388 



LIGHT AND LIFE 



R 



2) {HOU){Enz) 



"Singl. 



O 



H H 



O 



HO— C— C— CHs 



O 

 H 



R 



{Enz OH) 



H 



H 

 O 



I 

 HO— C 



O 



H 



I 

 - C— CH3 



I 

 O 



H 



The purpose of the first photochemical reaction (represented by the chemi- 

 cal symbols of the upper half of the figure) is to store temporarily chemical 

 energy at the cost of the metastable state of chlorophylls. The reaction is 

 supposed to be a simultaneous keto-enol transition in the fifth ring of 

 chlorophyll-a and in the photosynthetic oxidant. In the diagram, PGA 

 is chosen, but other keto acids would show the same isomer transition. 

 A complex of chlorophyll with keto acids should easily be formed by hydro- 

 gen bonds. The energy of the metastable excitation of chlorophyll should 

 be sufficient to promote by an exchange of two hydrogen atoms the two 

 enolization processes. The shift of the hydrogen atoms during the transfer 

 is small. Thus, losses by heat of activation should be small and enough 

 energy would be available to store about 20 kcal. If one uses the conven- 

 tional bond energies for the keto and enol forms, each of the latter should 

 be about 10 kcal. abo\e the energy levels of the keto forms. The symbol 

 Enz. in the diagram stands for cytochromes which in our theory are direct 

 neighbors of each water-exposed chlorophyll. Neither will the cytochrome 

 be changed chemically during this reaction, nor will water be split. However, 

 one water molecule can be formed by dehydration of the two hydroxyl 

 groups attached to the same carbon of this acid. The second step is initiated 

 by arrival of singlet excitation at the complex. Its energy, together with 

 the 20 kcal. stored in the enol states and released again during the second 

 photochemical step, represents ample energy for the splitting of water, con- 

 nected with a transfer of an H-atom to the PGA, and of an OH to the 

 cytochrome. Two of the radicals formed by addition of one hydrogen atom, 

 to PGA, are supposed to dismutatc into one phosphotriose molecule and one 

 PGA. The disnuitation reaction is exothemiic to an amount of ca. 10 kcal. 

 The energy liberated (an be used for ATP formation from ADP and phos- 

 phate, if the disnuuatioii occurs on a surface of the enzymatic system promot- 

 ing phosphorylation. This type of phosphorylation is probably the one which 

 Dr. Arnon calls primary phosphorylation connected with the reduction 

 process. This picture fits in with all the observations on fluorescence and 

 quenching and so on. 1 believe it is also not in contradiction to any 



