176 V. HEMATIN COMPOUNDS 



been claimed for pyridine hemochrome above pH 10 (180); it is, 

 however, difficult to understand how such a polymerization could 

 occur. 



4.1.2. Stereochemistry of Base Combination. Only substances 

 which have their nitrogen atoms in exposed positions can combine 

 with the iron of heme without steric hindrance due to collision 

 between the remainder of the molecule and the large porphyrin plate. 

 Thus pyridine combines readily, a-picoline less easily, and substances 

 like quinoline, a,a'-dipyridyl, and o-phenanthroline not at all (1292). 

 The two last-named substances have a high affinity for iron, forming 

 complex salts by chelation, in this way removing the iron even from 

 ferrocyanide. They are unable to form complexes with heme, since 

 only one iron valency is available on each side of the porphyrin 

 plate, the latter being relatively so large that chelate ring formation 

 cannot occur around it. A similar situation occurs with ethylene- 

 diamine, only one amino group of which can combine with the iron; 

 the hemochrome formed thus contains two molecules of this sub- 

 stance. Only substances of very specific sterical properties, with 

 large crevasses between suitably situated exposed nitrogen atoms, 

 can combine with heme in such a manner that two nitrogen atoms 

 of the same molecule are linked to one heme iron atom (1683, p. 427). 

 W,e find this type of linkage in denatured protein hemochromes and 

 in cytochrome c (cf. Chapter VI and VIII). 



4.1.3. Absorption Spectra. The hemochrome structure is charac- 

 terized by the extreme sharpness of the a-band lying in the region of 

 550-560 niM. Since all hematin compounds can be readily transformed 

 to hemochromes by the addition of alkali, base, and reducing agent, 

 this band provides one of the best means for the spectroscopic detec- 

 tion of hematins. The position of the bands is influenced by both 

 the base and porphyrin. The effect of varying the base has been 

 investigated by a number of workers (cf. Anson and Mirsky, 65). 

 Thus the first band of ammonia and of hydrazine hemochrome lies 

 at 555 m/x, that of pyridine and of denatured globin hemochrome at 

 558-560 m/ix. One must remember, however, that the position of 

 the bands depends on the solvent and on the degree of association 

 (cf. H75) and can be very different in supersaturated solutions in 

 which precipitation of the hemochrome is impending. The position 

 of the absorption bands of mesohemochromes differs from that of 

 the protohemochromes by the usual 10 m/x. 



