12 L. E. Orgel 



metal complex and a reactive positively charged species which preserves and 

 can utilize the free energy of the redox reaction. I hasten to add that the 

 minor details which I have given are not essential, for example, the meta- 

 phosphate need never be liberated as such, but might be transferred directly to 

 an appropriate acceptor in a concerted reaction, the mediator need not be a 

 quinol, the transferred group need not be a phosphate, etc. 



This type of system for utilizing redox free energy has much in common 

 with ones previous proposed except that 



, , , , _ Oxidation Transfer 



M++— Ri > M+++— Rj > M+++— Rx— Ra 



\ \ 



the "electron mediator" and the transferred group are part of the same 

 molecule. The advantage of this in ensuring the efficient coupling between 

 the redox and transfer processes is obvious. 



If mediated electron transfer is important in oxidative phosphorylation, 

 then the details of the electronic configurations of the metal enzymes con- 

 cerned will be critical to the understanding of the process. Here I can 

 summarize only a few of the most important considerations : 



1 . The effect of a metal ion in inducing the hydrolysis of a bond as in the 

 reactions (ii) and (iii) will be large if the metal has a maximum number 

 of spins paired after reaction, e.g. if the Fe^++ ion produced is low- 

 spin rather than high-spin (of course it will also be greater for a 

 trivalent than for a divalent ion, other things being equal). 



2. The rate of transfer will be greatest if there is no need for a change of 

 spin configuration during the process, i.e. if it occurs between two 

 high-spin or two low-spin complexes. 



3. The rate of transfer also depends on the degree of overlap between 

 metal orbitals and mediator orbitals. This is greatest for low-spin 

 complexes. 



4. The most effective mediator molecules are likely to be ones which can 

 act either as good electron donors or as good electron acceptors. 



In conclusion I should like to note that even if oxidative phosphorylation 

 does not involve steps which have an obvious appeal to the theoretician, but 

 rather a sequence of conventional coupled redox reactions, both the redox 

 potentials and the rates of reaction will certainly depend critically on the 

 spin-states of the metal ions and hence on the details of their interaction with 

 their environments, 



REFERENCES 



George, P. & Griffith, J. S. (1959). The Enzymes, 2nd edition, Ed. Boyer, Lardy and 



Myrbiick, Chap. 8. 

 Gibson, J. F. & Ingram, D. E. (1957). Nature, Lond. 180, 29. 



