186 FIXATION OF CARBON DIOXIDE CHAP. 8 



Werkman (1943) ; and good evidence of reversibility obtained. Werkman 

 and coworkers found, for example, that if nonradioactive oxalacetic acid 

 is allowed to lose by enzymatic action in an atmosphere of radioactive 

 carbon dioxide about one-half its carbon dioxide content, and the remain- 

 ing portion is analyzed for radioactivity, a measurable quantity of active 

 carbon is found in the acid (in the carboxyl adjoining the CH2 group). 

 It thus seems that, in the case of oxalacetic acid, the equilibrium lies 

 further on the side of carboxylation than it does in other organic acids. 

 It would be interesting to check this conclusion directly by the car- 

 boxylation of pyruvates. 



Baur and Namek (1940) suggested that the carboxylation equihbrium can be 

 shifted towards association not only by the formation of salts (as discussed above) but 

 also by the formation of esters: 



(8.24) 0C=0 + R'OH + R"H > R"COOR' + H2O 



Experiments, by which the occurrence of reaction (8.24) was allegedly proved, consisted 

 in determining the effect of phloroglucinol, C6H3(OH)3, and of rosohc acid (both rep- 

 resenting R"H) on the carbon dioxide absorption by glycerol (representing R'OH). 

 A certain increase in absorption was observed in the case of phloroglucinol, but no 

 glycerate of the phloroglucinol-carboxylic acid, C6H2(OH)3COOH, could be isolated. 

 The addition of 0.7 g. rosohc acid to 5 ml. of glycerol caused an increase in the carbon 

 dioxide absorption by 2.5 ml.; this, as well as certain color and fluorescence effects, was 

 interpreted as evidence of the formation of a dyestuff derivative of triphenylmethyl- 

 carboxylic acid by the carboxylation of about 5% of added rosohc acid. As in the case 

 of his work on the mechanism of photosynthesis (c/. Chapter 4), the conclusions of 

 Baur run far ahead of the very rough experiments. 



Organic compounds which are known to absorb carbon dioxide eagerly, with the 

 formation of carboxyl groups, are metal alkyls, e. g. Grignard reagents. These reactions 

 can be interpreted as additions of R — M (M = metal) to 0=0 



R 



/ 



(8.25) 0C--0 + RM > OC 



OM 



The instabihty of the carbon-metal bonds and the stabihty conferred on the salts by 

 ionic dissociation offer sufficient explanation of why, in this case, the equihbrium Ues 

 far on the side of synthesis. 



We have reviewed the reversible addition of carbon dioxide to O — H, N — H, 

 N — M, C — H and C — M bonds. Before applying these results to observations on the 

 carbon dioxide fixation by plants, it might be worth while to mention one important 

 example of reversible carbon dioxide fixation in nature — the equihbrium between carbon 

 dioxide and carbonic anhydrase. According to Roughton and coworkers (1940), the 

 equihbrium constant: 



where E = enzyme, is of the order of 0.1 atm. at 0° C, and 1 atm. at room temperature. 

 Thus, the energy of formation of the E-C02 complex is of the order of AH = — 15 kcal, 

 while the free energy is about AF = + 1.85 kcal at 25°. The chemical nature of this 

 complex is unknown. 



