168 V. HEMATIN COMPOUNDS 



since Haurowitz {1157) found that etiohemin, which has no carboxyl 

 groups in its side chains, as well as hemin esters, migrate toward the 

 cathode in slightly acid alcoholic solutions. If this formula is correct, 

 the various hemins, containing, for example, bromide, iodide, thio- 

 cyanate, azide, formate, or acetate, instead of chloride {861, p. 380), 

 do not differ in solution but only in solid form, and are to be considered 

 salts of the base hematin or hydroxyhemin. This is assumed by 

 Pauling and by Clark and Drabkin, as the nomenclature shows, and 

 also by Lindenfeld {17^8). Richter {22If8) raised the objection that 

 very little chloride ion can be removed from hemin crystals by boiling 

 water {111 If), but in view of the slight solubility of hemin in water 

 and the shielding of the chloride ions from the water by the large 

 planar molecules, this objection does not appear to be serious. 



The hemins with other radicals resemble a-chlorohemin. Deniges 

 {555), for instance, obtained azide hemin in typical Teichmann 

 crystals. With cyanide, however, hematin combines in a different 

 manner {cf. below). 



Different crystal forms of chlorohemin have been observed {17^8, 

 22If8). Qf-Chlorohemin crystallizes in various forms {224-8), but all 

 show oblique extinction and dichroism and differ in habit only. 

 /3-Hemin crystals, with straight extinction, are obtained from alcoholic 

 solutions {885,1113,11U,1600, 1604,2247,2248) while i/^-hemins are 

 soluble in alcohol acidified with sulfuric acid. It is still uncertain 

 whether these forms are only crystallographic modifications caused 

 by impurities (for example, by partial esterification of the propionic 

 acid side chains with the alcohol), polymorphous modifications {1748), 

 or whether they have a different structure of the complex. A linkage 



H. 



()■ 



;('H, 



-x^ 



Fig. 2. Stereochemical improbability of ring formation between iron atom an<l 

 carboxyl group of the same molecule in hematin compounds: bonds lying in porphyrin 

 plane ( — ); bonds lying outside porphyrin plane (•••). 



of the iron to the carboxylic acid groups of the propionic acid side 

 chains of the same molecule has been assumed by Kiister and Richter, 

 but is stereochemically excluded. Richter {2248) claims erroneously 

 that the formula shown in Figure 2 contains an eight-membered ring 



