J. A. BASSHAM AND M. CALVIN 



plus about 30 to 40 kcal. be about the same as the bond energies 

 of broken bonds, if the reaction is to provide a means of efficient 

 energy conversion. The products of the reaction should not be 

 able directly to recombine easily in such a way as to produce 

 water again (back reaction) but should be able to react separately 

 to produce ultimately oxygen and reducing power. Finally, 

 the quantum converter molecule should be able to return to its 

 orginal state, after having transferred its electrons (reducing 

 power) and oxygen to other molecules. 



In addition to the requirements imposed by the mechanism, 

 there is the necessity that the quatum converter be present in 

 sufficiently high concentration in the chloroplast to account for 

 the observed rates of quantum conversion, both in steady-state 

 photosynthesis and in flashing-light experiments. This require- 

 ment of concentration will depend, of course, on the time 

 required for the quantum converter to undergo one cycle, from 

 acceptance of the quantum of electronic energy and to return to 

 its original state. 



Thioctic acid (lipoic acid) has been proposed as a com- 

 pound which might satisfy all the above requirements (3,16). 

 It was suggested that following the absorption of a quantum 

 by chlorophyll, this energy is transferred to thioctic acid, 

 causing the S — S bond of the latter molecule to break to 

 give a diradical. It was postulated that this diradical then 

 reacts with water, forming a sulfhydryl and a sulfhydroxyl 

 group. Dismutation of this reaction product results in a dithiol 

 molecule and a disulfenic acid. The dithiol would then reduce 

 DPN or TPN and would itself be reoxidized to thioctic acid, 

 while the disulfenic acid would undergo a series of reactions 

 resulting in the reformation of thioctic acid and the liberation of 

 oxygen. All of these reactions from the dismutation onwards 

 would probably involve catalysis by metalloproteins. 



The lipophilic properties of thioctic acid and its small 

 molecular size would permit close association with the chloro- 

 phyll aggregate and might account for the apparent lipid 

 requirement of the Hill reaction. The formation of the diradical 



48 



