PHOTOSYNTHESIS 



by breaking the strained ring has been suggested as the mechanism 

 for storing the accepted electronic energy. Estimates of the bond 

 energy for the S — S bond in simple open-chain molecules ranging 

 from 50 to 70 kcal. together with estimates for the reduction of 

 the dissociation energy of the S — S bond due to ring strain in 

 the 6,8-trimethylene disulfide ring of 10 to 25 kcal. indicate the 

 possibility that formation of the free radical could store 30 to 40 

 kcal. (3). A number of studies with model systems (6,8-trimethyl- 

 ene disulfide in u.v. light) were presented which indicated, 

 among other things, an ability of the trimethylene disulfide to 

 react with ROH (alcohol) or water under illumination to form 

 a thiol and a sulfenic acid or ester. 



+ light + ROH 

 S — S 



H OR 



Two pieces of biological evidence were off'ered to support 

 the suggestion that thioctic acid participates in the quantum 

 conversion. It had already been observed that in photosynthe- 

 sizing plants the conversion of photosynthesis intermediates to 

 Krebs cycle intermediates is inhibited during illumination, but 

 occurs rapidly in the dark. Since this conversion involves the 

 formation of acetyl CoA from pyruvic acid, a reaction which 

 requires thioctic acid as a cofactor (14,33,44,48,49), it had been 

 suggested (18) that this reaction is blocked in the light because 

 most of the thioctic acid is maintained in the reduced dithiol 

 form within the chloroplast. The reduction of thioctic acid 

 could occur, of course, as a secondary reaction resulting from the 

 formation of some other primary reducing agent in the light- 

 energy conversion. However, the favorable characteristics for 

 quantum conversion which the thioctic acid possesses together 

 with the indication that it was reduced in the light led to the 

 supposition that it might be in the primary reaction. 



When algae are allowed to take up added thioctic acid in the 

 dark for several minutes and then killed with quinone and the 



49 



