10 L. E. Orgel 



section, like others in the field, are hypothetical and are only meant to draw 

 attention to some of the recent work on electron transport which may be 

 relevant. The detailed schemes proposed are meant to be illustrative, and 

 not as detailed mechanisms for oxidative phosphorylation, etc. 



We distinguish two types of mechanism by means of which redox energy 

 may be transformed for example to pyrophosphate energy. In the first 

 place some "low-energy" phosphate is oxidized to a product with a high 

 phosphate transfer potential. The oxidized phosphate is then used to 

 generate pyrophosphate (e.g. ATP), and finally the oxidized, dephosphory- 

 lated fragment reacts with the next member of the electron transport chain 

 in its reduced form. Recent interest has been turning to various benzo- 

 quinones and naphthaquinones as possible carriers in the above scheme. 

 Systems of this kind may achieve considerable chemical subtlety but they 

 require little in the way of theoretical analysis. 



In the remainder of this paper I shall discuss an alternative mechanism, 

 not because I believe it to be intrinsically more (or less) plausible but because 

 it requires some theoretical interpretation and if operative might depend in 

 detail on those features of haemoprotein structure which I have already 

 discussed. 



Taube and co-workers (1959) in a series of briUiantly conceived experi- 

 ments have shown that if Co+++ and Cr++ ions are placed in solution with 

 fumaric or terephthalic acid, electron transport takes place as illustrated : 



Co+++(H20)5 



\ 



O O 



O O— Cr++(H,0)5 



Co++ (aqu) 



O O 



/ \=J \ 



O O— Cr+++ (aqu) 



This shows in a particularly clear way that the bridging group involved in 

 many redox reactions may be a conjugated molecule. The qualitative theory 

 of electron transport through bridging groups has been discussed elsewhere 

 (Orgel, 1956) and particularly for conjugated molecules by Griffith (1959). 



Taube further observed that if fumaric or terephthalic acid is replaced by 

 its monomethyl ester this is hydrolyzed during the course of the reaction, 

 and the chemical behaviour of the solution suggests that the methyl group 



