188 V. HEMATIN COMPOUNDS 



dicyanide ferriporphyrin is formed, at sufficiently high cyanide con- 

 centrations, to a pll at least as low as 6. It may be seen from Hogness' 

 results that the reaction is assisted by increasing acidity until the 

 region of the pK of the hydroxyl group of the hematin is reached. 

 The /)K of hydrocyanic acid, however also lies in this region, with 

 conseijuent fall in cyanide ion concentration with further reduction 

 in pll. It would therefore be expected that with constant total 

 cyanide, an optimum of formation of dicyanide ferriporphyrin would 

 occur in the pll region 9-10. Zeile {3157) noted that color and 

 catalatic activity of cyanide hematin at pH 6 differed from those of 

 cyanide hematin in more alkaline solution. 



When crystalline hemin is dissolved in water-free hydrocyanic acid, 

 the spectrum produced is that of dicyanide ferriporphyrin (Haurowitz, 

 1157,1166). 



5.3.2. Cyanide Ferroporphyrins. The reaction of heme with 

 cyanide is one of stepwise association (69,393,620). At low cyanide 

 concentrations the spectrum is similar to that of hemochromes; at 

 high concentrations there is likewise a sharp two-banded spectrum, 

 but the bands are shifted toward the red, and the order of strength 

 is reversed (see Table \TI). At cyanide concentrations intermediate 

 between those necessary for these two types of spectrum, a diffuse 

 four-banded spectrum appears. Holden (1317) suggested that the 

 latter represented a third c\anide compound, but more recent work 

 (393) has shown that this is not correct. 



From the work of Anson and Mirsky (6,9), Drabkin (620), Hill 

 (1277), Holden and Freeman (1322), and Callaghan and Giovanelli 

 (393), it is reasonably certain that two heme cyanide compounds 

 exist, containing one and two cyanide groups per iron atom, respec- 

 tively, and therefore meriting the names monocyanide and dicyanide 

 ferroporphyrin.* The latter is the well-known "cyanide hemochro- 

 mogen." Its structure is probably: 



CN -i2- 



N^ I N 



Fe 



CN 



with two negative charges more than normal hemochromes. 



*Tlie same was recently found l)\- Sliack and Clark {'25-i8a). 



