The Electronic Structure of Haemoglobin 



a positive charge is transferred to the iron atom as a result of the 

 attraction of the strongly electronegative atom oxygen for the electrons 

 in the double bond, this attraction being transmitted through the carbon 

 atom in the case of carbonmonoxyhaemoglobin. The iron atom would 

 then be enabled to form a bond with partial covalent character with 

 another nitrogen atom, receiving from this bond enough negative 

 charge to neutralize the positive charge given to it by the partial ionic 

 character of the double bond. The positive charge transferred to the 

 nitrogen atom would then be shifted to the other nitrogen atom of the 

 imidazole ring, through the imidazole system of conjugated double 

 bonds, and would have the effect of increasing the acidity of the 

 imidazole group. 10 



Table I 

 Haem-linked Acid Groups in pH Range 4-5 to 9 a 



Hb+ pK x = 5-3 Mo] pK 2 = 665 Si, Mi,PopK 3 = 8-105/, Mo) 



\Pi \Pi 



Hb pK x = 5-25 Mi) pK» = 7-83 Si, Mi ) 



Hb0 2 

 HbCO 



pK x = 5-75 Mi, Po pK 2 = 6-80 Si, Mi, Po 



a S, M, P mean spectrophotometrically , magnetically, and potentio- 

 metrically, respectively ; o means operative, i inoperative. All acid 

 groups are of course potentiometrically operative, except when cancellation 

 of the effects of two groups occurs, as indicated by Pi for bracketed pairs. 



There is another haem-linked acid group in haemoglobin, with acid 

 constant pK x — 5-25 for ferrohaemoglobin, 5-3 for ferrihaemoglobin, 

 and 5-75 for oxyhaemoglobin. It has been suggested 1 ' 9 ' 10 that this 

 group also is an imidazole group of a histidine residue, held by the 

 structure of the globin molecule in a position somewhat removed from 

 the iron atom ; but the nature of the interaction between the haem 

 group and this acid group has not yet been determined. No explanation 

 has been offered of the fact that ionization of this group changes the 

 magnetic moment of ferrihaemoglobin significantly, but does not have 

 an observable magnetic effect on ferrohaemoglobin and its compounds. 



The acid group giving/?^ = 8-10 for ferrihaemoglobin presumably 

 is a coordinated water molecule, which is converted into a hydroxide 

 ion by loss of a proton. 



61 



