360 Discussion 



dependent on purity. However, it may not overcome the difficulty of the formation 



of rather firm linkages between the porphyrin and lipids which we have found. 

 Estabrook: I would like to ask Lemberg about the 2-a hydroxy in the structure of haemin 



a. The presence of such a hydroxy group confers assymmetry upon the associated 



carbon and should introduce optical activity to haemin a. Is haemin a optically 



active ? 

 Lemberg: We have no evidence on optical activity. 

 Morrison: We have looked for optical activity in many ways, but have been unable to 



detect it. 

 Williams : Since the — CHOH group sits on a formylporphyrin ring, racemization might 



easily occur. 

 Estabrook: What is the evidence for the presence of the 2-a hydroxy group? 

 Lemberg : The evidence is as follows : 



(a) Acetylation with alterations of Rp. 



(b) Dehydration with /7-toluene-sulphonylchloride in benzene, shifting absorption 

 to longer wavelengths. 



(c) Oxidation (by chromic acid) of CHOH — R to CO — R with the formation of 

 an a-ketonylporphyrin. 



The Properties of Haem a^ and Cytochrome a<^ 



By J. Barrett (Sydney) and J. P. Williams (Oxford) 



Williams: We should like to make certain observations about cytochrome a^. The 

 prosthetic group is a dihydro-porphyrin and therefore has one p/C value much lower 

 than either of those of simple porphyrins (Conant and Dietz, /. Amer. chem. Soc. 

 56, 2185, 1934). Reduction in p/r of the ligand reduces the stability of the iron com- 

 plexes, ferric much more than ferrous (Williams, Chem. Rev. 1956; Falk and Perrin, 

 this volume, p. 56). Thus where the further co-ordination of an iron-protoporphyrin 

 and an iron-dihydroporphyrin to a protein are the same, equilibration between the 

 ferrous and the ferric form of the chlorin should favour the ferrous. The addition of 

 two hydrogen atoms to the beta position of one of the pyrrole rings results in an 

 altered resonance pathway which is now markedly asymmetrical. As a consequence 

 the stability of the low-spin forms relative to the high-spin states will be reduced. 

 These changes from a simple porphyrin make the removal of iron from haem a^ 

 relatively easy, as is observed. The model compounds of the iron-dihydroporphyrin 

 are also of interest. Using Barrett's data (this volume, p. 355) we note the following: 



(a) CO and CN~ move the peaks to longer wavelengths relative to the pyridine 

 haemochrome but less than expected from comparison with protoporphyrin and 

 mesoporphyrin haemochromes. 



(b) The pyridine haemochrome is rather unstable. 



Both (a) and (b) are in keeping with the suggestion that the pyridine haemochromes 

 are not 100% low-spin complexes. 



(c) The acid haematin gives a band at 740 m/< which could be the charge-transfer 

 band in these complexes. If this is so then the anomalous band-position of the 

 hydroxide at 662 m/i would be a charge-transfer band too. 



We now consider cytochrome a^ itself. The following points suggest that it is at 

 least partly in the high-spin state in both the reduced and oxidized forms : 



In the reduced form: (1) the band max. 628-630 m/< is at a considerably longer 

 wavelength than the pyridine haemochrome; (2) a CO complex is readily formed; 

 (3) an O2 complex is obtained (Horio, this volume, p. 315). 



In the oxidized form: there is a band at 750 m/< similar to that in the acid haematin 

 compound. This is the charge-transfer band typical of ionic ferric species. 



Following the remarks made about model complexes which pick up oxygen rever- 

 sibly (Williams, this volume, p. 49) we conclude that the protein binds iron through 

 an imidazole group. If this conjecture is correct the redox potential of cytochrome a^ 

 will be about -f 0-30 V. 



We predict that (1) the value of A m// (Soret band) between the Fe++ and Fe+"'""'' 



