76 



Discussion 

 Table 1. a-BANDS of some haem complexes 



Lemberg: There is considerable difficulty in finding suitable models for haemoproteins 

 among simpler haem compounds. Thus amino acids at neutral pH are zwitterions 

 and therefore unsuitable. Research with amino acids at a physiological pH should 

 therefore be carried out with amino acid esters or poly-aminoacids, but such data are 

 missing. Moreover, even then the affinity to haem may be far smaller than if the 

 combining group is held in the protein in a restrained position close to the haem iron. 

 In this regard Kaziro's approach offers perhaps more hope than model experiments. 

 I was interested to note, in Falk's discussion (this volume, p. 74), the lack of vari- 

 ability of the diformyldeuterohaem spectrum with different ligands. This is quite 

 different from the great variability of band position of nitrogenous compounds of 

 monoformyl haems such as haem a. 



Falk: Lemberg reinforces the point I have made — that relatively few model haem com- 

 pounds have been studied; those which have been are largely of one class, in which 

 the ligand atom is — N^. The reasons for this are partly the difficulties, as Lemberg 

 points out — we are, in fact, studying amino acid esters and small peptides — and 

 partly, perhaps, that we have all been over-impressed with the suspected role of 

 histidine, and have modelled our models upon it. I agree that model studies can 

 never entirely solve the problems of haemoprotein structure, but I believe that there 

 still remains a great deal to discover about the chemistry of the combination of haem 

 with new types of ligands, and that knowledge on this level will be a prerequisite for 

 our eventual understanding of the haemoproteins. 



Not only do we need data on compounds of haems with primary and secondary 

 nitrogen atoms, with thiols, etc., but as Perrin and I have pointed out, there is a par- 

 ticular need for studies of mixed compounds, with one ligand of these types and the 

 other of the — N== type. In this regard Wang's model (this volume, p. 98), in which 

 the haem is held down on one side to a — N= bond, should be particularly valuable. 



Phillips: It is interesting to note that although free amino acids form haemochromes 

 only with great difficulty, the corresponding amino acid esters react readily. The 

 reluctance of the free amino acids to react is not simply due to electrostatic repulsion 

 betv^een the amino acid anion and the carboxylate anions on the porphyrin side 

 chains, since a similar behaviour is observed in detergent solutions using porphyrin 

 esters. 



Carbon Monoxide-Pyridine Complexes with Haems 

 By J. H. Wang (Yale) 



Wang: In connexion with Falk's remarks on the special significance of mixed haemo- 

 chromes, I should like to discuss some interesting results obtained in our equilibrium 

 studies on the combination of carbon monoxide with haem in aqueous solutions 

 containing small amounts of pyridine. 



