326 



RADIATION BIOLOGY 



ide or to oxidized products formed in the dark period in the beginning of 

 the experiment. 



It was tempting to try to obtain evidence for the generation of a reduc- 

 ing power also from direct potential measurements. Wassink (1949) and 

 Wassink and Kuiper (in preparation) obtained a distinct potential shift to 

 the reduced side only when suspensions of Chromatium were illuminated in 

 Nz + 30 per cent Ha at a pH of about 8.0 (Fig. 5-16). Under these 

 conditions the uptake of gas that could not be suppressed by the pres- 

 ence of potassium hydroxide and thus was taken to be hydrogen was 



-300 



o 



D 



UJ 



^- 

 <t 



(T 



2o 



< o 

 <f) ir 



t- O 

 </) UJ 



Z _l 



< ^ 



UJ 



I- 

 o 

 a 



- -400 



■500 



-600 



TIME 



Fig. 5-16. The same as Fig. 15, but at pH 8.0. Heavy lines: potentials; light lines: 

 gas exchange. Curves No. 1: N2 + 30% H2; No. IC: the same, but in the presence 

 of KOH, (IC) glass electrode record in the same vessel; No. 2: N2 -f- 30% Ha -|- 5% 

 CO2. {From Wassink and Kuiper, in preparation.) 



much higher and more resistant than at a lower pH, at which initially 

 only a small uptake of gas was observed. In the beginning of the illumi- 

 nation a potential shift to the oxidized side is often observed which seems 

 due to the presence of some carbon dioxide produced in the previous 

 dark period. 



Because the combined measurements of fluorescence and photosynthe- 

 sis had shown that cyanide inhibits preferably the carbon dioxide side of 

 the reaction chain, it was tempting to try to demonstrate directly the 

 reducing effect of light under conditions of cyanide inhibition. It turned 

 out, however, that in the presence of cyanide the potential shift to the 

 reduced side (in N2 + H2) was converted into a shift to the oxidized side, 

 so that no appreciable difference existed with the curve obtained in 

 N2 + H2 + CO2. It was concluded that a hydrogenase that is especi- 

 ally susceptible to cyanide is activated by light (at pH 8.0). This theory 



