SPECTRA AND REDOX POTENTIALS OF 

 METALLOPORPHYRINS AND HAEMOPROTEINS 



By J. E. Falk* and D. D. PERRiNf 



Division of Plant Industry, C.S.I.R.O., Canberra and Department of Medical 

 Chemistry, Australian National University, Canberra 



Why is it that there is no relationship between the oxidation-reduction 

 potential of the cytochromes a, b and c and their absorption spectra (Table 1)? 

 The porphyrin side-chains of these cytochromes increase in electron-attracting 

 power in the order c, b, a. This sequence is reflected in the spectra of the 



Table 1 



Here and throughout this paper, the data, for the cytochromes of animal mitochondria, 

 are taken from Morton (1958). Side-chains other than those shown are methyl and 

 propionic acid groups, which have little effect upon the properties discussed. For side- 

 chains in haem a, see Lemberg, Clezy and Barrett, this volume, p. 344. 



* The Eo' for a reaction is the electrode potential for 50% oxidation at a stated pH. 

 t From Colpa-Boonstra and Holton (1959). 



cytochromes and of the pyridine haemochromes of their prosthetic groups. 

 But while the pyridine haemochrome of haem c has a spectrum very like 

 that of the cytochrome itself, the spectra of cytochromes b and a are displaced 

 far to the red of their respective haemochrome spectra. The redox potentials 

 of the three cytochromes follow no sequence whatever in relation to their 

 spectra, or the chemistry of their haem prosthetic groups. In the absence of 

 protein, however, the electron-attracting power (Falk and Nyholm, 1958) of 

 porphyrin side-chains is correlated with changes of both spectrum (Table 2) 

 and redox potential (Table 3). 



* C.S.I.R.O. 



t Australian National University. 



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