CONCENTRATION OF REDUCTANTS 



945 



curve" has the same typical "saturation shape" as the curves representing 

 the rate of photosynthesis as function of carbon dioxide concentration; 

 half saturation is reached at ca. 20 mm. (2.5%) H2, full saturation above 

 75 mm. (10%). Wassink et al. (1942) found, with Chromatium in nitrogen 

 containing 5% CO2, no signs of hydrogen saturation up to about 2% (cf. fig. 

 27. 11 A); single experiments at higher pressures indicated that saturation 

 was reached probably at about 10%, certainly below 15%. (The rates at 

 15% and 30% H2 were identical.) These hydrogen concentrations are 

 approximately 100 times higher than those required for saturation of photo- 

 synthesis with carbon dioxide (about 0.03% CO2 is needed for half satura- 

 tion). In analogy with the interpretation of carbon dioxide saturation. 



E 

 E 



O 



o 



Ll. 

 O 

 UJ 



< 



I- 

 0. 



3 



200 



Scale of concentratjon 

 10 times reduced 



100 



200 



300 400 



AMOUNT OF Hj/VESSEL, mm^ 



Fig. 27.11. Rate of carbon dioxide reduction by Chrornatium (after Wassink 1942): 

 (A) effect of [H2]; {B) effect of [H2S]. 5% CO2 in N2; pH 6.3; 29° C; strong light. 



given in the first part of this chapter, the simplest explanation of hydrogen 

 saturation is to assume reversible formation of a hydrogen acceptor com- 

 pound, with a dissociation constant of the order of 0.02 atmosphere (as 

 compared with 3 X 10 ~^ atmosphere for the carbon dioxide acceptor com- 

 pound). Thermodynamically, reversible hydrogenation (i. e., hydrogena- 

 tion with energy close to zero) presents no difficulties, since the free energies 

 of hydrogenation of organic compounds can be either positive or negative, 

 depending on the degree of resonance stabilization of the individual com- 

 pound in the hydrogenated and the dehydrogenated form (c/. Vol. I, page 

 217). Kinetically, however, the problem is less trivial, since in vitro, no 

 example is known of organic compounds behaving like metallic palladium, 

 i. e., taking up hydrogen under high pressure and releasing it when the 

 pressure is reduced. 



Quantitatively, the results of French with Streptococcus varians and of 

 Wassink with Chromatiujn are similar enough to justify the suggestion 

 that the hydrogen acceptor is the same in both species. This common ac- 

 ceptor may be either the enzyme hydiogenasc, or the compound, desig- 



