Some Physical Properties and Chemical Reactions of Iron Complexes 45 



while in ionic complexes it is approximately 1:12, as in haemoglobin. (2) 

 The reaction of cytochrome a^ with carbon monoxide is rapid and only ionic 

 forms of ferrous complexes can undergo rapid substitution reactions of this 

 kind (Table 1). All ferrous complexes which react rapidly with carbon 

 monoxide also react rapidly with oxygen. (3) The ferric form of cytochrome 

 ^3 has a weak absorption maximum at '^ 650 m/^, which only appears when 

 at least some of the ferric couple is in the ionic state. A similar discussion of 

 the same spectroscopic features would lead us to suppose that cytochrome a 

 is largely covalent with but a small percentage of the ionic form. 



The correlations between Soret band position and the magnetic moments of 

 the ferrous complexes permit comment on the interaction between the ferrous 

 ion and the protein. For example, Gibson (1959) observed that on intense 

 illumination of carboxyhaemoglobin (HbCO) a short-lived species Hb* was 

 produced. This species has its Soret band at slightly longer wavelengths than 

 Hb itself. The discussion presented here would lead us to conclude that the 

 protein groups interacting with the ferrous ion must be more weakly bound 

 in Hb* than in Hb. In keeping, the reaction of Hb* with CO is forty times as 

 fast and of lower activation energy. Again, the Soret band of myoglobin 

 (438 m/<) is at a longer wavelength than that of haemoglobin (431m//) 

 which are to be compared with MbCO 424 mju, Mb02 416 m/^ and HbCO 

 418 m//, HbOa 414 m/< (Lemberg and Legge, 1949). The values suggest that 

 the protein groups are less strongly bound to myoglobin than to haemoglobin. 

 This is in keeping with the more rapid reactions of myoglobin. 



Before leaving this point it is clear that the spin state of a ferrous or ferric 

 complex is very sensitive to environment. We must expect that extraction of a 

 cytochrome will sometimes alter its properties either through a minor 

 denaturation of the protein or even through a change of the medium di-electric 

 constant. 



OXIDATION-REDUCTION POTENTIALS 

 We have made several comments upon the redox potentials of ferrous/ferric 

 couples (Williams, 1959; Tomkinson and Williams, 1958). The general 

 impression of the variations in the FePX, potential with change of X is that 

 either by (1) a continuous adjustment in the nature of X from water to 

 increasingly improved donors such as ammonia or (2) a gradual reduction in 

 the Fe-X distance for a group X which is a good donor, the redox potential 

 can be made to go through a continuous series of values which show a 

 maximum, see Fig. 2. The maximum is reached because the ferrous as 

 opposed to the ferric ion undergoes electron rearrangement at lower effective 

 electronegativities of the group X. At low electronegativities, increase in 

 electronegativity favours ferrous over ferric, while at higher effective electro- 

 negativities increase in donor properties of the ligand favours ferric over 

 ferrous. The maximum will be most accentuated for ligands which are 



