MECHANISM OF REDUCTION OF CO2 243 



preparations) and adsorbable on talcum, charcoal, or glass powder. 

 It is not precipitated by protein-coagulating agents (heat, trichloroacetic 

 acid), or by reagents which precipitate basic amino acids. It is colorless 

 in solution, and not extractable from water by organic solvents. 



In these rapid experiments (as contrasted with the first-mentioned 

 experiments of longer duration), less than 1% of active carbon was found 

 in sugars, and none at all in phosphorylated sugars, starch, or cellu- 

 lose. Hydrazone tests revealed the absence of active carbonyl groups, 

 while the Schotte-Baumann test with benzoyl chloride showed the pres- 

 ence of at least one alcoholic hydroxyl group. Most of the active 

 material could be precipitated (from 80% alcohoHc solution) by barium, 

 calcium, or lead ions, thus indicating the presence of a carboxyl (or 

 another acid) group. In decarboxylation experiments (heating dry 

 barium salts to 250° C), only 5% of the barium-precipitated active 

 carbon was found convertible into barium bicarbonate. (As mentioned 

 on page 204, a much higher yield in active barium bicarbonate — 30-50% 

 — was obtained in experiments with the C*02 absorption product formed 

 in the dark.) 



Ultracentrifugation revealed no essential difference between the active 

 intermediates formed in light and in the dark. For example, the sedi- 

 mentation velocity constants were 6.2, 6.1, 5.7, and 7.5 X 10"^^ in light 

 (after 4, 10, 10, and 20 min. exposure to C*02, respectively) and 

 8.6 X 10-^^ (after 20 min. exposure to C*02) in the dark; the diffusion 

 coefficients were 0.44, 0.35, 0.43, and 0.37 X 10"^ cm.^ per sec. in light 

 (10, 20, 30, and 30 min. exposure, respectively) and 0.44 X 10"^ in the 

 dark (15 min. exposure). The calculated molecular weights were from 

 1000 to 1600 for the photochemical intermediate and 1500 for the complex 

 obtained in the dark (Ruben and Kamen 1940). 



It thus appears that the active intermediates present in Chlorella 

 cells after a few minutes exposure to light are very similar to the com- 

 plexes obtained in darkness with one important difference — that most, if 

 not all, of the active carbon is no longer present in the carboxyl group. 

 It seems natural to assume that this group has been reduced in light; 

 but if so, we must account for the continuous precipitability of the 

 complex by barium salts. The explanation may be trivial — the presence 

 of a second carboxylic (or generally acidic) group not concerned in the 

 carbon dioxide transformation; but another and more interesting possi- 

 bility is that, after the first carboxyl group has been reduced, another 

 one is formed by the addition of a second molecule of carbon dioxide, 

 to be reduced in its turn, and so on — until a carbon chain of the length 

 nc has grown upon the acceptor molecule, and the proportion of radio- 

 active carbon present in the form of carboxyl groups, has declined from 

 100% to 100/nc% (c/. reaction sequence 9.17). 



