DIFFERENT OXIDANTS 15G5 



and 3.5 X 10~^ mole/1, (c. g., in one set of experiments 70%); the initial 

 rate also seemed to be independent of chromate concentration. 



(c) Oxygen as Hill Oxidant 



One of the puzzles of photosynthesis is how the photochemically trans- 

 ferred hydrogen manages to avoid the abundantly available strong oxidant, 

 molecular oxygen, in favor of the sparse and extremely unwilling oxidant, 

 carbon dioxide. Only under certain abnormal conditions — such as carbon 

 dioxide starvation, or excessive illumination (c/. chapter 19) — does photau- 

 toxidation replace photosynthesis. It was suggested (c/. scheme 19.1, p. 

 544) that in these cases, oxygen acts as a "substitute hydrogen acceptor" 

 replacing carbon dioxide, while the primary photochemical hydrogen trans- 

 fer remains the same as in photosynthesis. However, as long as water is 

 the reductant, the Hill reaction with oxygen as oxidant is a circular process, 

 leading to no net chemical change, and therefore unrecognizable except by 

 isotopic tracer experiments (photocatalysis of isotopic equilibration of free 

 oxygen and water). For it to become observable by ordinary chemical 

 methods, it has to lead to the oxidation of compounds other than water^ 

 such as benzidine (page 528) or chlorophyll itself (page 537). This may 

 occur by direct substitution of these reductants for water in the photochem- 

 ical process (as suggested in scheme 19.1), or indirectly by the action of 

 hydrogen peroxide formed as intermediate in photochemical hydrogenation 

 of oxygen (see below). 



The isotopic tracer method was applied to the "hidden" Hill reaction 

 (with oxygen as oxidant and water as reductant) by Brown (1953). Using 

 water containing only 0^® and oxygen gas enriched in O''' and 0^^, he found 

 that in light an exchange of the two isotopes took place at a rate comparable 

 to that of the Hill reaction at the same light intensity. When chloroplasts 

 deteriorated (by aging or phenanthroline poisoning) their capacity to 

 photocatalyze the isotopic exchange of oxygen between H2O and O2 de- 

 clined in the same proportion as their capacity for Hill reaction with qui- 

 none as oxidant. 



An indirect but ingenious chemical proof that oxygen can serve as Hill 

 oxidant with chloroplast suspensions was supplied by Mehler (1951''^, 

 1952). He used for this purpose the system ethanol-catalase, which can 

 "trap" hydrogen peroxide (expected to arise as intermediate in the hydro- 

 genation of oxygen). In the presence of ethanol, the oxidation-reduction 

 reaction 



catalase 



(35.31) H2O2 + C2H4OH > C2H4O + 2H2O 



in which catalase acts as a "peroxidase," competes with the more common 

 "catalatic" dismutation of H2O2 to H2O and O2. First, Mehler had to show 



