CYTOCHROME C 



355 



The lack in ferrocytochrome c of the pK values ascribed to imidazole is 

 not explained. It appears likely to us that in ferrocytochrome c the resonance 

 form A greatly prevails over B or intermediate forms. 



NH 



The dissociation of a proton from form A can be compared with that of 

 pyrrole as acid, while that of a proton from form B is comparable to the 

 dissociation of a pyridinium ion (cf. Chapter VI, 3.2.2.4.). If form A greatly 

 prevails, the pK may become very high. 



Form II resembles hemoglobin in its structure, its absorption spectrum 

 (with the shift of the bands toward the ultraviolet caused by the saturation 

 of the unsaturated side chains of the hematin) and its magnetic susceptibility. 

 Like hemiglobin it forms a fluoride compound the absorption spectrum of 

 which resembles that of hemiglobin fluoride. 



Form III of ferricytochrome as formulated by Theorell would be expected 

 to have the same high pK value as ferrocytochrome c. Its lower pK value 

 may be due to prevalence of the resonating forms 111 A and IIIB. 



lUA 



+ 

 ti NH 



N. 



UIB 



NH 



N. 



N" 



HI 



Fe 



N 



U NH 



