50 R. J. P. Williams 



others. In both cases the same ferric complex is always obtained, 

 Fe+++(DMG)2(H20)OH. We suggest the mechanisms: 



Fe+ ^(DMG)2X2 Fe++(DMG)2(H20)02 -> Fe+++(DMG)2(H20)OH 

 Fe++(DMG)2X02 



Fe++(DMG)2X+02- -> Fe+++(DMG)2(HoO)OH 



m^ 



(Reaction (2) is a catalysed autoxidation of X.) No complex of a saturated 

 base X is known which carries oxygen (cf. cytochromes b and d) ; however, all 

 the complexes of unsaturated bases X can carry oxygen (cf. myoglobin, 

 haemoglobin, cytochrome a). It is of great importance here to note that 

 Fe(DMG)2 (imidazole)2 can even pick up molecular oxygen in the presence 

 of borohydride, sodium formaldehyde sulphoxylate, or sodium dithionite. 

 This reaction is common to cytochrome a (Sekuzu, Takemori, Yonetani and 

 Okunuki, 1959) but not to haemoglobin. It implies that the iron complex 

 undergoes slow dissociation of its ligands as free oxygen reacts rapidly with 

 borohydride or sulphoxylate. Haemoglobin undergoes very rapid de-oxygena- 

 tion under these circumstances. We can now make some comments about the 

 binding in haemoglobin. We note first that covalent ferrous complexes 

 (Class III, Table 1) undergo slow ligand exchange. Magnetic data show 

 oxyhaemoglobin to be covalent, yet it takes part in fast reactions involving 

 ligand replacement. The iron must be in an energy state which is only 

 slightly more stable than its high-spin states. This is in agreement with 

 spectroscopic evidence as well as with the value of its redox potential. Now 

 if the oxygen is labile in HbXOg the group X, the histidine, must be labile also. 

 What is it then that prevents the autoxidation of haemoglobin in accord with 

 equation (1) above? The answer which we suggest to this question is that it 

 is the high activation energy of the rearrangement of the protein which 

 prevents a water molecule replacing X and thus prevents autoxidation. On 

 the other hand, the cytochrome a oxygen complex dissociates slowly to a 

 covalent haemocliromogen (judged by the spectra) whence there is little 

 danger of dissociation of groups X leading to autoxidation. In cytochrome a 

 we suggest iron is more strongly bound to imidazole than in haemoglobin. 

 Elsewhere (WiUiams, 1958) we have reached this conclusion from a very 

 different argument. 



Amongst cytochromes some of the cytochromes b appear rapidly autoxidiz- 

 able. We believe that this observation is irrelevant to biological function. If, 

 as we suppose, cytochromes b (d) are amine ( — NH2 -^ Fe) complexes, then 

 bringing them out of a cell environment to a pH of about 7-0 in free solution 

 may well dissociate the NHg -^ Fe link. We can show this easily with 

 Fe(DMG)2(NH3)2 which is fairly stable to autoxidation at pH 100 but 



