170 



V. HEMATIN COMPOUNDS 



phyrin in alkaline alcoholic solution is uncharged and does not 

 migrate in the electric field {1157) : 



-lO 



ROFe- 

 H 



CO2M 



— OH 



CO2M 



If hemin is dissolved in excess alkali and titrated with acid, precipi- 

 tation occurs when approximately one equivalent of alkali remains 

 unneutralized. The resulting hematin or hydroxyhemin is usually 

 formulated as: 



-lO 



CO2H 



Fe OH 



CO,H 



The pK of the dissociation (FeOH) + H+ ;±: (Fe • OHa)^ lies, how- 

 ever, at a more alkaline pH than the pK of the dissociation of the 

 carboxylic acid groups {Jf51)* One would, therefore, rather expect 

 the following sequence of reactions on neutralization of alkaline 

 hematin solutions: 



-.2- 



+ H^ 



CO2 



H2O • Fe • OH. 



CO2 



+ H+ 



-lO 



CO2 

 H2O • Fe • OH2 

 CO,H 



It is probably the last complex, which, having lost its electric charge, 

 precipitates with loss of water and with polymerization. 



According to the conditions, the resulting solid may have either the 

 composition of hydroxyhemin or of partial or full anhydrides (Hamsik, 1117, 

 1120,1122). The formula given for hematins in Table I is thus strictly 

 correct only for alkaline hematin solutions. Amorphous hematin freshly 

 precipitated from alcoholic alkaline solution by acetic acid, or "crystalline 

 hydroxyhemin" prepared from crystals of pyridine hemichrome by removal 



* Cf. also Shack and Clark {2538a), who found a pK value of 7.6 for ferf iproto- 

 porphyrin. 



