HEMICHROMES 183 



since it gives the impression that imidazoles have a far greater 

 affinity for hematin than for heme. 



Using alkaHne solutions, Holden {1322) found heme to combine 

 less readily with 4-methylimidazole than with pyridine and from his 

 data it may be calculated that half dissociation of the resulting 

 hemochrome occurs at a base concentration of 7 X 10"^ M, a value 

 comparable with that for other hemochromes. From Langenbeck's 

 results the corresponding figure for 4-methylimidazole hemichrome 

 in 1% aqueous sodium carbonate solution may be estimated as about 

 7 X 10"* M, comparable with other hemichromes. 



The data on combination of hematin with histidine are very scanty 

 and somewhat contradictory. Although Hamsik {1122) found histi- 

 dine hemichrome easily dissociable in neutral methanol solution, and 

 Haurovvitz {1175) observed no reaction with histidine, Barron's data 

 on oxidation-reduction potentials as a function of nitrogenous sub- 

 stance concentration {ISO, p. 296) indicate that in alkaline solutions 

 abov^e pH 9 histidine and pilocarpine are bound more firmly to heme 

 than to hematin. No observations have been made on the combi- 

 nation of hematin with histidine peptides. From the results quoted, 

 it is clear that although the difference in dissociation between 4- 

 methylimidazole hemochrome and 4-methylimidazole hemrchrome 

 may be much smaller than that between pyridine hemochrome and 

 pyridine hem/chrome in neutral solution, the affinities in alkaline 

 solution of imidazoles for heme and hematin bear the same relation 

 as those of other nitrogenous bases. The available data do not show 

 whether this relationship is maintained with decreasing alkalinity or 

 whether, like the cyanide compounds, the affinities become more 

 nearly equal or perhaps reverse {cf. Section 7.2.). 



4.2.5. Absorption Spectra. The absorption spectrum of pyridine 

 hemtchrome in neutral solution has two bands similar in position to 

 those of pyridine hemochrome, a fact which has confused earlier 

 workers. The fact that pyridine hemochrome is reduced to the 

 hemochrome by impurities present in crude pyridine has contributed 

 to this confusion. Actually the two absorption spectra are very 

 different. Pyridine hemichrome has only a weak band at 558 m/x 

 («mM = 8.5) and a stronger ban<l at 530 m^ (e,„M = 10.2). The 

 quantitative data are derived from a paper of Barron {ISO). 



Barron has also .shown that the absorption in alkaline solution, i.e., 

 that of pyridine hemichrome hydroxide, is (juite different from that 



