J. A. BASSHAM AND M. CALVIN 



oxidants required for the Hill reaction could be explained, since 

 they could be reduced directly by electrons supplied by the 

 photoactivation of chlorophyll rather than by some specific 

 primary reducing agent. During actual photosynthesis, a 

 specific reducing agent would undoubtedly be formed and would 

 provide a more efficient transfer of the energy available from the 

 electrons obtained from the photochemical reaction. We would 

 expect that this compound would have special properties which 

 would particularly suit it to the task of accepting electrons and, 

 in its reduced form, transferring energy to the carbon reduction 

 apparatus. It might be expected that this compound would 

 stimulate the Hill reaction owing to its special qualifications for 

 receiving electrons. For example, if thioctic acid were the com- 

 pound receiving the electrons from the chlorophyll during 

 photosynthesis, it might be expected that thioctic acid would 

 stimulate the Hill reaction under suitable conditions even though 

 not required for the Hill reaction to function. 



Such a stimulation has been observed by Bradley and 

 Calvin (9), who studied the rate of oxygen evolution in the Hill 

 reaction as a function of added thioctic acid. When quinone 

 was added as an oxidant at a concentration which produced the 

 highest rate of evolution of oxygen with quinone alone, it was 

 found that the addition of thioctic acid in a molar concentration 

 only 0.1 as great asthatof the quinone produced a 35% stimulation 

 of the initial rate. Moreover, this stimulation was observed with 

 6,8-dithiooctanoic acid (6-thioctic acid) only, whereas 5-thioctic 

 acid and 4-thioctic acid were not eff'ective. The reduced form 

 (dithiol) of thioctic acid and the more oxidized form (the sulf- 

 oxide) were also found to be ineffective. It appears that, barring 

 some enzymatic specificity which seems unlikely in the Hill reac- 

 tion, the special property of 6-thioctic acid which is responsible for 

 its activity is the ring strain of the five-membered ring which 

 facilitates breaking of the sulfur-sulfur bond. 



One advantage of considering the chlorophyll as an aggre- 

 gated system is that it permits a more reasonable mechanism for 

 the transfer of one electron at a time, with each electron re- 



52 



