

/'. 



\^ 



Chapter 11 



THE NONPHOTOCHEMICAL REACTIONS IN 

 PHOTOSYNTHESIS 



III. LIBERATION OF OXYGEN * 



In chapter 7, it was assumed that one — and perhaps the only — 

 primary photochemical process in photosynthesis consists in a direct 

 oxidation of water (or in the oxidation of an intermediate reductant, HZ, 

 which in its turn oxidizes water) by an intermediate oxidant, X (which 

 in its turn reduces carbon dioxide). In chapters 8 and 9, we dealt with 

 the catalytic mechanism of reduction of carbon dioxide by the "primary 

 reduction product," HX. Now, we shall deal with the catalytic mecha- 

 nism of oxidation of water, that is, the evolution of oxygen from the "pri- 

 mary photochemical oxidation product," designated by {OH} or Z in 

 chapter 7. 



In chapter 6, we concluded from the phenomena of "adaptation" 

 and "de-adaptation" of green algae that the conversion of the primary 

 photochemical oxidation product into free oxygen involves the formation 

 of (at least) one intermediate (which was designated there as {O2}), and 

 therefore requires the assistance of (at least) two catalysts, which were 

 designated as Ec and Eo, respectively (c/. Gaffron 1944). We are at 

 present concerned with the nature of these intermediates and enzymes. 



1. The Peroxide Problem 



The problem of the conversion of the primary oxidation product into 

 oxygen can be called the "peroxide problem" because the main question 

 is whether the intermediate in this transformation is a free peroxide 

 and, if so, whether it is hydrogen peroxide or another compound con- 

 taining an O — O bond. A similar problem is encountered in the study 

 of the reverse processes (respiration and combustion). There, the 

 transfer of hydrogen atoms to oxygen leads, in the simplest case, to the 

 primary formation of hydrogen peroxide: 



(11.1) 2 RH + O2 > H2O3 + 2 R 



which is either reduced to water in a second oxidation-reduction step: 



peroxidase 



(11.2) 2RH + H2O2 >2H20 + 2R 



* Bibliography, page 298. 



281 



