78 Discussion 



TT-bond formation between the C-atom and Fe++ decreases the probability-density of 

 the d^x and dy^^ electrons on the other side of the haem plane, it enables the pyridine 

 molecule to be bonded more effectively, i.e. the CO-molecule also enhances the 

 aflRnity of Fe++ for the pyridine molecule attached to it. Thus because of this com- 

 plementary electronic nature of carbon monoxide and pyridine, the mixed complex 

 pyridine-haem-CO is not only thermodynamically more stable than the complex 

 HjO-haem-CO, but also more stable than bipyridine haemochrome. 



(H^ 



(17 KD 



Weaker 

 cr —bond 



/9 » (^[^ 



(£> 



^ 



Identical J'^P^S^ 



ligands (c) <r-bond ^^j 



Fig. 2. Cross-section of ^^.^-orbital in octahedral complexes. 



If we assume that the ligand-field of the co-ordinating electron-pair of carbon 

 monoxide is so weak that 77-bonding between the Fe++ of haem and the C-atom of 

 the CO-molecule is essential for the binding of the latter by haem, it would then be 

 easy to see why each haem binds only one carbon monoxide molecule. The hypo- 

 thetical complex OC-haem-CO should be unstable, since the two CO-molecules on 

 opposite side of the haem plane would compete for the same d^^. ai^d dy^ electrons in 

 order to be bonded to the Fe++. 



Pauling suggested that this type of w-bond formation between the ligand and haem 

 also plays a significant role in the binding of nitric oxide and oxygen by haemoglobin 

 and related compounds. We would like to suggest further that this type of w-bonding 

 could be the major cause for the haem-linked imidazole group to exhibit the famous 

 Bohr effect. As illustrated by diagram (5) in Fig. 1 , the vr-bond formation causes the 

 d^x, and dy^ electrons of the Fe++ to drift toward the -\-Z direction. This would cause 

 the iron nucleus to be very incompletely shielded on the — Z side of the A'K-plane 

 and hence to exert an unusually large polarizing influence on the attached imidazole 

 group. 



Equilibrium Constants for Reactions of Haems with Ligands 



By J. N. Phillips (Canberra) 



Philups: The question of mixed haemochromes is a rather interesting one. Mixed ligand 

 complexes of the cyano-pyridine type are known for Fe++, Fe+++, Co+++ and Mn+++ 

 porphyrins. Where quantitative equilibrium constants have been determined the 

 stability of the mixed complex has been found to be greater than the stability of the 

 corresponding pure complexes — as Wang also found with the mixed carbon monoxide- 

 pyridine complex. 



Figures 1 and 2 summarize the equilibrium constants at 25°C for the reaction 

 between Fe++ and Fe+++ protoporphyrin and the ligands, water, hydroxide ion, cyanide 

 ion, and pyridine. These results have been extracted from that very extensive series 

 of papers by Clark and his co-workers and illustrate the danger of attempting to 

 interpret in any simple fashion haemochrome formation in alkaline solution. 



