SOME PHOTOCHEMICAL CONSIDERATIONS 29 



Chi* + Oo ^ Chl,„ + HO... 

 Chl„, + .1 -^ Chi + A„, 



H02« -^ ^' 2 HoO + '/iOo 



A + 1/4O2 -^ A„, + V/'iHoO 



According to Schenck (41), a photosensitizing reaction produces a biradical 

 Chlj^ of chlorophyll 



Chi + /if -^ Chlu 



The biradical Chlj^ has a pronounced affinity to Oo 



Chlu + 0,> -> ChlH---02 



This compound reacts with an acceptor A 



ChlR---02 + A ^ Chi + .4O2 



It would lead too far to discuss the arguments put forward by Schenk to 

 support his hypothesis. They are based upon interesting investigations on 

 the photosensitized autoxidation of ce-terpinene, found in chenopodium oil, 

 and which, in the presence of chlorophyll, results in the production by photo- 

 synthesis of ascaridol. This is an example of a particular photochemical 

 action of chlorophyll characteristic of the chenopodium plants (43, 44, 45). 



The question arises whether Schenck's findings can be linked with normal 

 photosynthesis. He assumes that Oo is involved in the photolysis of water, 

 a hydroperoxide base being formed 



Chli;---02 + H2O -^ HO— ChlH---OOH 



The hydroperoxide base is dehydrogenated by a hydrogen-transferring 



enzyme or a cytochrome to produce the free radical HO — Chip O — 0». 



This compound splits off Oo, forming a new free radical HO — Chlj^*, which 

 is then split into normal chlorophyll and an HO radical 



HO— Chla* -^ Chi + HO« 



Finally, four HO radicals yield water and Oo, the intermediately formed 

 H2O2 being decomposed by catalase 



4 HO. -^ 2 H,02 ^ 2 H2O + O2 

 For the sake of clarity the reactions involved are depicted again below 



4 Chi + 4 //f/ ^ 4 Chin 



4 Chlu + 4 O2 ^ 4 ChlR O2 



4 CWr O2 + 4 H2O -^ 4 HO— Chhi OOH 



4 HO— Chlu OOH -^ 4 HO— Chin O— O. + 4 H 



4 HO— ChlR O— O. -^ 4 HO— Chhc + 4 O2 



4 HO— Chlu* -^ 4 Chi + 4 HO- 

 4 HO« -> 2 H2O2 

 2 H2O2 -^ 2 H2O + O2 



