48 



R. J. P. Williams 



myoglobin, and the other group is water we imply that there is no other 

 co-ordinating group very near to the iron. Between this extreme and the case 

 where there are two groups equally and strongly co-ordinated, as in cyto- 

 chrome c, every possible intermediate may arise through the inability of the 

 protein to satisfy simultaneously the stereochemical requirements of the iron 

 and those of hydrogen-bonding in its own structure. 



* See also Williams, p. 72 of this volume. 



The 'Imidazole' Hypothesis 



Pauling and Coryell (1936) considered that the two dissociation constants 

 of haemoglobin in the pH range 5-8 could be accounted for by assigning one 

 ^K to a dissociation of type (1), of an imidazole group which was at a con- 

 siderable distance from the iron atom and a second to the reaction (2). 



Basicity 



Fig. 3 



This hypothesis is readily tested by examining the ionization of Fe(DMG)2 

 (imidazole)2. We find that the imidazole ionizes as in reaction (1) but that 

 reaction (2) does not occur up to a pH of 11-0 (Croft and WiUiams, unpub- 

 lished). We add the following evidence against any such ionization. 



