EXPERIMENTS WITH HEAVY WATER 295 



however, that it is a radical derived from chlorophyll, and that a hemin 

 complex is the next hydrogen (or electron) donor in the series, which 

 restores oxidized chlorophyll (Z) to its original state (HZ). Possibly 

 not one, but several, reversibly oxidizable hemin derivatives are interpo- 

 lated between the primary oxidation product Z and the terminal "de- 

 oxidase," occupying positions similar to those of the cytochromes in 

 respiration. 



Thus, the intermediate, {O2}, postulated in chapter 6 (Scheme 6.1), 

 may be not a "free" peroxide, but a complex of the type of ferricyto- 

 chrome or one of the series of intermediates in (11.26) which, even if 

 they contain — O bonds, do not have the characteristic instability of 

 hydrogen peroxide and the common organic peroxides. 



As described in chapter 16, chlorophyll in vitro reacts with excess 

 ferric chloride and is restored by excess ferrous chloride. This seems to 

 indicate a reversible oxidation, and a normal potential close to that of 

 the ferro-ferri system. The latter is, in neutral solution, only 0.03 volt 

 more positive than the potential of an oxygen electrode. It is thus 

 conceivable that, in vivo, a photochemically produced "oxy chlorophyll" 

 radical may oxidize a complex iron compound, which in turn is capable 

 of oxidizing water through the intermediary of a deoxidase. 



5. Experiments with Heavy Water 



One could expect to obtain some information as to the catalytic 

 mechanism of water oxidation in photosynthesis from experiments with 

 water containing the rare isotopes of hydrogen or oxygen. Experiments 

 with radioactive oxygen have already been discussed (in Chapter 3) because 

 they provided the main experimental basis for the interpretation of 

 photosynthesis as hydrogen transfer from water to carbon dioxide. The 

 results of experiments with heavy hydrogen have been mentioned in 

 passing, in chapter 7 (page 157). We shall now describe them in more 

 detail. 



Reitz and Bonhoeffer (19350 found that deuterium is assimilated by 

 algae {Chlamydomonas and Scenedesmus) , grown in mixtures of ordinary 

 and heavy water, at a rate 2.3 times smaller than that of the assimilation 

 of ordinary hydrogen. Later (1935^, they observed that Scenedesmus 

 cannot grow in 38.4% heavy water. On the other hand, Meyer (1936) 

 found that Chlorella grows well even in 99% heavy water; this was 

 confirmed later by Trelease and coworkers. In good agreement with 

 Reitz and Bonhoeffer, Curry and Trelease (1935) found that the rate of 

 photosynthesis of Chlorella in pure deuterium oxide is 2.5 times smaller 

 than in ordinary water. (Shibata and Watanabe, 1936, found, with 

 Chlorella ellipsoidea, only a difference of 20%.) The change in the rate 

 was complete after a 30-minute immersion into heavy water, and re- 



