PROPERTIES OF FERRIC COMPOUNDS 221 



2.3.5. Hemiglobin Azide. This compound was discovered by Smith and 

 Wolf {2576) in 1904 and later studied by Keilin in 1926 {U82). One molecule 

 azide combines with one iron atom. On adding sodium azide to hem/globin 

 the color changes to bright red, the spectrum showing a feeble band at 630 

 m)u. with bands in the green at o7o and 542 m/x. The spectrum is insensitive 

 to changes in pH. The magnetic susceptibility (^08,501) corresponds to one 

 unpaired electron per iron atom, with covalent bonds. 



Since hydrogen azide is a strong acid one would not expect to find pH 

 dependence in the acid range, but only for the reaction HiOH + N3" ^ 

 HiNa + OH". The reaction Hi+ + N3" ^ HiNs is independent of pU. 

 Keilin found the pK value for the latter reaction to be between 5.1 and 5.2, 

 while Kiese and Kaeske {1527) found yK = 5.16. The affinity of hemoglobin 

 for azide is thus almost as great as that for cyanide. 



2.3.6. Nitric Oxide Hemiglobin. This compound was first observed by 

 Keilin and Hartree in 1937 {14S9). Solutions are red in color, with bands in 

 the green at 568 and 531 m;u. The magnetic susceptibility of the compound 

 has not been measured. The dissociation constant has not yet been reported, 

 but the nitric oxide may be replaced by cyanide to form hem/globin cyanide. 

 Nitric oxide hem/globin is unstable, undergoing slow autoreduction to nitric 

 oxide hemoglobin. 



2.3.7. Hemiglobin Hydrosulfide. Keilin {147S) observed this compound in 

 1933. Solutions are red in color, with bands in the green at 570 and oio m/z. 

 The dissociation of hemoglobin hydrosulfide and its dependence on pH have 

 not been reported since, like nitric oxide hem/globin, it undergoes auto- 

 reduction. In the presence of oxygen, further changes take place, which are 

 discussed in Chapter X. The magnetic susceptibility corresponds to one 

 unpaired electron per iron atom, with covalent bonds. 



2.3.8. Hydrogen Peroxide Hemiglobin. This compound was dis- 

 covered by Robert {1558) in 1900. It is red in solution with bands at 

 589 and 545 ni/u. Hem/globin forms analogous compounds with 

 ethyl hydrogen peroxide and methyl hydrogen peroxide {11^85,2658). 

 The hydrogen peroxide compound has been studied by Haurowitz 

 {1166) and Keilin and Hartree {1^85). Haurowitz has shown that 

 in alkaline solutions the peroxide may be replaced by the hydroxyl 

 ion according to the equation Hi"^H202 + 0H~ — ^ HiOH + H2O2. 

 The hydrogen peroxide is therefore less fi^-mly bound to the hemiglobin 

 than the cyanide ion and, on addition of cyanide to solutions of the 

 hem/globin hydrogen peroxide compound, hemiglobin cyanide is 

 formed. Hemiglobin can compete with catalase for hydrogen perox- 

 ide, since the hydrogen peroxide hemiglobin can be detected spectro- 

 scopically when hydrogen peroxide is added to a solution containing 

 the two hemoproteins. 



Hydrogen peroxide hemiglobin is an unstable compound; the 



