Chapter 10 

 INTERMEDIATES IN THE REDUCTION OF CARBON DIOXIDE 



A. The Problem of Intermediates in Photosynthesis; 

 The Hypotheses of Liebig and Baeyer* 



In studying the transformation of carbon dioxide (nc = 1, L = 0) 

 into a carbohydrate (wc = 6, L = 1) it appeared natural to look for 

 intermediates among the compounds with carbon chains between nc = 1 

 and 6, and reduction levels between L = and 1; and much work has 

 been spent on this search in the past. 



It may be asked now, whether, in consideration of the probability of 

 a mechanism of photosynthesis not involving a separation of the reduction 

 substrate from a large carrier molecule during the whole reduction 

 process, a discussion of the intermediates of photosynthesis, based on 

 properties of molecules with short carbon chains, is of any use at all. 

 The answer is that speculations of this kind certainly cannot be considered 

 as important now as they once used to be, but that they are not entirely 

 useless. Some of the chemical characteristics which the future carbo- 

 hydrate molecule possesses at the different stages of its growth may be 

 essentially the same whether it is free or attached to a carrier. Experi- 

 ments with radioactive indicators (pp. 241 et seg.) indicate that separation 

 of the substrate from the carrier occurs before its conversion into a sugar 

 is completed. Finally, equilibria may exist between free and bound 

 intermediates (similar to that between free carbon dioxide and the 

 complex, {CO2}). For example, if a large molecule of a carboxyHc acid 

 is reduced by hydrogenation first to an aldehyde and then to an alcohol, 

 the corresponding small molecules— carbon dioxide, formic acid and 

 formaldehyde — may be found in the free state in consequence of the 

 equilibria: 



(10.1a) RCOOH . RH + COj 



(10.1b) RCHO + HoO . RH + HCOOH 



(10.1c) RCH2OH . RH + HCHO 



For the "old-fashioned" chemist or biochemist who had no sensitive 

 spectroscopic, potentiometric or radioactive tools with which to discover 

 fleeting intermediates, the way to identify intermediates was to prove 



* Bibliography, page 273. 



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