Electronic Structure and Electron Transport Properties of Metal Ions 17 



example of this is electron transport transverse to the polypeptide chains of 

 protein where the transport occurs through the CO and NH group via a hydrogen 

 bond. 



Orgel: This could be compared to a condenser. 



LocKWOOD : Yes. The picture of the cytochrome change in particular preparations where 

 the cytochromes are situated spatially side by side is a legitimate one and the distinction 

 between the two types of models becomes important. The transport of the electrons 

 through the members of the cytochrome chain is a process which is repeated an 

 indefinite number of times and the model, to be satisfactory, should belong to the 

 copper wire type. It appears to me that the condenser type of model would be useless 

 to explain the transport of electrons through the cytochrome chain. 



Orgel: We cannot be sure that electron transport will never take place through aliphatic 

 side chains. However, I myself would be very surprised to find transport through 

 more than at most three or four carbon atoms. We are currently investigating this 

 problem by magnetic resonance methods. Transport through the a helix or similar 

 protein structure via a long series of hydrogen-bonded C=0 and NH groups is more 

 problematical; again I suspect that this process is not favoured except in systems 

 which have been excited optically. 



Chance: Although we have been discussing in some detail the mechanisms by which 

 electrons might be transferred through the peptide chain of the protein, an experi- 

 mental test of this possibility suggests that an insufficient conduction rate would occur 

 at least in the case of cytochrome c. Experiments carried out by my collaborator 

 Patrick Taylor on dried cytochrome c in an atmosphere of nitrogen, show that less 

 than 1 ,000th of the conductivity w ould be obtained when compared with the rate at 

 which electrons are transferred in the cytochrome chain. While this experiment may 

 not be conclusive it is certainly indicative of the difficulty of applying this approach. 

 Our early experiments on the reaction of cytochrome c and the peroxidase intermediate 

 have been reviewed and considerably extended by John Beetlestone. He finds that an 

 active centre of the size of 5 A would be adequate to explain the observed kinetic 

 data. This size is larger than that of the iron atom but would fit nicely with the idea 

 that a histidine group is involved. Thus to within the accuracy that is possible with 

 this determination, some group on the outside of cytochrome c may be responsible 

 for the interaction. 



George: I would like to add a few comments to those of Chance on the subject of kinetic 

 data for haemoprotein reactions. Even though some of the velocity constants are 

 quite low, i.e. 10^ to 10^ M~^ sec^^ in comparison with high values of 10® to 10* M~^ 

 sec~^, these low values are often found to originate in large (unfavourable) activation 

 energies E, so that when the temperature independent factor A in the Arrhenius 

 equation, k = A e-^l^^'^, is evaluated it is found to have remarkably high values of 

 the order lO^^ to lO^'. 



Now in terms of the simple collision theory for bimolecular reactions A is equated 

 to PZ, where Z is the collision frequency, 10^\ and P is the steric factor. Considering 

 "target areas" for haemoprotein reactions one would expect P to be a fraction, yet 

 it is apparent that P can in fact be several powers of ten. 



It would seem that other features are extremely important in these reactions of 

 which we know very little at present. For example, the haem plate is hydrophobic in 

 nature and undoubtedly alters the structure of the liquid water in its vicinity. In 

 addition, around tlie haem plate, there is a constellation of ionic charges on the protein, 

 which may be very important when a reaction between two haemoproteins occurs. 



Chance: I agree with George that temperature-independent factors in haemoprotein 

 interactions are high and variable and thus the accuracy with which one can determine 

 tlie size of the active centre is definitely limited. However, the results are useful 

 indicators nevertheless. 



In this connexion, increasing knowledge of cytochrome c structure is of importance 

 and the apparent inaccessibility of the haematin, due to the surrounding structures, 

 provides independent support for the idea that the active centre of cytochrome c in 

 the peroxidase reaction may have to exceed the size of the iron atom. 



