Evolution of Photos) nthctic Mechanisms 25 



reversibility of at least some of the steps of oxidative phosphorylation. 

 Thus, there is evidence that in mitochondria it is possible to produce 

 the reaction 



Fe 11 (cytochrome) + l/ 2 DPN + (n) ATP -> 



Fe 111 (cytochrome ) + i/ 2 DPNH + (n) ADP + nPi 



If an independent (nonredox) method of dehydration could be found 

 for producing ATP according to the reaction, 







II II II II 



AMP-0-P-0H + HO-P-0 > AMP-0-P-0-P— + H,0 



II II 



0~ OH 



I 

 H 



ADP Pi , ATP 



then both ATP and TPNH could be photoproduced without calling 

 upon a photoinduced direct electron transfer reaction. 



We already have a precedent for the idea that an optically excited 

 pi-electron system can have an increased affinity for water leading to 

 its hydration by an only very slowly reversible process so that energy 

 may be trapped in this manner. 26 







H 

 / 



C C 



HN^ ^CH , HN^ ^C'-OH 



| || + H,0 -^-> I I 



o^ ^n^ o^ ^r \ 



H 



H H 



For example, if the 9-10 enol in chlorophyll were to add orthophos- 

 phate (when excited), an enol phosphate could be produced, which 

 presumably would be capable of phosphorylating ADP to make the 

 required ATP. 27 Part of this would then be used to reverse the 

 DPNH-cytochrome reduction to produce the ultimately necessary 

 separation of oxidant and reductant (water splitting I required for 

 Oo production and C0 2 reduction. 



The not inconsiderable difficulty with such a plan as this is the 

 necessitv for producing a good many more than one ATP for each 

 quantum absorbed by chlorophyll. Even if a way of circumventing 

 this difficulty were found, it remains fairly clear that such a device 



