54 Discussion 



our own interpretation of the data (Tomkinson and Williams, /. chem. Soc. 2010, 

 1958) for the reason given in that paper. 



I do not agree with Perrin's interpretation of ligand field theory as presented in the 

 question and in his paper (see discussion of Orgel's paper, p. 13). I think that 

 bonds, both in energy and in length, undergo considerable changes on change of spin 

 type and that in biological systems these changes are of greater importance than 

 almost any other factor. I was well aware of the data on the redox potentials of iron 

 porphyrin complexes and have tried to use them properly and with due reservation. 

 Perrin : The experimental values of E^ for metal porphyrin complexes with bases which 

 Falk and I list represent probably the most complete series available in the literature. 

 They show a general decrease with ^K of the ligands and so cannot be quoted as 

 support for Williams' suggestion that they should pass through a maximum. I think 

 it is dangerous to attempt to argue too finely from redox potential differences. For 

 example, the porphyrin in cytochrome c differs from mesoporphyrin (with which it 

 was compared) by having two CH3CHSR groups instead of ethyls. It would seem to 

 be better to take haematoporphyrin, with two CH3CHOH groups, rather than meso- 

 porphyrin, as an approximation to porphyrin c. The difference in redox potentials 

 is then much smaller for the reactions : 



Fe++ (porph)-Py2 + OH- = Fe+++ (porph)-Py-OH + Py + e 



at pH 9-6, £'0 is + 15 mV for protoporphyrin, + 4 mV for haematoporphyrin, and 

 — 63 mV for mesoporphyrin. (Lemberg and Legge, Haematin Compounds & Bile 

 Pigments, 1949.) Potentials are even more different depending on whether the reaction 

 is for the loss of a molecule of base from the ferric complex and its replacement by 

 OH", as in the example just quoted, or simply for the loss of an electron from the 

 Fe(porph)-B2 complex. The difference between the pyridine and the histidine complex 

 of iron-pro toporphyrin is < 80 mV in the first case and about 210 mV in the second 

 case (calculated from Barron, /. biol. Chem. Ill, 285, 1937, and Shack and Clark, 

 /. biol. Chem. 171, 143, 1947). In the absence of other evidence as to the nature of the 

 extra ligands in cytochromes and other metalloporphyrins, any suggestions from 

 redox data must be almost entirely speculative. It should also be pointed out that there 

 is not one, but many, members of each of the cytochrome families. There are, for 

 example, many cytochromes c which, although similar in absorption spectra, do not 

 have the same redox potential — compare cytochromes Cj and c^ where the Eo-values 

 differ by 0-545 V (Morton, Rev. pure appl. Chem. 8, 161, 1958). 



Which of these should we assume from spectra or redox potentials to contain two 

 imidazole groups bound to the metal, and how are mixed ligands to be ruled out? 

 Williams : In all my discussions of redox potential data I have been fully aware of Perrin's 

 points. I therefore restate that 



(i) All conclusions about structure are made taking into account spectra, redox, 

 and magnetic properties (see Chem. Rev. 56, 299, 1959 for the way in which I do this). 



(ii) A maximum in redox potential is only expected on change of spin type. None 

 is expected for the compounds in the series of Falk and Perrin (p. 69) except in the 

 sequence water, pyridine, imidazole, NHg. 



(iii) Mixed complexes are treated later in this discussion (p. 55). The cytochromes 

 c of different kinds are discussed by myself and Chance in Disc. Faraday Soc. 27, 

 269, 1959. Mixed complexes are included in that discussion. 

 George: In answer to Williams' question about the £'0 for the Fe+++/Fe++ tetrapyridyl 

 couple, I would like to report on the results of an investigation recently carried out 

 in collaboration with G. Haight and A. Bergh, 



We thought originally, following Morgan and Burstall, that both ferrous and ferric 

 derivatives were square planar complexes containing one tetrapyridyl molecule, 

 somewhat analogous to haem and haemin. But while the ferric complex has the 

 composition Fe(tetrapy)i+++, two ferrous complexes are formed with K^ >-^2» 

 Fe(tetrapy)i++ and Fe(tetrapy)2++. K for the ferric complex is greater than K^ for the 

 first ferrous complex, so that upon the addition of tetrapyridyl, E'q first falls below 

 the value of 0-77 V for the Fe+++/Fe++ aquo-ion couple. But, in principle at least, as 



