PHILIP GEORGE 



complex formation also show that the formation of a neutral 

 complex can be highly favored. In reactions accompanied by 

 a decrease in charge, as there would be if the ferric hemoprotein 

 having a net charge of +1 combined with a singly charged 

 anionic ligand, there is an increase in entropy because the water 

 molecules are less tightly held in the solvation shells of the 

 products than in those of the reactants. The formation of ion- 

 pair complexes by inorganic cations, for example, Fe^^, Cu^"^, 

 Cd^+j Pb^"*", provides a very good example (46). AS° values for 

 five such complexes of the ferric ion are given in Table I, where 

 the majority of the values are seen to be large and positive. If 

 one of the reactants, however, is effectively neutral as in the case 

 of a ferrous hemoprotein, then the entropy change does not 

 favor unequivocally any one type of reaction. 



With the ferric form of hemoproteins all three considerations 

 favor combination with a singly charged anionic ligand in 

 preference to a neutral molecule. It is therefore surprising that 

 the pa variations of velocity constants and equilibrium constants, 

 which provide the best indirect evidence for the structure of the 

 complexes, can be taken to suggest that the HCN molecule is 

 bonded to the iron in the cyanide complexes of both ferrihemo- 

 globin and ferrimyoglobin (2) . On the basis of similar evidence 

 it would appear that ferricatalase forms complexes by combining 

 with HF, HN3, HCO2H, and HCN (2) ; and the scanty equilib- 

 rium data available for the fluoride and cyanide complexes of 

 ferriperoxidase likewise indicate bonding of HF and HCN (17). 

 However, in the analysis of these results, concurrent reaction 

 paths, one via the undissociated acid and one via the anion, 

 were not considered, nor was the ionization of heme-linked groups. 



344 



