IIEMICHROMES 179 



Clark and collaborators have attacked the problem by means of 

 an ingenious combination of potentiometric and spectrophotometric 

 titrations {If51-Jf53,536,27Jt9,2872: see also Chapter II). Despite the 

 wealth of information their work has produced, the results did not, 

 for the reasons mentioned above, succeed in eliminating all discrep- 

 ancies. Thus their potentiometric results for nicotine ferriproto- 

 porphyrin {536) indicate that this substance contains one molecule 

 of base, while the spectrophotometric study of pyridine ferriproto- 

 porphyrin (45i) is only in harmony with the assumption that in this 

 case two molecules of base are concerned. While in the latter case 

 spectrophotometric methods cannot distinguish between the two 

 possible reactions: 



[Fe] + 2B^[FeB2] 

 [Fe2l + 2B;^[Fe.2B.,] 



their results for pyridine ferricoproporphyrin showed no evidence for 

 polymerization. 



The evidence from the analytical composition of solid hemichromes 

 is also contradictory. Pyridine hemichrome has been obtained crys- 

 talline by precipitation from pyridine with ether or petroleum ether 

 {886,1122) but loses its pyridine too readily to give reliable data. A 

 somewhat more stable collidine compound had the composition 

 C34H3204N4FeCl • CgHiiN thus containing one molecule of base {838). 

 Rather stable compounds of hemin and hematin are formed with 

 imidazole compounds (imidazole, 4-methylimidazole, pilocarpine) 

 {1122,1643). From hematin in neutral chloroform-ethanol solution 

 Langenbeck {164-3) obtained compounds of the composition C34H33O5- 

 N4 • 2 B where B represents one molecule of base. Hamsik {1122), 

 however, isolated methylimidazole compounds of chlorohemin and 

 formylhemin which contained three molecules of base per atom of iron. 

 In the face of such conflicting evidence, no really satisfactory 

 picture of the structure of the hemichromes can as yet be presented. 

 The most probable structure on the acid side of the pK of the iron- 

 attached hydroxyl group may be represented by formula I, Figure 5, 

 B representing one molecule of the coordinating nitrogenous base. 

 Formula II of the same figure would be suggested in alkaline solution, 

 one molecule of base being removed by the entry of the hydroxyl 

 group. However, on the basis of their results, Clark and collaborators 

 incline to the view that even in the presence of the hydroxyl group, 

 two molecules of base still remain. Unless the hydroxyl group is 

 bound to some part of the porphyrin molecule other than the iron 



