2'-20 VI. HEMOGLOBIN 



in 1899 {1100). When carefully neutralized cyanide is added to 

 hemiglobin, the color changes from brown to bright red and the 

 spectrum shows a single absorption maximum at 540 m/x. The sub- 

 stance was first crystallized by von Zeynek (3177) in 1901, who 

 showed that one molecule of cyanide was combined with one iron 

 atom (cf. also Haurowitz, 1166). The magnetic susceptibility corre- 

 sponds to one unpaired electron per iron atom, signifying covalent 

 bonds. 



Holden {1315) assumed tliat cyanide is bound irreversibly to protein in 

 liem/globin cyanide. The dissociation constant has, however, been measured 

 by several workers. Recently it has been shown that hemoglobin cyanide 

 gradually inhibits the cytochrome oxidase system, which also indicates a 

 slow dissociation {Sli); the affinity of cytochrome oxidase for cyanide seems to 

 be higher than that of hemi'globin.- The cyanide is also removed by prolonged 

 dialysis against running water {392), although some alteration of the free 

 hemi'gloljin may occur. 



Many of the reagents which combine with hem/globin are them- 

 selves completely ionized at pH 5; at a pH greater than this we need 

 only consider the combination of the ion with hemiglobin or hemo- 

 globin hydroxide. Hydrogen cyanide however is a weak acid {pK. = 

 9.1) and we must therefore consider the influence of pH on the 

 dissociation of the reagent as well as on the hemzglobin ^ hemiglobin 

 hydroxide ^equilibrium. The three cases are: 



Hi+ + HCN ^ HiCN + H+ (a) 



HiOH + HCN ^ HiCN + H2O (6) 



HiOH + CN- ^ HiCN + OH- (c) 



Reactions a and c * are sensitive to pH while h is not. 



For equilibrium (a), Coryell, Stitt, and Pauling {502) found a pK value of 

 — 1.'25 by measurements of magnetic susceptibility, while Havemann {1187, 

 1188) found — 1.0'-2 by photoelectric colorimetry.' For equilibrium (^), Hauro- 

 witz {IIGC)) and Havemann {1187) found a yjK value of 0.0. Reaction (c) has 

 not yet been studied. The experimental y)K value for reaction {b) disagrees 

 with that calculated from the pK value of reaction (0) and the j)K of the 

 dissociation of hem/globin hydroxide. Reaction (6) cannot be measured accu- 

 rately, since there is no pH at which it is unaccompanied by either (a) or (c). 

 Similar divergencies have, however, been reported for the equilibria of hemi- 

 globin hydroxide with other anions {1188, 1756; cf., however, 502). They 

 are prol)ably due to the interaction of the dissociation of heme-linked groups 

 of globin with the dissociation of anions from the iron {cf. Section 3.2.2.3.). 



* Unless heptacooniiiiation occurs: HiOH + CN-;i± Hi(OH)(CN). 



