case of peptide 



* The contribution of electrostatic forces to the stability of a compact configuration 

 is, of course, the difference between the figures calculated and the corresponding inter- 

 action energies in a randomly coiled configuration. The latter, however, are comparatively 

 small because the space between charges in solvent-permeated configuration is filled by 

 material of high dielectric constant. 



