PHILIP GEORGE 



useful criteria in comparing and contrasting the various reactions, 

 as has already been found in the case of simpler inorganic and 

 organic systems. The A//" values give the balance between the 

 energies of the old bonds broken and the new bonds formed. 

 The A^"" values reflect the accompanying changes in configuration 

 and solvation. The structure of the products may entail a 

 looser, or a more closely knit, configuration, and the entropy 

 increases or decreases accordingly. If ionic charges are created 

 or destroyed in the reaction, the solvation or desolvation of 

 water molecules about these charges will result in a substantial 

 decrease or increase in entropy. 



Ferrous and Ferric Hemoprotein Complexes 



It is self-evident that the correct elucidation of the mecha- 

 nism by which these complexes are formed depends on knowing 

 their structure; for this purpose the nature of the amino acid 

 residue in the protein to which the iron protoporphyrin is 

 attached is immaterial, but it is essential to know precisely the 

 nature of the ligand joined to the iron in its sixth coordination 

 position. This is not always obvious. Many years elapsed 

 before oxyhemoglobin was recognized as a complex of ferro- 

 hemoglobin in which oxygen was the ligand, partly because the 

 distinction between reversible oxygenation and irreversible 

 oxidation yielding ferrihemoglobin was not readily appreciated. 

 The simplest approach is unfortunately inapplicable. The large 

 size of hemoproteins, whose molecular weights range from 

 16,200 for myoglobin through 41,000 and 68,000 for peroxidase 

 and hemoglobin to 224,000 for catalase, makes it impossible to 

 determine the composition of the complexes by the analytical 

 methods in use for simpler coordination compounds. Hence 

 there can be no direct evidence for the replacement of a water 

 molecule coordinated to the iron by oxygen when hemoglobin 

 forms oxyhemoglobin, and no direct evidence that GN~ and not 

 HCN is bonded when ferrihemoglobin forms its cyanide complex. 

 The hypothesis has long been accepted, however, that in the 



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