THE CHEMISTRY OF PHOTOSYNTHESIS 145 



Later, Calvin (19) regarded lipoic acid as a hydrogen acceptor only 



/S /SH 



2[H] + L< I ^L< 



\S \SH 



The electron transfer of the process is best represented by the following re- 

 actions 



HoO ^ 2 H+ + 2 e + \/. 0-, (a) 



{AG = +37500 cal) 



/S /SH 



L/ I + 2H+ + 2e^L< (b) 



\S \SH 



(AG = +13800 cal) 



Calvin and Barltrop (20) assume that lipoic acid reacts with chlorophyll as 

 follows 



S S 



L<{\ + Chi* -^ h(^ ' + Chi 



\s \s. 



The biradical produced accepts, according to Bickel and Kooymans (12), the 

 H atoms of a hydrogen donor HD 



,^S. /SH 



L< + 2 HD -> L< + 2 D. 



\S. \SH 



2 D. ^ D> 



It is evident that another compound must serve as the oxygen acceptor, i.e., 

 the acceptor of electrons from O2. According to these views, illumination 

 creates an excited state of chlorophyll (oxidation) and the energy of this state 

 (electron energy) must be converted to chemical energy by means of the 

 acceptor. Levitt (36) postulates the formation of the oxidized chlorophyll as 

 follows : 



light 

 Chl-Mg — ^ Chl-Mg+ + e 



Thus, the acceptor molecule removes an electron from the chlorophyll so 

 that the remaining positive chlorophyll molecule can accept another electron 

 from another source. This is the process represented in Figure 52. Herisset 

 (28), comparing chlorophyll and Mg-free pheophytin, considers the Mg atom 

 should play an important role in photosynthesis. 



In discussing the oxidative decarboxylation of pyruvic acid (§ 45) we learnt 

 that lipoic acid acts as an indispensable cofactor. Lipoic acid is considered 

 to be a part of the pyruvic acid dehydrogenase, so that it is supposed that this 

 enzyme also plays an important role in photosynthesis. Calvin (18) ob- 

 served that upon illumination practically no labeled carbon is found in inter- 

 mediate products of the tricarboxylic acid cycle, in contrast to what happens 



