200 V. HEMATIN COMPOUNDS 



and hydroxyl ions for a position of attachment to the iron atom of 

 the ferriporphyrin does not take place at the concentration of cyanide 

 used in the determinations. If, however, hepta coordination is 

 admissible as the explanation of the hydroxyl ion of the hemichromes, 

 it would have to be assumed that in the cyanide ferriporphyrins, 

 electrostatic repulsion by the negatively charged cyanide groups 

 renders the introduction of an additional negatively charged ion 

 impossible. 



It should, however, be understood that in the systems where: 



- ^ = 0.06 

 ApH 



all the oxidation-reduction potential measurements require is the 

 presence in the oxidant of one more dissociable hydroxyl ion than in 

 the reductant, or the equivalent of this. The nature of the dissociable 

 group is left for other methods to determine. 



Figures 10 and 11 show the pH dependence of the oxidation-reduc- 

 tion potentials of several heniatin systems. 



The relationship between the oxidation-reduction potentials of the 

 heme-hematin system and the cyanide iron porphyrin system is of 

 great importance in connection with the cyanide inhibition of hematin 

 catalysis. Reference to Figure 10 shows that the potential of hematin 

 decreases with increasing pH, in consequence of which the potential 

 of cyanide iron porphyrin, being invariant, becomes relatively more 

 positive. The assumption which has occasionally been made of a 

 fixed difference of characteristic potential (£^) between the two 

 systems is thus quite without foundation as is also the widely accepted 

 belief that combination of heniatin compounds with cyanide invari- 

 ably lowers the oxidation-reduction potential. 



At biological pH values, the relationship between the two systems 

 is not entirely clear. The curves dejjicted in Figure 10 approach one 

 another with decreasing pll; whether they will actually cross, and 

 the affinities of oxidant and reductant for cyanide thus become 

 reversed, depends on the position of the /;K of the hydroxyl dissocia- 

 tion of hematin, since there is no reason for assuming any change of 

 slope of the curve of the cyanide system in this region. According 

 to Barron's data (Fig. 11), the two curves actually meet at pH 8.2, 

 and the slope of the hematin curve is unchanged even below this 

 value. With the more positive hemochromes, however, the pH at 

 which the affinity of the ferrous form for cyanide becomes greater 



