236 VI. HEMOGLOBIN 



The changes which take place in the distal imidazole group on 

 oxygenation are discussed as follows: 



"We assume ,that the 3-nitrogen atom of this ring is restrained by the 

 configuration of the hemoglobin molecule to a relatively unfavorable position 

 for electrostatic coordination with the iron atom so that a proton can be 

 added, breaking the bond to the iron atom, at high enough acidity. These 

 assumptions explain the occurrence of the low pKi value 5.25 in ferrohemo- 

 globin for the imidazolium structure postulated for acid group I. The 

 coordination of the iron atom with an oxygen molecule on the same side of 

 the porphyrin ring would be expected to prevent the interaction with the 

 iron atom and thus to make the imidazolium group show more nearly the 

 same pK value as in histidine itself {pK = 6.04)." 



3.2.2.3. Linkage in Hemiglobin. Further data on the heme- 

 protein linkage became available on investigation of the hemoglobin- 

 hemtglobin equilibrium. Taylor and Hastings (2751) showed that 

 on oxidation of hemoglobin to hemiglobin the curve relating oxidation- 

 reduction potential to pH showed no inflexion corresponding to the 

 pK value of 8.1 found for the reaction Hi"^ + 0H~ ;;::i HiOH. Instead, 

 the potential (£^o) appeared to depend on an ionization with a pK 

 value of 6.65. 



Coryell and Pauling (^99) then reinvestigated the pH dependence 

 of the magnetic susceptibility of hemoglobin and were able to interpret 

 the variation in magnetic susceptibility with pH in terms of a group 

 with an approximate /)K value of 5.3, and the pK. value of 8.1 for 

 the hemiglobin-hemiglobin hydroxide equilibrium. They assumed 

 that the ionization corresponding to the pK value of 6.65 found by 

 Taylor and Hastings from oxidation-reduction potential measure- 

 ments had no effect on magnetic susceptibility. They explained the 

 variation of potential with pH by assuming that the ionization of 

 the group in hemoglobin with pK value 7.8 is replaced in hemoglobin 

 by the ionization of the hematin (hem/globin hydroxide — * hemi- 

 globin) with pK value 8.1. There is therefore no measurable effect 

 on the Eo/pH dependence. While the group with a pK value of 6.65 

 appeared to influence the potential, there was no evidence for a 

 spectral change of hem/globin in this pH region. The group with a 

 pK value of 5.3, which they found to influence the magnetic suscep- 

 tibility, appeared to be without effect on the oxidation-reduction 

 potential. 



The hemc-linked groups, the existence of which had been shown 

 by various methods, were summarized by Coryell and Pauling (Table 

 HI). In the table the symbols following the pK values describe their 



