536 



LIGHT AND LIFE 



I 



-J 



0.2 



0.1 



0.0 



0.1 



■0.2 



400 450 500 550 600 

 K(mfi) 



Fig. 16. Reduction of Chroinatium cytochromes by succinate in a cell-free sys- 

 tem. Reaction mixture included, in a final volume of 3.0 ml of 0.2 Af Tris buffer, 

 pH 7.8, chromatophores (P) containing 0.06 mg bacteriochlorophyll and super- 

 natant fluid (S) corresponding to 0.3 mg bacteriochlorophyll. 10 /i,moles of suc- 

 cinate were added to one of a pair of cuvettes, and the resulting difference spec- 

 trum was measured in a Gary spectrophotometer at the indicated time intervals 

 (Nozaki, Ogata, and Arnon, 114). 



evolving hydrogen ga,s, is represented by Fig. 17. HA represents an 

 inorganic or organic electron donor which gives low-energy electrons 

 to the cytochrome-chlorophyll complex, where with, the aid of ab- 

 sorbed light, they are raised to the energy level of electrons in molecu- 

 lar hydrogen. 



With succinate as the electron donor, the proposed sequence of 

 reactions for the non-cyclic electron flow which results in the photo- 

 production of hydrogen (Fig. 17) , is represented as follows: 



Succinate + 2Fe3 + cyt -^ fumarate + 2Fe2 + cyt + 2H + 

 2Fe2+cyt + 2[Chl]+ -» 2Chl -f 2Fe3+cyt 



2Chl 



light 



2e- + 2H 



+ 



-> 2[Chl]+ + 2e- 



hydrogenase 



H., 



Sum: succinate 



light 



■^ fmnarate -|- H2 



If the electron donor (HA) in Fig. 17 is thiosulfate, an analogous 

 sequence of reactions woidd, as is discussed in the companion paper 



