CYANIDE COMPOUNDS 



189 



The equilibria involved in the formation of the heme-cyanide derivatives 

 are not clearly understood. The compounds have been investigated princi- 

 pally at high pH values. Here the two steps of association are quite distinct. 



TABLE VII 



Spectrophotometric Data and Dissociation Constants 

 of Cyanide Compounds 



Substance 



Absorption maxima 

 CmM 



mfi 



References 



Dissociation 

 constant 



Dicyanide 

 ferriproto- 

 porphyrin 



545 



422 



11.2 to 12 180,893,620,621,1307,1579 0.53" 



{1307) 

 88 180,1307 



Pyridine cyanide 

 ferriproto- 

 porphyrin 



545 



11.8 



620 



Monocyanide 

 ferroproto- 

 porphyrin 



552-555 

 524 

 415 



15.3 

 10.3 



620,393 

 393 



ca. 4 X 10"' 

 (pH 10.6)'' 

 (393) 



Dicyanide 

 ferroproto- 

 porphyrin 



565-570 



537-540 



430 



10.6 to 11 

 14.0 to 15.2 



180,393,620,621 



180 



1.5 X 10" 



(536) 



Globin cyanide 

 ferroproto- 

 porphyrin 



562 



7i 



Dicyanide 

 ferrospiro- 

 graphis 

 porphyrin 



593 

 550 



182 



»K 



[ Fe2(0H),] [CN]^ 

 [Fe(CN)2]2 [0H]2 



''K = 



[ Fe] [CN] 

 [FeCN] 



= K = 



[Fe] [CNP 

 [Fe(CN),] 



the monocyanide compound being formed almost to completion before the 

 formation of the dicyanide compound is evi<lent. At /;H values between 7 

 and 8, both compounds are still formed, but higher cyanide concentrations 

 are necessary. The decrea.se in formation of the monocyanide with reduction 

 of pH cannot be accounted for sim{)ly by association of cyanide ions to 



