ON THE NATURE OF HEMOPROTEIN REACTIONS 



TABLE V 



Thermodynamic Data for Ferric Ion, Ferrimyoglobin, and Ferrihemoglobin 

 Complex Formation (15,18,40,43,46) 



Ligand OR- F^ CN^ N^ CNS" 



• Calculated from an estimated K = IC, a little greater than one-sixth of K for 

 the formation of Fe(CN)|", and a value for AS° of 16.1 e.u., based on the assumption 

 that S° for FeCN2+ is 44.3 e.u., equal to the mean of the S° values for the other 

 ferric ion complexes listed in Table I. 



values of AH^ in all cases favor the hemoprotein reactions, to 

 the extent of between 7 and 14 kcal./mole. The bonding is 

 stronger in the hemoprotein complexes. If the entropy changes 

 were the same for both ionic iron and hemoprotein complexes, 

 the equilibrium constants for the latter would be between 10^ 

 and 10^° times greater. However, this is not so; the entropy 

 changes greatly favor the formation of the ionic iron complexes, 

 to the extent of 11 to 17 e.u. for the ferrous derivatives and 23 

 to 57 e.u. for the ferric derivatives. On this account alone the 

 equilibrium constants for the ionic iron complexes would be 

 greater by between lO^-^ and 10^2.5 xhe actual differences in 

 affinity depend upon whether differences in A//° or A^"" predomi- 

 nate, and there is no simple correlation with ionic or covalent 

 bonding within the complex. 



This behavior is what would be expected on qualitative 

 considerations, for stronger bonding might be anticipated with 

 the iron in a coordination complex rather than a solvation shell 

 of water molecules, and, correspondingly less desolvation and 

 less change in the tightness of the structure might be anticipated 

 when the iron is already held in the rigid planar ring system of 

 the protoporphyrin. It is of interest, however, to examine the 



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