Ferrihaemoprotein Hydroxides 



137 



Table 7. A^ and A5° values for hydroxide formation* 



* These values have been calculated from the corresponding data for 

 the ionization reaction, and for the ionization of water (A// = -1-1 3-4 

 kcal/mole and AS^ = —19-2 e.u.). The references for ferrimyoglobin 

 and ferrihaemoglobin are given in Table 1 ; for ferrileghaemoglobin, 

 see George, Hanania and Thorogood (1959). 



On the other hand, in the acidic form of ferricytochrome c the iron is bonded 

 in an intricate crevice structure to nitrogenous base groups of tlie protein at 

 both the fifth and sixth co-ordination positions. One of the crevice bonds 

 must be broken if 0H~ is to replace one of the groups, and the ionization 

 reaction therefore takes the form, 



Prot. - Fe+++cyt.c - N(base) + H2O v^ Prot. - Fe++cyt.c - OH N(base) + H+ (21) 



With ferriperoxidase the nature of the reaction is rather more obscure, 

 because in less alkaline solution the pH variations of the equilibrium con- 

 stants for complex formation with cyanide, fluoride, azide, etc., differ systema- 

 tically from the corresponding variations for ferrimyoglobin, ferrihaemo- 

 globin and ferricytochrome c. The difference lies in the consumption of a 

 proton accompanying the formation of the complex. George and Lyster 

 (1958) have discussed various explanations that have been advanced, and 

 suggested as a further possibility that acidic ferriperoxidase also has a crevice 

 structure but with the labile bond, which is broken in complex formation and 

 upon ionization, to some group other than a nitrogenous base, with a ^K 

 of about 10 in horseradish peroxidase. Whatever the true explanation may 

 be, it is clear that no strict comparison can be made between thermodynamic 

 data for the ionization of ferriperoxidase and ferricytochrome c and the 

 corresponding data for the haemoglobins. 



Finally, the question naturally arises as to whether any other haemo- 

 protein derivatives are mixtures of high- and low-spin forms, Scheler, 

 Schoffa and Jung (1957) and Havemann and Haberditzl (1958) have suggested 

 that this may be the case for several other derivatives where the magnetic 

 moments differ appreciably from the usual high or low values. However, no 

 quantitative correlation of magnetic and spectroscopic properties, of the 



