46 



R. J. P. Williams 



TT-electron acceptors (Note 2). In order to develop further the discussion of 

 iron-porphyrin redox potentials we present some new data, Table 3, on the 

 redox potentials of Fe(NiOx)2X2 couples in water where NiOx is cyclo- 

 hexanedionedioxime. From the table we see that the potential of the complex 

 with pyridine is higher than that with imidazole which is higher than that with 

 ammonia. All these complexes except the hydrates in both ferrous and ferric 

 forms are covalent (of low spin). The values fall with the increasing donor 

 character of the groups as shown by the p^ values, pyridine 5-2, imidazole 7-1 



Table 3. Redox potentials of some iron nioxime complexes 



The redox potential in water alone is pH-sensitive, falling to about —250 mV at pH 

 approximately 100. 



NiOx is cyclohexanedionedioxime. 



and ammonia 9-1. Now the difference between the redox potential of 

 Fe(NiOX)2, (NH3)2 and Fe(NiOx)2 (imidazole)2 is +400 mV. This difference 

 is close to that between cytochrome c and cytochrome h, +200 mV, but 

 cytochrome Z) is a protoporphyrin complex whereas cytochrome c is a meso- 

 porphyrin complex. The redox potential difference produced by the different 

 porphyrins is '-^ +80 mV in their pyridine forms (Lemberg and Legge, 1949). 

 This factor should be added to the observed difference between cytochromes 

 h and c giving +250 — 300 mV difference between the h and c type cyto- 

 chromes had their porphyrins been identical. We conclude that if cytochrome 

 c is an imidazole (histidine) complex, cytochrome h is likely to be an amine- 

 imidazole complex. We will show later that the reactions of Z>-type cyto- 

 chromes are in accord with this hypothesis as are their spectra (WiUiams, 

 1958). 



We now turn to cytochromes d (using the nomenclature of Morton, 1958) 

 which appear to us to be related to h cytochromes structurally in much the 

 way haemoglobin is related to cytochrome c. For although the cytochromes 

 h and d have similar redox potentials they have very different chemical 

 properties. The cytochromes d are very readily autoxidized for example. 

 As we have noted in Fig. 1 the value of AA (Soret) between the Fe++ and 

 Fe+++ forms of cytochromes d suggests that they are largely ionic complexes. 

 This is supported by the easy autoxidation, by the uptake of carbon monoxide 

 in the reduced state, and by the position of the Soret band of the Fe+++ form 



