136 



P. George, J. Beetlestone and J. S. Griffith 



by comparing the differences in partial molal entropies of the complexes and 

 the parent haemoprotein (George, 1956). 



Table 6. IS.H and A^" values for the formation of ferrimyoglobin 



FLUORIDE, HYDROXIDE AND CYANIDE: AND THEIR MAGNETIC MOMENTS 

 (GEORGE AND HANANIA, 1952, 1956: THEORELL AND EHRENBERG, 1951) 



* See footnote to Table 7. 



A more rigorous correlation can be sought, if, for the same ligand, data are 

 available for closely related haemoproteins. But if the whole range from 

 high- to low-spin complexes is to be covered, this would clearly be restricted 

 to those ligands capable of giving theimal mixtures in some cases. For 

 example, the data in Table 7 for the various haemoglobin hydroxides show 

 that the increase in the fraction of the low-spin form is accompanied by more 

 negative (i.e., favourable) values of Ai/, which was to be anticipated from the 

 overall trend illustrated in Table 6. Furthennore, with a series of derivatives 

 of this type, where the ligands are identical and the structure of the complex in 

 the immediate neighbourhood of the iron is presumably very similar, the 

 values of AS* can be taken as a true indication of a general trend paralleling 

 the trend in Ai/. As the fraction of the low-spin form increases, IS.S assumes 

 more negative (unfavourable) values, while A// assumes more negative 

 (favourable) values. This trend in AS* is entirely in accord with the entropy 

 change obtained above for the conversion of the high-spin to the low-spin 

 hydroxide in the case of ferrimyoglobin, and it can likewise be associated 

 with a greater structural rigidity in the vicinity of the iron atom of the 

 low-spin form. 



The T^K values for the ionization of ferriperoxidase and ferricytochrome c 

 are so much higher than those for the haemoglobins (see Table 1) that 

 inevitably either the A// values, or the A^* values, and very probably both, 

 would show marked deviations from the correlation set out in Table 7. This 

 is not unexpected because the acidic forms of these haemoproteins have 

 different structures, and as a consequence the formation of the hydroxide is a 

 different type of chemical reaction. 



In the case of the haemoglobins, the reactions of the acidic form can be 

 very adequately expressed by the hydrate structure, e.g., Prof. — Fegb"'^++(H20), 

 and the ionization is accordingly the simple dissociation of a proton. 



Prot.— FeHb+++(H20) ^ Prot.— Fc 



Hb 



-OH + H+ 



(20) 



