Some Physical Properties and Chemical Reactions of Iron Complexes 47 



(>^max !^ 400 m-fi). Again there is evidence of bands at about 640 m/t in the 

 Fe+++ complexes, the band expected for ionic Fe+++ haems. 



On the basis of this analysis of the properties of the cytochromes we 

 need to modify the picture which we have given previously for the relationship 

 between redox potential and the basicity of the co-ordinating groups amongst 

 these compounds. Figure 2 gives our present impression. For all groups of 

 haem-proteins there is a range of redox potentials and a range of otiier 



Ligand basicity (effective) 



Fig. 2. The suggested relationship between redox potentials and ligand basicity 

 for the cytochromes. The top three curves are for haem a, protohaem and 

 mesohaem complexes in descending order of potential. All these complexes are 

 assumed to be imidazole-coordinated. The lowest curve is for either protohaem 

 or mesohaem complexes where the further binding of the haem from the protein 

 is assumed to be through amino-nitrogen. By effective ligand basicity we imply 

 that basicity which obtains under the conditions of steric hindrance realized in 

 the protein and we suggest that steric hindrance increases from right to left in 



all the cases. 



physical properties, such as magnetic moments and absorption spectra, as 

 well as a range of chemical properties such as affinity for oxygen and carbon 

 monoxide and rate of autoxidation. There is no reason to think that these 

 groups of haem proteins can be rigidly differentiated, in fact. 



The variation in combination between porphyrins and the type of group X so 

 far suggested would then be (see Williams, 1958), as shown in table on page 48. 



Inspection of these combinations leads to two immediate comments. Far 

 from every possible intercombination between different porphyrins and 

 different groups X or between different combinations of the two X groups 

 for any one porphyrin has been postulated, let alone demonstrated. Second, 

 where it is stated in the table that only one group is involved, say under 



