Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 69 



complexes with amino-acids, 8-hydroxyquinoline and o-phenanthroline 

 (Perrin, 1959a). 



Table 5. Complexes of protohaem with bases 



* Fe+++B2 + OH- 



t This study. 

 + Barron (1940). 



Fe++BOH + B + e. 



Much less information is available for reactions of the type 



Fe++B2 ^ Fe+++B2 + e 



which become important in neutral solutions. For iron-protoporphyrin we 

 find the values shown in Table 6. As would be expected the anion, CN"", 



Table 6 



* For the reaction Fe^+Bg 

 t Shack and Clark (1947). 

 t Barron (1937). 



Fe+++B2 + e. 



stabilizes the ferric state to a greater extent than any of the neutral ligands : 

 among the latter the ferric state is favoured the higher the p/f^ of the ligand. 

 Compared with the cyanide complexes, potentials for these reactions appear 

 to be much more sensitive to change in the porphyrin in the complex if the 

 ligand is neutral (pyridine. Table 3). 



SUMMARY AND CONCLUSIONS 

 We have outlined some theoretical aspects of the absorption spectra of 

 porphyrins and metalloporphyrins, of the co-ordination chemistry of metallo- 

 porphyrins, and of the redox potentials of haems and haemochromes. 



