Electronic Structure and Electron Transport Properties of Metal Ions 9 



If this is so then we have a very sensitive tool for measuring the closeness 

 of association of the histidine group with the Fe+++ ion; any increase in the 

 closeness of this approach will cause an increase in the anisotropy of the 

 resonance in the haem plane. Such information should be most important 

 in understanding how the protein configuration changes the chemical properties 

 of porphyrin derivatives. An illustration of this may be found by comparing 

 the forms of ferrihaemoglobin hydroxide obtained at pH 7 with those 

 obtained at pH 8-5, the former being substantially less anisotropic. We must 



Fig. 7. Double bonding between a metal atom and the tt orbitals of a conjugated 



iminazole ring. 



conclude that there is a proton in the structure which, if removed, causes a 

 weakening of the haem-protein interaction. Such a proton cannot be the 

 histidine proton, for if removed it would strengthen the interaction. We 

 presume then that the dissociation which occurs at about pH 8 is associated 

 with a change in protein configuration which must make it more difficult 

 for the histidine to co-ordinate closely to the metal. 



This incidentally raises a general point in haemoprotein chemistry which 

 seems to have received little attention. It is often supposed that the ability of 

 similar haem groups to react with reagents such as CO or CN~ is a function 

 of their accessibility; haem groups near the surface of proteins for example 

 reacting more readily than those in the interior. While this is quite correct 

 there is a second variable which makes uncertain stereochemical arguments 

 based on the reactivity of haem groups, for this depends also on the closeness 

 and rigidity of fit of the ligand group with the metal ion. The latter depends 

 in turn on the details of protein configuration near the haem-protein linkage. 

 I should like to emphasize that this is a much more sensitive dependence than 

 is implied by the distinction between accessible and inaccessible positions in 

 the protein; it is a matter of replaceability when the reactant has access to 

 the metal ion, and is related to the extra thermodynamic stability conferred 

 by chelation in simpler systems. 



ELECTRON-TRANSPORT BETWEEN METAL IONS AND THE 

 UTILIZATION OF THE REDOX ENERGY 



While a great deal of work has been done on the electronic properties of 

 haem compounds it has yet to be established that this has any direct relevance 

 to the elucidation of their biological functions. The suggestions in this 



