ORGANIC PEROXIDE HYPOTHESIS 289 



which then decomposes into formic acid, and oxygen: 



OOH 



I light 



(11.13b) 0=C— OH y HCOOH + O2 



As an experimental basis for this hypothesis, Baur quoted Thunberg's and his own 

 experiments on the formation of formaldehyde by distillation of percarbonates in the 

 presence of lead dioxide {cf. page 79). 



The second scheme was suggested by Gaffron and Wohl (1936), after Gaffron 

 (1927) found evidence for the formation of amine peroxides in the chlorophyll-sensitized 

 photoxidation of amines (cf. page 511). Gaffron and Wohl suggested that amine 

 O OH 



/ \ I 



peroxides, RN O or RN— OOH (with R possibly standing for a protem radical), 



may occur as precursors of oxygen in photosynthesis, e. g., according to the reaction 



scheme : 



O 



(dark) II light 



(11.14) RNH2 + CO2 > RN— C— OH > 



(carbamination) | + H20 



H 

 H+ 



RN— O— O- + {H2COJ > RNH2 + O2 + IHoCOl 



H 



These theories fail to conform with the requirement, derived from experiments with 

 radioactive carbon dioxide, that all oxygen should originate in water. They therefore 

 claim only an historical interest. However, the concept of organic peroxides as inter- 

 mediates can be utihzed in modern oxidation-reduction theories of photosynthesis as 

 well, although the separation between the oxidant (CO2) and the reductant (H2O), 

 which characterizes these theories, leads one to the consideration of catalyst peroxides, 

 instead of the peroxides derived from the reduction substrate itself. 



When Franck and Herzfeld (1941) abandoned the hydrogen-hydroxyl 

 exchange mechanisms (11.12) for the hydrogen transfer mechanism (7.12), 

 they retained the assumption that an organic peroxide, ROOH, is formed 

 by the substitution of an organic compound, ROH, for water as hydrogen 

 donor in the primary photochemical reaction (cf. Scheme 7.VA). In 

 some other schemes in chapter 7, a water-acceptor complex, designated 

 by {H2O}, was postulated as the primary reductant, and its oxidation 

 product, {OH}, was assumed to form a peroxide {OH} 2 before decom- 

 posing into free acceptor, water and oxygen. These are two examples 

 of theories which postulate "catalyst peroxides" as intermediate oxida- 

 tion products. 



The difference between the hypothesis of a complex, { H2O } , and the 

 assumption of Franck and Herzfeld of an intermediary catalyst, ROH, 

 consists in the reversal of the order of two reactions. To show this, let 

 us assume that the "water acceptor" is an organic double bond com- 

 pound, R'=R", so that the reaction, H2O . {H2O}, becomes: 



(11.15a) 4 R'=R" + H2O * 4 HR'— R"OH 



