70 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



The experiments on isolated chloroplasts, described on page 63 

 et seq., as well as considerations based on bacterial metabolism, (cf. 

 Chapter 5), make it feasible that one (and perhaps the only) photo- 

 chemical reaction in photosynthesis may be the transfer of hydrogen from 

 water to an intermediate acceptor. Consequently, in our search for a 

 model of photosynthesis in vitro, we are concerned, in the j&rst place, 

 with the photochemical oxidation of water by substances thermodynami- 

 cally incapable of achieving this oxidation in the dark. 



The liberation of oxygen from water can occur by "self-oxidation" 

 (dismutation), 



light 



(4.3) H2O + H2O > 2 H2 + O2 - 137 kcal or 



light 



(4.4) H2O + H2O > H2 + (H202)aq. - 91 kcal 



or (if hydrogen is taken over by an "acceptor") by an oxidation- 

 reduction: 



light 



(4.5) H2O > I O2 + {2H } or 



light 



(4.6) 2 H2O y H2O2 + { 2H } 



where brackets indicate acceptor molecules. 



Reactions (4.3) to (4.6) can be brought about by direct absorption of 

 ultraviolet light by water, or they can be sensitized. If, in (4.5) and 

 (4.6), the acceptor itself is the light-absorbing species, the reaction is a 

 photoxidation of the acceptor, rather than a true photocatalysis, and can 

 only be called "sensitized" in the wider sense defined on page 56. 



For the sake of simplicity, we do not speak here of the possibility 

 that an acceptor can be provided also for hydroxyl radicals or oxygen 

 atoms, so that the primary products of oxidation will be complexes of 

 the type {OH}, {OH} 2 or {O2}, rather than free molecules of oxygen or 

 hydrogen peroxide (cf. Chapter 11). 



Any of reactions (4.3) to (4.6) can provide an appropriate first step 

 towards artificial photosynthesis. To complete the process, carbon diox- 

 ide must be reduced by hydrogen or by the hydrogenated acceptor { H } . 



In addition to the photodismutations, (4.3) and (4.4), and the phot- 

 oxidations, (4.5) and (4.6), we shall also consider here the "photaut- 

 oxidation": 



light 



(4.7) H2O + I O2 > (H202)aq. - 23 kcal 



(The term " autoxidation " for "oxidation by molecular oxygen" is ugly, 

 but has come into general use.) This reaction could be useful as a first 

 step in artificial photosynthesis only if hydrogen peroxide were able to 

 reduce carbon dioxide. This question will be discussed on page 79 

 and answered in the negative. We have nevertheless added (4.7) to 

 the other forms of photochemical oxidation of water, because this reaction 



