PHILIP GEORGE 



of ferromyoglobin and ferrohemoglobin in aqueous solution can 

 also be calculated. This is the energy required to effect electron 

 removal, 



Fe(H20) — > Fe+(H20) + e" (4) 



and the values are 96 and 97 kcal./mole, respectively, compared 

 with 95 kcal./mole for the ferrous ion itself. 



From the entropy change in the cell reaction, the sum 

 A^"^ + 1/2 -5?!, gives (5^e(H.o) - -^Fe+cmo)) because 5?!. is, by con- 

 vention, taken as zero. For myoglobin and hemoglobin the 

 provisional values are —21.4 and —23.4 e.u. These differences 

 in partial molal entropy, which are large and negative, are quite 

 remarkable because the corresponding quantity for ionic iron is 

 -27.1 - (-70.1) = +43 e.u. (31). Figures 2 and 3 show this 

 contrast very clearly. 



The large and positive difference in the case of ionic iron 

 is attributable to two factors. There is a favorable change in 

 the Born charging entropy which is purely electrostatic in origin, 

 arising in the transfer of a charged particle from vacuo to the 

 solvent, and also a favorable change in the entropy of solvation, 

 because around the ion of smaller charge and larger size the 

 water molecules are less tightly bound. Even for complex 

 cations where the ligands are bulky organic molecules, e.g., the 

 tris-dipyridyl complexes of Fe^+, Ru^"*", and Os^^, the corre- 

 sponding ►S'" difference falls only to +3 =•= 4 e.u., which suggests 

 that in these coordination complexes, the metal ion, held 

 already in a rigid chelate structure, is shielded to such an extent 

 that solvent interactions are very small indeed (19), 



The large negative values of (S^ecmo) — ^emiiO)) for 

 myoglobin and hemoglobin point to a common configurational 

 difference between the ferrous and ferric states, the ferric state 

 having the more random configuration. If the heme were held 

 in a crevice in the protein in the ferrous state, but not in the 

 ferric state, this could account for the large negative values. 

 But this explanation receives no support at all from the (^^l ~ 

 Sm) values for the ferrous complexes, which should, on this 



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