244 REDUCTION OF CARBON DIOXIDE CHAP. 9 



Experiments of greater precision on the change in the proportion of 

 radioactive carbon present in carboxylic groups as a function of the 

 illumination time may make possible a decision between these two 

 hypotheses, and perhaps give further information as to the reduction 

 mechanism. The use of the weakly active but long-lived carbon isotope 

 C^*, instead of the highly active but short-lived C^^, should make possible 

 a less hurried and more thorough analytical work in this field, even 

 though it will require more sensitive measuring devices. 



Repeated carboxylation of the acceptor-bound substrate is not the only way in 

 which the carbon chain may grow without ever forming free intermediates of low mo- 

 lecular weight. Carrier-bound intermediates may polymerize, each remaining attached 

 to its original carrier, or may be transferred in the polymerization reaction to a common 

 carrier molecule. Schemes of this type, in which the carbon skeleton is built up by 

 the association of Unks of equal degree of reduction (instead of addition of nonreduced 

 hnks to an already reduced chain) remind one of the earher hypotheses of Maquenne 

 and Wohl. 



In the scheme of Maquenne (1923), a carbon chain was supposed to grow along a 

 chain of chlorophyll molecules held together by the residual valencies of its magnesium 

 atoms. Wohl (1937, 1940) thought that a synthesis of glucose can be achieved by the 

 reduction of six carbonic acid molecules attached to six "reduction centers" on a circle, 

 e. g., six nitrogen atoms in a protein "cyclol" pattern. (The formation of a "cyclose" 

 would be a logical outcome of such a process; it was mentioned on page 46 that Crato 

 and Kogel thought that inositol rather than glucose is the first product of photosynthesis.) 



Bibliography to Chapter 9 

 The Mechanism of Reduction of Carbon Dioxide 



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1924 Ruhland, W., Jahrb. wiss. Botan., 63, 321. 

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Friedheim, E. A. H., and Michaelis, L., /. Biol. Chem., 91, 355, 

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1938 Michaelis, L., Proc. 16 Intern. Physiol. Congr., Zurich, 1938. 

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