ORGANIC PEROXIDE HYPOTHESIS 



287 



Willstiitter and Stoll (1918) as well as its elaboration by Franck and 

 Herzfeld (1937), must now be considered as obsolete because it was 

 based on hydroxyl-hydrogen exchange as the elementary photochemical 

 process, a concept which has been proved false by the demonstration 

 that all oxygen evolved in photosynthesis originates in water (c/. Chapter 

 3, page 55). 



In consideration of the historical importance of these theories, we 

 shall give a brief account of them. Willstatter and Stoll suggested that 

 photosynthesis consists of two photochemical hydrogen-hydroxyl ex- 

 changes in the carbonic acid molecule, alternating with the catalytic 

 dismutations of the two peroxides — performic acid and performalde- 

 hyde — formed by these exchanges. 



(11.9a) 



OH H-OH 0-OH ..eatalatic" OH 



I exchange | dismutation | 



C=0 > C=0 * C=0 + ^02 



I i2h.) I I 



OH H H 



(carbonic (performic (formic acid) 



acid) 



(11.9b) 



acid) 

 O 



H— C 



hydration 

 > 



OH 



I 

 H— C— OH 



H— OH 

 exchange 

 > 



(2 A.) 



OH 

 (formic acid) 



OH 



(formic acid 



hydrate) 



H 



H— C— OH + I O2 



in 



(formaldehyde hydrate) 



H 



I 

 H— C— OOH - 



OH 



(performaldehyde) 



"catalatic" 

 dismutation 

 > 



dehydration 



> CH2O + H2O + ^ O2 



Willstiitter and Stoll added to hypothesis (11.9) another and independent hypothesis 

 -that of a reversible chlorophyll-carbonic acid association {cf. Chapter 16): 



(11.10) PhMg (Mg-pheophytin = chlorophyll) + H2CO3 ^ 



=^ HPhMgHCOa 



and the H — OH exchange, carried out in (11.9) with/ree H2CO3 molecules, was applied 

 to the chlorophyll— carbonic acid compound. The reduction product was supposed to 

 separate itself from chlorophyll after the last photochemical step : 



H 



I 

 (11.11) HPhMg— O— C— OOH 



I 

 H 



H 



-)- PhMg + HO— C— OOH 



I 

 H 



Franck and Herzfeld (1937) have attempted to elaborate scheme (11.9) by dividing 

 each two quanta process into two thermochemically feasible single quantum reactions. 

 Since they thought free radicals would present insurmountable energy barriers, these 

 authors introduced an intermediary hydrogen donor, ROH, which they substituted for 

 water as the first hydrogen donor and hydroxyl acceptor, and suggested that, if the 

 R — OH bond in this catalyst is considerably weaker than the H — OH bond in water, 



