CARBON MONOXIDE AND CYANIDE COMPOUNDS 187 



5.3. Cyanide Compounds 



Cyanide ion has been found to form compounds with heme, 

 hematin, and hemochromes. The well-known substance cyanhematin 

 was known to Hoppe-Seyler as early as 1865. "Dicyanide hemo- 

 chromogen" is of more recent discovery, and compounds containing 

 both cyanide and base attached to heme or hematin hav^e been found 

 later still. The state of knowledge with regard to all of these is, 

 however, far from satisfactory, a serious deficiency in view of the 

 fundamental importance of cyanide inhibition as a tool in enzyme 

 chemistry. 



These substances show marked differences from the hemochromes 

 and hemrchromes and are therefore treated separately. For the 

 same reason we shall use the Clark-Drabkin nomenclature, not the 

 terms "cyanide hemochrome" or "cyanide hemichrome." 



5.3.1. Dicyanide Ferriporphyrin. Hogness et al. (1307) have shown 

 spectrophotometrically that cyanide combines with hematin in alka- 

 line solution (pH 10.7 to 13.2) according to the equation: 



[Fe2(OH)2] + 4 CN- ;=± 2 [Fe(CN)2]- + 2 OH- 



The compound so formed thus evidently has the structure: 



N' 



CN 



Fe 



CN 



.N 



In this figure the charges due to the side chain carboxyl groups are 

 omitted; the complex has one negative charge more than normal 

 hemochromes. 



Drabkin (620) has suggested that the addition of cyanide to 

 hematin may occur stepwise. However, he gives no evidence to show 

 that this is so. It has recently been shown (393), in conformity with 

 Hogness, that the absorption curves of mixtures of hematin and 

 dicyanide hematin of constant total hematin concentration and at 

 pH 10.6 show only one isosbestic point in the visible region, and that 

 therefore only one compound of hematin with cyanide exists, at 

 least in the pH region investigated. 



The reaction between hematin and cyanide at low pH values has 

 not been investigated to any extent. It is known, however, that 



