Some Physical Properties and Chemical Reactions of Iron Complexes 55 



the tetrapyridyl concentration is increased Eq will eventually increase again when the 

 oxidation-reduction reaction becomes 



Fe(tetrapy)i+^ + + tetrapy + t'~ -;-=^ Fe(tetrapy)2++ 

 bright yellow reddish-violet 



Using Courtauld atomic models we found that co-ordination of all four TV-atoms to 

 give square planar complexes is not possible. In all probability the structure of the 

 Fe(tetrapy)2++ complex is that of a distorted octahedron with only three of the four 

 nitrogens of each tetrapyridyl molecule co-ordinated to the iron. Its absorption 

 spectrum resembles that of Fe(tripy)2++ very closely, which supports this hypothesis. 

 The 1 : 1 complexes probably have only three bonded N-atoms, and we suppose that 

 steric hindrance prevents the formation of the ferric complex corresponding to 

 Fe(tetrapy)2++. 



Margoliash: For cytochrome c the evidence we have obtained from a study of the 

 chemical and physico-chemical properties of the denaturation products, as well as of 

 those of the pepsin digested 'core', indicates that cytochrome c is probably not a 

 di-imidazole haemochrome, but probably a mixed haemochrome with a primary 

 amino-group and an imidazole group bound to the haem-iron. Moreover, by denatur- 

 ation it is possible to obtain products having Eq values ranging from that of native 

 cytochrome c down to not far from V. In cytochrome c the E^ value seems to be 

 an expression of the effect of the protein configuration on the haem iron-ligand bonds 

 rather than an intrinsic property of the particular groups involved. I should therefore 

 think it would be difficult to ascribe specific ranges of E'q values to specific haemo- 

 chrome-forming ligands in haemoproteins. 



Williams : I consider that Margoliash has studied a series of complexes, often mixtures 

 varying from di-imidazoles through mixed complexes, to di-amines. No simple 

 explanation of his results is possible. 



