PHILIP GEORGE 



AF^, AH^, and AS° for the Formation of Ionic Iron, 

 Myoglobin, and Hemoglobin Complexes 



In the formation of some of their complexes myoglobin 

 and hemoglobin show a greater affinity for the ligand than the 

 ferrous or ferric ion. It is of interest to see how this arises in 

 terms of the relative magnitudes of AH^ and A^"", for since AF" = 

 A//° — TAS'^, the more negative the value of A//°, and the more 

 positive the value of A^"", the greater is the decrease in free 

 energy, AF°, which provides the quantitative measure of the 

 affinity between the metal and the ligand. 



The data at present available have been assembled in 

 Tables IV and V. For the ferrous complexes no direct com- 

 parison is possible yet, but, since NO combines with both ferrous 

 ion and the ferrohemoproteins, data for FeNO^^ may be com- 

 pared with data for the ferromyoglobin complexes with O2 and 

 CO. In the case of these complexes and the F~, CNS", and 

 N7 complexes of ferrihemoglobin, no allowance was made in 

 the analysis for the participation of heme-linked ionizations 

 (41,43), but any discrepancy is unlikely to exceed 2 kcal./mole 

 in AF° or A//°, or 5 e.u. in AS'^. Only when the ligand is covalently 

 bonded, vis., O2, CO, CN", and Nj (28), does the hemo- 

 protein have the greater affinity: with the ligands OH~, F~", 

 and CNS~ ferric ion has the greater affinity, decreasing in this 

 order. 



TABLE IV 



Thermodynamic Data for Ferrous Ion and Ferromyoglobin Complex Forma- 

 tion (33,43) 



AFo AH" AS" 



Reaction (kcal./mole) (kcal./mole) (e.u.) 



Fe»+ + NO -3.5 -9.2 -19.1 



Ferromyoglobin + O2 -7.5 -16.4 -30.0 



Ferromyoglobin + CO -9.2 -20.0 -36.2 



An examination of the A//° and AS° values reveals that the 

 differences in affinity arise through opposing effects. The 



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