8 L. E. Orgel 



resonance studies in the mm region would give valuable and detailed 

 information about metal-protein interactions. 



The low-spin ferric complexes are perhaps the ones which offer the greatest 

 hope of yielding useful information by means of conventional resonance 

 experiments. So far no data have been reported for the cytochromes but 

 three ferrihaemoglobin derivatives have been studied, namely the azide and 

 the two forms of the hydroxide. The rest of the discussion of resonance 

 experiments will be concerned with these compounds. 



r ' " — "■ rfyz 



~800 cm-' 



~800 cm-' 



1 



Fig. 6. The energy level diagram of the t^g orbitals in ferrihaemoglobin azide. 



The magnetic behaviour of crystals of ferrihaemoglobin azide naturally 

 differs for directions parallel and perpendicular to the haem plane, for we have 

 seen that the dy.y orbital is no longer equivalent to the d^^ and ^j,, orbitals 

 (the haem plane is the xy plane here). More surprisingly the spectrum is 

 anisotropic within the haem plane. 



By analysing the spectrum Griffith has estimated that the d^y orbital lies 

 lowest with the ^4^ and d^^ orbitals about equally spaced 600-1000 cm~i 

 above it as shown in Fig. 6. We must ask what it is that distinguishes the x 

 from the y direction in the haem plane. There are two obvious explanations, 

 namely that the asymmetry of the porphyrin molecule is responsible or that 

 the other groups attached to the iron have less than cyhndrical symmetry. 

 On the v/hole I feel that the former explanation is unlikely on account of the 

 following evidence, v/hich, however, is not conclusive : 



1. The degree of asymmetry differs markedly between the two hydroxide 

 complexes. 



2. The spectrum of the high-spin ferric complexes shows no sign of 

 asymmetry. 



If we reject this explanation we are led to the attractive hypothesis due to 

 Ingram that the asymmetry is associated with the direction of the plane of the 

 histidine group wliich is supposed to be attached to the metal ion. Double- 

 bonding, which is a well-recognized feature in the electronic structure of 

 these compounds, can affect the d orbital whose plane is perpendicular to 

 that of the histidine molecule (Fig. 7) but not the one that lies in the histidine 

 plane. This causes a marked change in the energy of only one of the pair of 

 f/j.^ and dyg^ orbitals, and so accounts for the calculated energy-level pattern. 



