Ferrihaemoprotein Hydroxides 



111 



has three unpaired electrons, and again raised the possibility that it is a 

 thermal mixture of high- and low-spin forms. 



Against the background of these developments in electronic theory certain 

 other experimental observations take on added significance. 



(1) Taube's comment, that the existence of high- and low-spin derivatives 

 of haemoglobin indicates a close balance between the energies of the two 

 forms in the case of iron porphyrin compounds, is further borne out by the 

 fact that with the same ligand, namely the azide ion, high- or low-spin 

 complexes are formed depending on the particular haemoprotein. The 

 magnetic moment of ferricatalase azide is 5-36 B.M., compared to 2-84 for 

 the ferrihaemoglobin derivative (Deutsch and Ehrenberg, 1952; Coryell, 

 Stitt and Pauling, 1937). A fairly close balance between the energies would be 

 a prerequisite for the two forms to exist in thermal equilibrium. 



(2) While the magnetic moment of ferrihaemoglobin hydroxide is 4-47 

 B.M., just a little in excess of the theoretical value for the spin contribution of 

 three unpaired electrons, the moments of the other ferrihaemoprotein 

 hydroxides are very substantially different, as shown in Table 3. Ferri- 



Table 3. Magnetic moments of ferrihaemoprotein hydroxides 



peroxidase and ferricytochrome c hydroxides come into the category of low- 

 spin complexes, with moments comparable to those of the CN~ and SH~ 

 derivatives; whereas the magnetic moment of ferrimyoglobin hydroxide 

 approaches that of a high-spin complex, the value of 5-11 B.M. being even 

 greater than the theoretical value for the spin contribution of four unpaired 

 electrons. Hence, although the three unpaired electron configuration could 

 still be invoked for ferrimyoglobin, by assuming a large orbital contribution, 

 it is clearly impossible in the case of ferriperoxidase and ferricytochrome c. 

 (3) For a given ligand, the spectra of ferrihaemoprotein complexes usually 

 have absorption bands at approximately the same wavelengths with com- 

 parable extinction coefficients, independent of the particular haemoprotein. 

 There are a few exceptions, and among them the hydroxides provide the most 

 striking examples. These spectra show marked variations, exhibiting a regular 

 trend from ferrimyoglobin at one extreme, through ferrihaemoglobin, to 

 ferriperoxidase and ferricytochrome c at the other. 



