Some Physical Properties and Chemical Reactions of Iron Complexes 51 



rapidly autoxidized at pH 7-0. The behaviour is also very similar to that of 

 Rhodospirilhim haemoprotein (cytochrome d) and could well arise in both 

 cases through the high acid dissociation content of the — NH3+ group. It 

 seems to us that some lower organisms may not have suitable histidine-con- 

 taining proteins to give rise to Fe-histidine cytochromes but must be content 

 with Fe-amine cytochromes. If this is the case and our discussion is valid 

 then these organisms are unlikely to be able to store or transport molecular 

 oxygen. Their cytoclirome oxidases and electron-transporting cytochromes 

 are amine complexes whereas those of higher organisms are both amine and 

 histidine complexes. 



One such conjecture about these compounds leads immediately to another. 

 The development of histidine cytochromes a and c in a cell gives the organism 

 the advantage over cells containing only amine cytochromes b and d that the 

 energy of the oxygen molecule can be more efficiently used. Some 300 mV 

 more energy (the difference in redox potentials) can be stored chemically for 

 each electron transported. 



ELECTRON TRANSPORT 

 There are two reasons for thinking that electron transport occurs across 

 the porphyrin of the cytochromes. If catalytic activity resided in the 

 imidazole-Fe-imidazole bonds then it should be demonstrable in Fe(DMG)2 

 (imidazole)2. The model complexes do not have the electron transporting 

 properties of cytochromes as far as we can discover. Again using the models 

 we have shown that although there is strong charge transfer interaction be- 

 tween Fe++ and (DMG) there is no evidence for it in Fe++-imidazole. On 

 the other hand, there is good evidence in Fe++(DMG)2 (pyridine)2. In this 

 complex there is a band (absent in other Fe++(DMG)2X2 complexes) at 

 '^ 400 m^a. For differently substituted pyridines it moves in the following 

 manner (Jillot and Williams, 1958): 



If it is assumed that this band is due to a partial charge transfer of an electron 

 from the ferrous atom to the pyridine, the band positions are explicable in 

 terms of the electron-acceptor properties of the substituents. The band 

 position is solvent-dependent, again suggesting a charge transfer band. The 

 absence of such a band in the imidazole complexes would suggest that charge 

 transfer and therefore electron-transport across the imidazole is not facile. 



Finally, if we are correct in saying that cytochromes b and d are amine 

 complexes then as amines are not unsaturated systems and presumably 

 could not carry out electron transport we must assume in these cytochromes 

 that the electron moves through the porphyrin. If this is so then it is very 

 likely that electrons are mobile in the porphyrin of FeP (histidine)2, but of 



