MECHANISM OF REDUCTION OF CO2 241 



Ruben 1943, Gardner 1943), as illustrated by reaction sequence (9.17): 



(9.17a) ECHO + CO2 > RCO • COOH 



(9. 17b) RCO • COOH + 2 HX > RCHOH • COOH + 2 X 



(9. 17c) RCHOH • COOH + 2 HX > RCHOH • CHO + 2 X + H2O 



(9. 17d) RCHOH • CHO + CO2 > RCHOH • CO • COOH 



(9. 17e) RCHOH • CO • COOH + 2 HX > RCHOH • CHOH • COOH + 2 X 



(9. 17f ) RCHOH • CHOH • COOH + 2 HX > RCHOH • CHOH • CHO + 



2 X + H2O 



(9. 17j ) RCHOH • CHOH • CHOH • CHO > RCHO + CH2OH • CHOH • CHO 



Scheme (9.17) includes three carboxylations, (9.17a, 9.17d, . . .), three 

 reductions of carboxyl groups to carboxyl groups (9.17c, 9.17f, . . .), 

 and three reductions of carboxyl groups to RHOH groups (9.17b, 9.17d, 

 . . .). Ruben (1943) suggested that the reactions of the first and 

 second type are assisted by transphosphorylations, while the thermo- 

 dynamically less difficult reactions of the third type do not require this 

 help. It is clear from the preceding discussion, that scheme (9.17) has 

 no basis except the postulated similarity between the mechanisms of 

 photosynthesis and respiration. 



10. Experimental Evidence Regarding the Mechanism of Reduction 



of Carbon Dioxide 



In respiration and fermentation, speculations of the type presented 

 above, are corroborated by experimental evidence — isolation of inter- 

 mediates, substitution tests, reactions of isolated enzymes and so forth. 



All this is lacking in photosynthesis. 



Attempts to identify the intermediates in photosynthesis by ordinary 

 analytical methods will be dealt with in chapter 10. They have produced 

 numerous data (some of which may yet prove to be related to photo- 

 synthesis), and have led to prolonged controversies (for instance, con- 

 cerning the occurrence of formaldehyde in plants), but did not yet reveal 

 a single chemical compound clearly associated with photosynthesis, either 

 as an intermediate or as a catalyst. 



Recently, some information concerning the nature of intermediates 

 in photosynthesis was obtained from experiments with radioactive carbon. 

 As stated in chapter 8, Ruben and coworkers found that, in the dark, 

 carbon dioxide is incorporated into a large molecule (containing about 

 100 atoms), probably forming a carboxyl group. Similar experiments 

 were also carried out by Ruben and coworkers with illuminated plants. 

 Ruben, Hassid, and Kamen (1939) illuminated barley leaves for 1^70 

 minutes in the presence of C*02. The leaves were then extracted with 

 warm water and analyzed as quickly as possible. Only about 25% of 

 assimilated C* was found in carbohydrates (which does not agree well 



