354 DANIEL I. ARNON 



effective electron carrier in reactions involving cytochromes. For example, 

 Massey [90] has found that cytochrome c is rapidly reduced in a non- 

 enzymic reaction with reduced phenazine methosulphate. In Fig. 5, the 

 intermediary electron carrier is vitamin K or FMN. 



The cyclic electron flow diagrams, illustrated by Figs. 4 and 5, are 

 components of the scheme presented earlier [94]. The key reaction in the 

 proposed mechanism, the photo-oxidation of chlorophyll by the loss of an 

 electron, is based on a type of reaction in photochemistry that was experi- 

 mentally documented by Lew'is and Lipkin [96]. They found, by 

 illuminating a variety of substances in rigid media, "that one of the 

 commonest photochemical processes is the mere loss of an electron by an 

 activated molecule" [96]. The evidence for the then (in 1942) "new and 





■^ Cof actor 



'~P 



ADP 



Chi '. V < ^>^ Cyt 



LIGHT ~P— ADP — Katp, 



Anaerobic cyclic photophosphorylation 



Fig. 5. Scheme for anaerobic cyclic photophosphorylation catalyzed by vitamin 

 K, or FMN. Details in the text. 



somewhat surprising phenomenon" [96] was, for example, "that chemical 

 oxidation at room temperature and photo-oxidation at liquid air tempera- 

 ture (of tri-/)-tolylamine) have given the same substance, namely, the 

 positive ion left, (/>-CH3C6H4)3N +, when one electron has been 

 removed" [96]. 



In the reactions studied by Lewis and Lipkin the fate of the ejected 

 electron was uncertain but, as they pointed out, "the electron must lie in 

 a potential hole which is deep enough so that the large electrostatic field 

 of the ion is unable to dislodge it" for considerable periods of time. This 

 was indicated by the fact that "the (blue) color ( of the 'odd ion' formed) 

 persists at liquid air temperature for several days, but at only slightly 

 higher temperatures the color disappears. Then presumably the electron 



