CAN CARBON DIOXIDE SERVE AS OXIDANT IN HILL REACTION? 1533 



Table 35. VI 



Temperature Effect on CO2 Reduction and Methylene Bltie Reduction by 

 Chloroplasts on Filter Paper Strips (after Boichenko 1947, 1948) 



Temperature, A(H2 + CDs), Relative rate of MB 



° C mm.' reduction 



20 125 15 



25 175 16 



30 250 16 



35 363 17 



A very peculiar observation was made: when the reaction was continued long 

 enough, it reversed itself, and both hydrogen and carbon dioxide were rapidly liberated 

 again! This occurred very early at temperatures above 35° C; consequently at 35° 

 (and even at .30° C.) no complete consumption of hydrogen or carbon dioxide could be 

 achieved. At 25° C, only hydrogen came out again, and at <25° C, no reversal of the 

 reaction took place at all. Boichenko associated these peculiar phenomena with the 

 thermodynamic reversilMlity of the reaction H2 + CO2 <=* HCOOH, but it is thermody- 

 namically impossible for a reaction to proceed first in the one and then in the other di- 

 rection. 



That formic acid actually was formed in these experiments was confirmed by chemi- 

 cal tests (reduction of AgNOs and HgCl2). The reducing power resided in the film, and 

 not in the solution; the reductant (formic acid?) thus must have been present in "bound 

 form." The amount of HgCl2 reduced agreed with the (manometrically determined) 

 amount of hydrogen taken up by the chloroplast film. 



Boichenko saw in these experiments an imitation, with isolated chloroplasts, of the 

 first step in the reduction of carbon dioxide in photosynthesis, but both the reliability 

 of the experiments and their interpretation are doubtful. 



Vinogradov, Boichenko and Baranov (1951) applied C^^ tracer technique to the 

 products of carbon dioxide fixation by Boichenko's chloroplast preparations in the dark, 

 with H2 as reductant, and found the tagged products entirely extractable by hot water,. 

 with up to 90% of it precipitable by barium chloride; about 75% of the precipitate con 

 sisted of uronic acids, the rest were highly carboxylated nonreducing acids. 



Boichenko and Baranov (1953) studied C" uptake by the same preparations in light 

 under anaerobic conditions. In the presence of hydrogen the uptake was 0.01 relative 

 units in the dark anil 0.032 relative units in light (as against 0.067 and 0.015 relative 

 units in the presence of 1.25% oxygen). In pure nitrogen the uptake was negligible 

 (0.0003 relative units in light, 0.004 in dark). The authors saw in these results the indi- 

 cation that the dark "chemosynthetic" fixation of CO2, which requires oxygen, is re- 

 placed in light by "photoreduction," similar to bacterial photosynthesis, which occurs 

 only under anaerobic conditions. Only a fraction of C*02 fixed in light was found in 

 carboxylic groups — an observation which was considered evidence that carbon dioxide 

 was actually reduced (and not merely taken up in a carboxyl). 



Franck (1945) noted a stimulating effect of carbon dioxide on oxygen 

 liberation by chloroplasts in the absence of added oxidants (i. e., on the 

 Friedel-Molisch phenomenon), and thought at first that this effect indi- 

 cated the capacity of chloroplasts to use carbon dioxide as oxidant, albeit 

 with a very low efficiency. Subsequent experiments with C(14) by Franck 

 and Brown (1947) caused him to abandon this interpretation. 



