THE CHEMISTRY OF PHOTOSYNTHESIS 149 



The significance of lipoic acid as the primary hydrogen acceptor still re- 

 mains doubtful. Lipoic acid occurs in chloroplasts in low concentrations 

 only. The chlorophyll concentration is about 1000 times that of lipoic acid, 

 so that the chlorophyll aggregate should be composed of at least 1000 chloro- 

 phyll molecules. Arnon et al. (2, 4) and Geller (25) point out that water 

 photolysis with isolated chloroplasts is not influenced by inhibitors of sulf- 

 hydryl groups (e.g., arsenite), findings which are certainly not in favor of the 

 lipoic acid hypothesis. Biswas and Sen (13) incubated cells o{ Scenedesmus and 

 Chlorella with labeled NaHCOa and various concentrations of lipoic acid. 

 They observed practically no stimulation of ClOo uptake upon illumination. 

 By contrast, with concentrations higher than 0.25 mg/ml, a marked inhibi- 

 tory effect of lipoic acid upon CG-y uptake was noted. Similar experiments 

 with simultaneous addition of quinone showed a slight increase of CO2 up- 

 take with low concentrations of lipoic acid. Paper chromatography revealed 

 that, with low concentrations of lipoic acid, intermediate products of the 

 tricarboxylic acid cycle and related amino-acids were more radioactive, and 

 sugars and sugar phosphates less radioactive than without lipoic acid. Ac- 

 cording to Calvin, if lipoic acid were reduced to its thiol form, a greater in- 

 corporation of '^C in sugars and sugar phosphates would be expected. It 

 follows from these investigations that it is very doubtful whether lipoic acid 

 is of significance in photosynthesis. Reed (48) arrives at the same conclu- 

 sion. 



S 



5^ The Significance of Vitamin K 



Wessels and Havinga (59, 60, 61) examined the Hill reaction with various 

 quinones and dyes as hydrogen acceptors. They found that the reduction 

 power of these substances is strongly dependent upon the redox potential. 

 This is not astonishing", as the redox potential of a system is a measure of its 

 reducing or oxidizing power. It was found that reduction does not occur 

 with quinone derivatives and dyes having a redox potential lower than 

 + 0.040 volt. It would therefore be questionable whether DPN+ and TPN+, 

 which have a redox potential of —0.300 volt, could serve as a Hill reagent. 



Wessels and Havinga suggest vitamin Ki instead of lipoic acid as the pri- 

 mary hydrogen acceptor, i.e., reduced vitamin Ki as the primary hydrogen 

 donor to TPN+. The redox potential of vitamin K] is about —0.030 volt, 

 whereas that of lipoic acid is about —0.300 volt. For a better understand- 

 ing of the energetic situation, the various reactions are indicated below, the 

 equations a and b of § 55 being repeated 



HoO^ 2 H+ + 2e 4- V2O. (a) 



(AG = +37500 cal) 



yS xSH 



L< I + 2H+ + 2.^ L< (b) 



(AG = +13800 cal) 



