HEMOCHROMES 175 



drazine, and hydrazine, and still lower for glycine. Lysine and 

 arginine do not form hemochromes, while histidine does {2776). 



It has been claimed (3165,1277) that the dissociation curve does 

 not conform to the assumed reaction equation: 



Fe + 2 B ^ FeBz 



but in the more extensive studies of Clark and collaborators it was 

 usually found to do so.* 



TABLE III 

 Dissociation of Hemochromes 



Substance pK 



Pyridine ferroprotoporphyrin 5.05 {1277), 6.3 {1322) 



Nicotine ferroprotoporphyrin 6.95 {1277), 5.5* {536)" 



a-Picoline ferroprotoporphyrin about 4.0 {1322) 



4- MethyUmidazole ferroprotoporphyrin about 5.3 {1322) 



Piperidine ferroprotoporphyrin about 5.0 {1322) 



Methylamine ferroprotoporphyrin about 3.5 {1322) 



Nicotine ferromesoporphyrin 5.9* {536)^, 5.7* {536)'' 



« At 30° C. * At 16° C. " At 23° C. 



In Table III the majority of values have been calculated from the 

 available data, using this equation. Da vies' values (536), marked 

 with an asterisk, were obtained from oxidation-reduction potential 

 measurements (cf. Section 5.) in ethanol-water solution, for which 

 the equation: 



[Fe] [BY- 



[FeB2 



= K 



was found to hold. However for nicotine ferroprotoporphyrin in 

 aqueous solution the equilibrium fitted the equation : 



Fe2 + 4B;=±Fe2B4 

 and the constant: 



[Fe,B^] 



had a value of 1.2 X 10"'". 



Dimerization of hemochromes, postulated here by Davies, has also 



* According to the recent investigation of Shack and Clark {2538a), dimeric ferro- 

 protoporphyrin combines with four moles of pyridine to give two moles of monomeric 

 dipyridine ferroprotoporphyrin; spectrophotometric evidence for a two-step association 

 was obtained. 



