CHLOROPHYLL PARTICIPATION IN THE PRIMARY PROCESS 553 



One objection can be raised to mechanism (19.13). Reaction (19.13c) 

 appears to be the same by which "substitute reductants" replace water 

 in the photosynthesis of bacteria and anaerobically adapted algae. If 

 this reaction can occur in all green plants (that is, if it does not need the 

 intermediary of an hydrogenase), why should all of them not be able to 

 reduce carbon dioxide at the cost of cellular or added organic hydrogen 

 donors, that is, to carry out " photor eduction" with organic reductants 

 instead of photosynthesis? (This question was asked once before in 

 chapter 6, page 145.) The answer may be that photoreduction is 

 possible, but that in photosynthetically active plants, the probabihty 

 that oChl will react with water is so much higher than that it will react 

 with another hydrogen donor (A in 19.13c, H2R in Scheme 6. Ill) that 

 the last-named reaction remains unnoticed. In photoxidation, on the 

 other hand, only the small fraction of oChl molecules which react with 

 A produce a net chemical change, since (as mentioned on page 543) the 

 reaction of oChl with H2O: 



(19.14) {oChlX} + I H2O > {ChlX) + I O2 



merely compensates for an equal amount of oxygen which is consumed 

 by the reoxidation of the intermediate HX by oxygen according to 

 (19.13b). 



Thus, photoxidation in vivo may represent a small irreversible residue 

 of a reversible photochemical process, which may be described as ''photo- 

 synthesis running in a circle" (because of the substitution of oxygen for 

 carbon dioxide as the final oxidant). The residual effect is caused by 

 the substitution of a small proportion of oxidizable cellular substrates for 

 water as final reductants (compare Scheme 19.1). 



Since photoxidation occurs also in boiled leaves, reactions (19.13b) 

 and (19.13c) must be catalyzed by heat-resistant catalysts of low mo- 

 lecular weight, rather than by true enzymes (c/. Noack 1925, 1926). 



(b) Photochemical Reduction, Nonphotochemical Reoxidation of Chlorophyll 



This alternative was suggested by Conant (c/. Conant, Dietz, and Kamerling 

 1931), who thought that extracted chlorophyll is the reduced form of the catalyst. 

 It oxidizes itself in air by forming allomerized chlorophyll. Conant suggested that, 

 in vivo, it could be oxidized by a thermal reaction with carbon dioxide and reduced by a 

 photochemical reaction with water. (Thus, the photochemically active form was 

 supposed to be identical with allomerized chlorophyll.) 



Willstiitter (1933) suggested that chlorophyll (H2R) is first oxidized by oxygen to 

 a radical, " monodehydrochlorophyll " (HR), thus accounting for the alleged necessity 

 of oxygen for photosynthesis (cf. Chapter 13, page 326) ; it then reduces carbon dioxide 

 by a thermal reaction, being itself converted into a "didehydrochlorophyll," R. (This 

 mechanism bears a certain similarity to the "energy dismutation" defined on page 165.) 

 Finally, R oxidizes water by a photochemical reaction and is itself reduced, first to 

 HR, and then to HjR. 



