182 V. HEMATIN COMPOUNDS 



occur if hematin is used, instead of hemin in organic solvents. In 1% potas- 

 sium hydroxide in methanol, Hamsik obtained a monopotassium mono- 

 imidazole hematin. The fact that only one of the two carboxylic acid groups 

 is ionized requires the dimeric formula ((\ Fig. 7) for this compound. A 

 similar formula (D) may also be postulated to explain the diimidazole com- 

 pound of hematin in neutral solution, but the analyses of Langenbeck and 

 Hamsik favor a formula with five, not four, oxygen atoms per atom of iron. 



4.2.4. Affinity of Hematin for Bases. It has been established by 

 Clark {5J/.2,54-S) that in alkaline solution linkage of bases to heme is 

 stronger than to hematin. It will be seen in Section 5. that this rule 

 may not apply for cyanide compounds close to the neutral point. 

 Owing to our lack of exact knowledge of the nature of the equilibrium 

 involved, it is not possible to use dissociation constants or their 

 corresponding pK values to express the affinity. In consequence 

 we have listed in Table V the negative of the logarithm of the base 



TABLE V 

 — log [B] at Whicli Half Dissociation of Hem/chrome Occurs 



Hemichrome ;>H — log (B| Ref. 



Nicotine proto 11.1 1.29 536 



Nicotine meso 11.10 1.34 536 



Pyridine copro 12.7 0.89 451 



concentration (— log B) at which 50% combination occurs. If it is 

 assumed that in the strongly alkaline solution used the equilibrium 

 is represented by the equation: 



FeOH + 2 B ;^ FeB.OH 



it follows that pK = 2 (— log B). This transformation makes the 

 values given comparable with the pK values given for hemochromes 

 in Table III. 



It will be seen by comparison of Tables III and V that most 

 hemichromes are still half dissociated at base concentrations of about 

 0.1 M, at which hemochromes are fully combined. 



The problem of the affinity of imidazole compounds to hematin 

 compounds is of particular importance in view of the role ascribed 

 to the heme-histidine linkage in hemoglobin and cytochrome c, but 

 it has received insufficient attention. The great affinity of imidazole 

 compounds for hematin observed by Langenbeck {164S) is often 

 unduly stressed, and a superficial study of the literature is misleading 



