CARBOXYLATION EQUILIBRIA 183 



dioxide acceptors than the nitrogen-hydrogen bonds? In other words, may we expect 

 the reaction: 



(8.19) 0C=0 + R=N— M > OC— NR 



OM 



where M = metal, to proceed more easily and completely than reaction (8.15a)? 



The importance of this question hes in the fact that chlorophyll contains two 

 nitrogen-magnesium bonds. The interaction of carbon dioxide with chlorophyll in 

 vitro will be discussed in chapter 16. One mole of sohd or colloidal chlorophyll ap- 

 parently can absorb up to two moles of carbon dioxide. In interpreting this uptake 

 (page 454), we shall have to consider a reaction of type (8.19) as one possibility. 



4. Carboxylation Equilibria 



The reaction which has aroused most interest in connection with the 

 primary carbon dioxide fixation in photosynthesis is carboxylation. It 

 can be interpreted as an addition of an organic compound RH to the 

 C=0 double bonds in carbon dioxide; in other words, the C — H bond 

 plays in carboxylation the same part which the N — H bond plays in 

 carbamination and the — H bond in the hydration of carbon dioxide. 



OH 

 / 



(8.20) OC =0 + RH > OC 



R 



Respiration ends with the elimination of carbon dioxide by decarboxylation 

 of certain keto acids. Since photosynthesis is the reversal of respiration, 

 one is tempted to consider the reversal of this last step in respiration as 

 a possible first step in photosynthesis (Thimann 1938). 



However, the analogy between the role of decarboxylation in respi- 

 ration and the role of a preliminary carboxylation in photosynthesis is 

 not quite so close as it may appear. In the respiratory process, decar- 

 boxylation is a step in the breakdown of the sugar molecule. Carboxyla- 

 tion would play a corresponding role in photosynthesis only if carbon 

 dioxide were added to an intermediate reduction product, and not to a 

 catalyst which must be restored at the end of the reaction. The car- 

 boxylation of chlorophyll or another temporary carrier may be useful for 

 kinetic purposes, but it does not constitute a first step in the building up 

 of a carbon chain. 



Carboxylations and decarboxylations do not change the average 

 reduction level of the reacting system and hence have only relatively 

 small heat effects (c/. page 216). Table 8. VIII shows that decarboxyla- 

 tions usually are slightly endothermal (AH > 0). If decarboxylation 

 leads to the disruption of a conjugation between the C=0 double bond 

 in the carboxyl and another C=0 double bond in the molecule (as in 



