172 Discussion 



PosTGATE : I think there exists some evidence against Drabkin's view that the 280 m/n peak 

 in haematins is mainly due to some frequency in the iron-porphyrin system and not 

 to residues of aromatic amino-acids. Cytochrome C3, which has a molecular weight 

 closely similar to that of cytochrome c, has two haemins/molecule. Yet in spite of 

 this the absorption of this material at 280 m/i is negligibly small; the 280 m/f peak of 

 cytochrome c is absent. 



On the other hand, we find from qualitative observations that the aromatic residue 

 content of c^ is very small, which would be consistent with the traditional view of the 

 significance of the 280 m/t band. 



Morton: As will be seen from the tables in my review (Morton, Rev. pure appl. Chem. 

 8, 161, 1958), there is a paucity of information concerning the influence of the state 

 of oxidation on the absorption of cytochromes in the ultra-violet region. Our studies 

 with cytochrome b.^ suggest that the position and height of the band in the 260-280 

 m/i region does change between the oxidized and the reduced compound. Could 

 Postgate comment on the diff"erence between ferricytochrome c^ and ferrocytochrome 

 C3 in the 280 m/< region. 



Postgate: We have never observed the 280 m/t range of ferrocytochrome c^; because of 

 its low Eq we have found no reducing agent which will maintain the reduced form 

 without absorbing strongly in this range. 



Drabkin: The situation Postgate describes in cytochrome C3 is certainly most unusual, 

 perhaps unique for haemin chromoproteins. I would not be astonished by the absence 

 of a well-defined maximum, but am puzzled by negligible absorption, particularly 

 since co-ordination complexes such as haemin dicyanide (my Fig. 5) absorb rather 

 strongly in this region. Perhaps the presence of a weak masked band could be brought 

 out by the type of analysis I have employed. What is the nature of the ultra-violet 

 absorption of reduced cytochrome Cg? That approximately 30% only of the total 

 absorption at 280 m/t can be ascribed to the specific absorption of the aromatic 

 amino acids in most haemoproteins is in essential agreement with my own deduction, 

 based upon the content of tyrosine, phenylalanine and tryptophane, as discussed in 

 my paper. I believe that these aromatic amino acids contribute to the absorption, 

 but the main contribution is owing to the haemin structure. It is misleading in the 

 case of the haemin derivatives to speak of the protein band (at 280 m/t). 



Williams: I wonder should not one re-examine, in respect to the absorption at 280 m/t, 

 the question of energy transfer from aromatic amino-acids to haemoproteins, e.g. in 

 the photochemical decomposition of CO-complexed haemoproteins (Weber, Disc. 

 Faraday Soc. 11, 1959). As I understand Drabkin's remarks, there is a co-operative 

 enhancement of the absorption of the aromatic amino-acids at 280 m/t by the haem 

 unit. 



Drabkin: I am sorry that I am not acquainted with the actual experimental work of 

 Weber to which you refer. In any event the energy would have to be quantized. In 

 my paper I do refer to Warburg's classical study, and believe that his photochemical 

 spectrum which includes a broad spectral coverage, with several maxima besides the 

 'protein' band at 280 m/t, must indicate that the same haemin structure is involved 

 and photochemically eff'ective in spectral regions which cannot be ascribed to protein. 

 This appears to support my proposal of a similar origin for the various bands. 



On the other hand, the total co-ordination complex includes residues from the 

 protein. Hence, the eff"ect of protein cannot be wholly separated from the haemin, 

 and some protein contribution may be present over the whole spectral range (see 

 discussion in paper). 



Lemberg: Most porphyrins have, indeed, a weak absorption in the region 260-280 m/t. 

 For protoporphyrin in dioxane we have found an £„,« of 14 at 280 m/t. The absorp- 

 tion is thus far weaker than that of the Soret band, whereas many haemoproteins 

 absorb at 280 m/t as strongly or more strongly than in the Soret region. 



