70 J. E. Falk and D. D. Perrin 



It has been shown that the following firm correlations exist among the 

 non-protein complexes : 



(a) The more electron-attracting the side-chains on the porphyrin nucleus, 

 the greater the shift to longer wavelength of the visible absorption 

 maxima of porphyrins, their square metal complexes, and their 

 haemochromes with double-bonding ligands. 



(b) The more electron-attracting the porphyrin side-chains, the more 

 positive the redox potential of the haemochromes formed by double- 

 bonding ligands. 



(c) The greater the electron-donation (higher pK^) of double-bonding 

 ligands, the more negative the redox potential of the haemochromes 

 formed by them. 



(d) For ligands, in the above group, of ipK^ from about 3 to 7, although Eq 

 varies by about 150 mV, the spectra of the haemochromes hardly differ. 

 Reasons for this have been discussed. 



Among the haemoproteins : 



(a) The effects of porphyrin side-chains upon both spectrum and redox 

 are obscured and overweighed by the effects of the proteins. 



(b) The haemochromes which have been studied in any detail are mainly 

 those formed by double-bonding ligands, and their spectroscopic and 

 redox properties may have little value as models for haemoproteins, 

 even in cases hke cytochromes c, which resemble haemochromes in 

 some ways. 



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