OXIDASE HYPOTHESIS 293 



the primary products. According to Baur (1936, 1937), carotene peroxide 

 is formed only in light, and its formation is at least partially reversible. 



Peroxides of quinonoid dyes have often been mentioned in the 

 literature, but have not been well investigated. It has been suggested, 

 for example, that the autoxidation of organic dyes proceeds through 

 primary peroxide formation (c/. page 499). Gaffron (1927) observed 

 the formation of reversible peroxides of organic amines in experiments on 

 chlorophyll-sensitized photoxidation (c/. Table 18.1). 



In the case of chlorophyll, evidence of oxygen absorption as the cause 

 of so-called " allomerization" (c/. page 459) was presented by Conant, 

 Hyde, Moyer, and Dietz (1931). This absorption was attributed to the 

 formation of a chlorophyll peroxide by Conant and H. Fischer. Fischer 

 represented it as an alkyl hydroperoxide R— 0— OH, with the peroxidic 

 group in position 10 (c/. Formula 16.III). Certain difficulties of this 

 hypothesis (in particular the fact that oxygen absorption can be observed 

 only in alcohol) are discussed in chapter 16. No definite proofs have as 

 yet been found that the oxygen uptake in the allomerization process is 

 reversible. 



The formation of a reversible (or almost reversible) peroxide, capable 

 of monomolecular decomposition, could be of great advantage for 

 photosynthesis if it would permit the saving of energy otherwise lost in 

 bimolecular dismutation. However, the question arises as to the way in 

 which such peroxides could be fitted into the reaction cycle of photo- 

 synthesis. The primary photochemical oxidation products, Z or {OH}, 

 probably are free radicals in which one hydrogen atom is "missing"; 

 jour such radicals must cooperate in the direct liberation of one mole- 

 cule of oxygen (while the recombination of two radicals is sufficient to 

 produce one-half an oxygen molecule by dismutation) . The mechanism 

 by which four radicals could transfer two oxygen atoms to a catalyst to 

 form a "reversible" peroxide, for example: 



(11.25) 4 lOH ! + R > ROo + 2 H.O 



is difficult to visualize, especially if the peroxide-forming catalyst is an 

 organic molecule with a double bond, or a quinonoid ring system. It is 

 somewhat easier to devise a similar mechanism if the peroxide-forming 

 catalyst is a hemin complex capable of reacting with four radicals in as 

 many "univalent" steps. The next section will be devoted to this 

 possibility. 



4. The Oxidase Hypothesis 



It was stated before that, in the main course of respiration, the 

 formation oi free peroxides is avoided by the action of "oxidases," and 

 that a reversal of the oxidase action may be the mechanism of oxygen 



