178 V. HEMATIN COMPOUNDS 



definite evidence that heraatin combines with pyridine in alkaline 

 solutions; it has even been shown that casein and denatured albumin 

 form hemichromes in alkaline, not in neutral, solution {1322). 

 Whereas the absorption spectra and stability of hemochromes are 

 little affected by pH changes above a certain pH value, absorption 

 spectra and (at least in some cases) stabilities of hemichromes depend 

 on the pH. This is explained by the presence of a hydroxyl group 

 which, bound to the ferric iron atom in alkaline solutions, dissociates 

 with increasing acidity. 



Clark and collaborators (4.51,2872) have reported the first measure- 

 ments of a ^^K for the dissociation of hydroxyl from a ferriporphyrin 

 hydroxide : 



[FeOH] 4-H+^ [FeOH2] + 



and a few values for the dissociation of hydroxyl from hemichromes. 

 The pK of ferricoproporphyrin hydroxide was found to be 7.44, that 

 of pyridine coprohemichrome hydroxide 9.94 (potentiometrically) or 

 10.3 (spectrophotometrically), that of nicotine coprohemzchrome 

 hydroxide 8.92. For pyridine protohemichrome a pK. value of 8.96 

 at 25° C. {1699) has been found by spectrophotometry. 



4.2.2. Bond Type in Hemichromes. The bonds in alkaline solu- 

 tion of pyridine hemichrome were shown by the magnetochemical 

 method to be of d-sp^ covalent type, with 1 unpaired electron {2219). 

 The same result was found for ferricytochrome in neutral and weakly 

 alkaline solutions (Chapter VIII). Hemin dissolved in neutral 

 aqueous 50% pyridine was found to have a magnetic susceptibility 

 of 3.1 Bohr magnetons, which Rawlinson explains as due to incomplete 

 hemichrome formation {2219). If this explanation is correct, it 

 follows that pyridine hemichrome dissociates much more readily in 

 neutral than in alkaline solution, since in alkaline solution containing 

 only 20% pyridine the bonds were found to be fully covalent. 



4.2.3. Composition and Structure. The facts that ferric hematin 

 compounds tend to polymerize and that they react with hydrions 

 and hydroxyl ions introduce an enormous complexity into the problem 

 of the composition and structure of hemichromes. The experimental 

 difficulties are increased by the small solubility particularly of the 

 uncombined hemes and hematins in water and the great oxidizability 

 of hemes. Thus it has not yet even been possible to establish whether 

 hematin combines with one molecule of base to form a hemichrome, 

 or with two. 



