72 Discussion 



to the CT-bonding power of the metal is, I believe, incorrect. The band positions are 

 partly affected by metal acceptor power through a-bonds and partly through the 

 77-bond system (see Williams, Cliem. Rev. 56, 299, 1956). However the total analysis 

 of the spectra of porphyrins given by Gouterman is the best available to date. 



The diagram in my paper. Fig. 1 (this volume, p. 43), is based on relatively little 

 evidence for protoporphyrin and mesoporphyrin complexes. Lemberg has pointed 

 out to me that it is inadequate for haem a complexes. While I deal with his complexes 

 later I should like now to state the general case (see Fig. 1). In the region of high-spin 

 complexes in both oxidation states Amax of both ferrous and ferric complexes moves 

 to longer wavelengths with increasing a-donor power of the ligands perpendicular to 

 the porphyrin plane. This is also true for the band positions of completely covalent 

 complexes. Both these statements have theoretical and practical support. In the 

 region of mixtures of high- and low-spin complexes the Soret band of Fe++ complexes 

 moves to shorter wavelengths while that of Fe+++ does not. The reasons I give for 

 these shifts remain unaltered but here the discussion is empirical first and theoretical 

 afterwards. In this region all the spectra are composite, part being due to low-spin 

 and part due to high-spin forms. 



Unlike Falk, I consider that some correlation between spectra and spin form must 

 be established by us for otherwise there is little hope of knowing in what state the 

 cytochrome is in the cell. It may be that the correlations I suggest are only partially 

 true but I know of few exceptions to them. 



In order to avoid terminological difficulties could I point out the equivalent 

 definitions: 



ionic = high-spin = weak field 

 covalent = low-spin = strong field 



The discussion of redox potentials given in my paper needs modification. In an 

 exchange with Chance (Faraday Society Discussion on Energy Transfer, 1959) I was 

 led to the conclusion that £)-type cytochromes are mixed imidazole, amine porphyrin 

 complexes. The diagram of redox potential against ligand basicity now should be 

 as in Fig. 2 of my paper (this volume, p. 47). 



I have attempted to show the relationships to simple haem complexes at the risk of 

 over-simplification. My table of conclusions on the basis of spectra and redox poten- 

 tials (this volume, p. 48) is incorrect now and should read: Cytochrome b, one amino, 

 one imidazole. From some work we have done recently (Brill and Williams, un- 

 published) we have good reasons for supposing that peroxidase is an amine carboxylate 

 complex rather than a simple carboxylate complex. 



The chemical reactions of model compounds with oxygen have recently revealed a 

 possible cause of the differences between cytochrome 03, uptake of oxygen and rapid 

 autoxidation, and haemoglobin, uptake of oxygen only. The reactions we have ob- 

 served are that whereas Fe (dimethylglyoxime)2 (imidazole)2 picks up oxygen reversibly, 

 Fe (cyclohexane dione-dioxime)2 (imidazole)2 first picks up oxygen and is then slowly 

 oxidized to a conjugated hydrocarbon. The oxidation is also possible with ferri- 

 cyanide, and iridichloride. Schematically we have: 



A B 



Fe++A'(imidazole)2 <^ Fe++A' (imidazole)02 ^ Fe+++A' (imidazole)2 



The reversible reaction. A, goes if X has no hydrogens which can react with O2 of 

 the complex, while B follows if X contains suitable hydrogen. If analogies are helpful, 

 the oxidation of cytochrome a^ by O2 is due to the removal of a hydrogen from a 

 sensitive group interacting with the porphyrin and the electron transport is initiated 

 by hydrogen oxidation. 

 Falk : In the paper with Perrin (this volume, p. 56), the haemochromes in our Table 5 

 are low-spin by magnetic susceptibility measurements; the ligands forming these 

 compounds are =N — atoms of pAT values varying from 3 to 7. The visible spectra 

 hardly differ from each other. I do not have Soret band measurements. In the com- 

 pounds with primary and secondary aliphatic amines described a little later in this 

 discussion, changes directly related to ^K occur in both X and e of the visible bands. 



