238 VI. HEMOGLOBIN 



imidazoles in hemoglobin in the same way as in the oxyhemoglobin- 

 hemoglobin system (Section 3.2.2.4.). The chief effect of oxidation 

 of hemoglobin to hemiglobin is the increase in the formal positive 

 charge by one unit, the bond type remaining essentially ionic. Both 

 oxidation and oxygenation decrease the dissociation of the proximal 

 imidazole from a pK of 7.9 to 6.7, about one pH unit. They deduce 

 from this the distance which separates the ionizing group from the 

 iron atom: 



"Assuming that the electrostatic interaction of the iron atom and the 

 proton of the acid group is solely responsible for this change in pK2, we can 

 estimate roughly the distance between acid group II and the iron atom from 

 comparison with some dipositive acids such as the salts of the alkaline 

 diamines. The first and second pK values of propylenediamine and butylene- 

 diamine differ by 2.04 and 1.54 units respectively. These values must be 

 decreased by log 4 or 0.60 for the symmetry effect, which does not operate 

 between the chemically unlike second and third acid groups of ferrihemo- 

 globin. The observed increase in pK2 on oxidation of the iron atom lies 

 between the corrected diamine differences (ApK) 1.44 and 0.94. The dis- 

 tance between the nitrogen atoms between the two diammonium salts, 

 assuming extended configuration and the angles and distances given by 

 Pauling {2125), are 4.94 and 6.27 A., respectively. We conclude that acid 

 group II of ferrohemoglobin and ferrihemoglobin is roughly o A. from the 

 iron atom, about the expected distance between an iron atom near one of 

 the ring nitrogen atoms of a histidine residue and the second nitrogen atom 

 of the ring." 



The behavior of the distal imidazole is explained as follows: 

 "On oxidation of the iron to the ferric form, the pK, value would at first 

 sight be expected to be lowered, as is the pKi value; instead, no appreciable 

 change is observed. If, however, after addition of a proton to the imidazole 

 group a water molecule coordinates (through dipole attraction) with the iron 

 atom in the ferric state more strongly than with it in the ferrous state, the 

 corresponding extra stabilization of the acid form by the water molecule will 

 tend to offset the expected decrease in pK i when the iron atom is oxidized. 

 The cancellation of these effects seems to be complete." 



3.2.2.4. Objections to the Imidazole Hypothesis. While this 

 hypothesis is able to account for a number of the properties of the 

 heme-globin linkage, Roughton {2362) has pointed out that it is 

 unable to account for at least one important reaction, the direct 

 carbamino combination of carbon dioxide with oxylabile groups in 

 the hemoglobin molecule. It has been established by a number of 

 workers {7 If5a,h;1882a:2586a) that more carbon dioxide is bound as 

 the carbamino compound in the hemoglobin of the ox, horse, and 

 human species than in the oxyhemoglobins. Now, only free amino 



