LINUS PAULING 

 THE HAEM-HAEM INTERACTIONS 



Reliable evidence about the nature of the structural mechanism of 

 the haem-haem interactions that are responsible for the sigmoid 

 character of the oxygen equilibrium curve of haemoglobin and that 

 affect other haemoglobin equilibria 11 has not yet been obtained. The 

 possibility that the interactions take place through the conjugated 

 system of double bonds, and involve the vinyl side chains of the proto- 

 porphyrin molecules, is made unlikely by the absence of detectable 

 spectroscopic or magnetic effects. 12 A more likely mechanism may be 

 suggested : steric hindrance (possibly by the imidazole ring responsible 

 for pKi), which interferes with the apposition of a diatomic molecule 

 to the iron atom of a haem group, and which is decreased in its effect 

 for a second haem by the loosening of the structure caused by the 

 conversion of a first haem to oxyhaem or carbonmonoxyhaem. 



THE MAGNETIC MOMENTS OF HAEMOGLOBIN 

 AND ITS DERIVATIVES 



The magnetic moments found by experiment for haemoglobin and its 

 derivatives correspond roughly to the values of the spin moment for 

 the numbers of unpaired electrons described in the foregoing dis- 

 cussion of their electronic structure, but the quantitative agreement is 

 poor, and no explanation of the discrepancy has previously been 

 advanced, except the general one that the total magnetic moment is 

 due in part to the orbital motion of the electrons. 



It may be expected that the theoretical interpretation of the observed 

 moments would throw further light on the structure of these molecules. 



Let us first consider the compounds of haemoglobin that are usually 

 thought to contain one odd electron per haem group. These include 

 ferrihaemoglobin cyanide, with magnetic moment 2-49 magnetons, 

 ferrihaemoglobin azide, with magnetic moment 2-84, ferrihaemoglobin 

 hydrosulphide, with moment 2-26, imidazole-ferrimaemoglobin, with 

 moment approximately 2, and probably also ferrihaemoglobin 

 hydroxide-ammonia, with moment 2-98. All of these moments are 

 considerably larger than the value 1-73 that corresponds to the spin 

 of one electron ; the only haem compound giving good agreement 

 with this value is nitric oxide haemoglobin, for which the reported 

 moment is 1-7 magnetons. Let us assume that in the approximately 

 octahedral field of the complex the orbital moment is not quenched, 

 but is combined with the spin moment to a resultant total angular 

 momentum, given by a total quantum number J. We also assume that 

 / is a constant of the motion of the system, but that the orbital 

 quantum number, L, need not be a constant of the motion ; that is, 



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