TRACER STUDIES OF SPECIAL FORMS 1701 



tized photoxidation of model compounds, such as trimethylene disulfide, 

 but as yet the findings do not seem to bear much relation to the suggested 

 function of lipoic acid in photosynthesis. 



One argument against Calvin's hypothesis that lipoic acid is the "quan- 

 tum acceptor" in photosynthesis is as follows: If it is assumed that the S — S 

 bond in lipoic acid is looser by 20 kcal (or more) tlian the standard S— S 

 l)ond, then the reducing power of the couple (3G.17) should be correspond- 

 ingly weaker (since there is no reason why S — H bonds in the dithiol : 



-SH 

 -SH 



should not have the normal strength). Assuming approximate parallelism 

 of free energies and total energies of reduction for the different disulfides, 

 the redox potential of the couple (36.17) should be V2 X 20:23 or about 

 0.45 volt less positive than that of a similar "standard" system (such as 

 cystine/cysteine). The normal potential of the latter pair is about +0.35 

 volt (close to that of pyridine nucleotides); that of the couple (36.17) 

 should then be negative, and thus quite insufficient to reduce TPN or DPN. 



14. Tracer Studies of Special Forms of Photosynthesis 



The observation of Tobert and Zill (1954) on the COa fixation and tracer distri- 

 bution in squeezed-out material from the giant Chara and Nitella cells were described in 

 chapter 35 (p. 1536). In the same chapter we briefly reported also the findings of 

 Arnon, Bell and Whatley (1954, cf. p. 1615) of the fixation of C*02and ATP '^ formation 

 by whole chloroplasts, and the apparently competitive character of these two processes. 



Some carbon tracer measurements have been made with hydrogen adapted algae 

 Gaffron, Fager and Rosenberg (1951) "stabilized" adapted Scenedesnms cells with phthio- 

 col {cf. chapter 6) to be able to observe photoreduction in relatively strong light. They 

 noted that under these conditions, the scattering of C* over the three fraction A, B, C 

 {cf. section 4) was almost as rapid as in true photosynthesis. The tagged primary 

 products thus could be converted into various metabolites, including fats and proteins 

 under strictly anaerobic conditions, i. e., without any help of respiratory energy. Badin 

 and Calvin (1950) made similar experiments, but did not inhibit "de-adaptation"; 

 they had therefore to work at very low hght intensities and use very long exposures; 

 the same kind of tagged compounds was found after photoreduction as after prolonged 

 photosynthesis in weak light. 



Comparatively Uttle C* was found, by Badin and Calvin (1950), to be fixed in the 

 oxyhydrogen-carbon dioxide reaction of hydrogen adapted Scenedesmus {cf. chapter 6, 

 section 3); apparently none of the tracer passed into the insoluble fraction (polysaccha- 

 rides, proteins). 



Benson (1950) stated that purple bacteria {Rhodospirillum rubrum) produced a 

 greater variety of tagged compounds in 5 min. photoreduction in H2 -1- C*02 than did 

 barley leaves in equal period of photosjmthesis. Unicellular algae were found to stand 

 midway between higher plants and bacteria in respect to the complexity of tagged prod- 

 ucts. Benson also noted that Rhodospirillum produced no sucrose, but formed a poly- 

 saccharide, "probably starch." 



