240 REDUCTION OF CARBON DIOXIDE CHAP. 9 



serve as oxidants in the chloroplast-sensitized photoxidation of water. 

 The normal potentials of these complex salts are far above those of 

 free ferric ions (— 0.77 volt). However, it is doubtful whether they 

 can be positive enough to allow the complex in the ferrous form to 

 reduce the hydrogenase (whose potential at pH 7 must be about + 0.42 

 volt), a reaction which was credited to the HX radicals in scheme 6. III. 

 In chapter 11, we shall consider the possibility that a ferrous iron 

 compound serves as reductant in the primary photochemical process. 

 This compound must have an exceptionally negative potential (even 

 below that of free ferrous ions) in order to be able to recover its electron 

 from water. This role of iron complexes is more in keeping with their 

 function in respiration (where they occur close to the "oxygen end" of 

 the "electron bucket brigade") than the above-suggested role as oxi- 

 dants in the primary photochemical process. However, it may be 

 worth while to keep in mind the possibility that the photochemical 

 process in photosynthesis may be the transfer of electrons from an iron 

 (or other metal) complex of exceptionally negative potential to another such 

 complex of an exceptionally positive potential. An hypothesis of this type 

 was suggested by Weiss (1937). 



9. Transformations of the First Reduction Product of Carbon Dioxide 



We have so far been concerned only with the first step of the 

 reduction of carbon dioxide — which is probably the conversion of a 

 carboxyl group in a large molecule {C02}(=RC00H) into a radical, 

 {HCO2} (=RC(0H)2). On page 158, we suggested that the rest of the 

 reduction process may be ascribed to dismutations (cf. Eqs. 7.8b, c) ; and 

 this hypothesis was retained in schemes 9. Ill and 9. IV. An alternative 

 is that the photochemically produced reductants (e. g., the radicals, HY), 

 are again called upon to reduce the intermediate products, in a series of 

 thermal oxidation-reduction processes similar to the sequence of photo- 

 chemical reactions assumed by Franck and Herzfeld (cf. scheme 7.VA). 

 This alternative allows the closest analogy between the processes of 

 photosynthesis and respiration since, in the latter, the dehydrogenases 

 (e. g., the pyridine nucleotides) are instrumental in removing hydrogen 

 atoms not only from keto groups (in the oxidation of glyceraldehyde to 

 glyceric acid and of acetaldehyde to acetic acid in Schemes 9.1 and 9. II), 

 but also from the more stable R'H-R"H — groups (in the oxidation of 

 acetate and succinate) and RHOH groups (in the oxidation of malate to 

 oxalacetate in Scheme 9. II). 



Pushing the analogy with respiration still further, one could suggest 

 that an alternation of hydrogenations and carboxylations continues until 

 a triose (e. g., glyceraldehyde) can be separated from the carrier (cf. 



