SOME PHOTOCHEMICAL CONSIDERATIONS 27 



evidence is lacking. Weigi and Livingston (63) established that all the 

 hydrogen-carbon bonds of the chlorophyll molecule are sufficiently stable to 

 prevent hydrogen exchange in neutral organic solvents in the dark. How- 

 ever, when cells of Chlorella or Scenedesmus are illuminated in water containing 

 3H2O, it seems that tritium is incorporated at the Cio atom of chlorophyll a. 

 These experiments were carried out by Vishniac and Rose (56), who suggested 

 that this light-dependent labeling of chlorophyll by tritium originadng from 

 water may be the first step in the hydrogen transfer reaction chain in photo- 

 synthesis; the adherents of the water photolysis hypothesis assume this takes 

 place. There is insufficient evidence to believe that this labeling of chloro- 

 phyll is related to photosynthesis and it must be very difficult to ascertain 

 that tritium is really incorporated into the chlorophyll molecule and not 

 into impurities. It is interesting to note that a distinct transfer of tritium 

 from chlorophyll a to TPN+ should occur in the light. However, why 

 should tritium not appear in TPNH, according to the reaction equations 

 discussed in § 50 and § 52 ? 



Another hypothesis is based upon tautomerism 



Chi* -^ Chi, 



Chi, being a tautomeric form. According to a further hypothesis, Chi* 

 reacts with the solvent, i.e. 



Chi* -\- A ^ Chl„, + A,,, 



or 



Chi* + A„, -^ Chi,,, + A 



Instead of the solvent A, a normal chlorophyll molecule may react with 

 Chi* and give rise to a kind of photodismutation 



Chi* + Chi -^ Chl„, + Chi,,, 



It may also be that tautomerism and photodismutation are linked together 



Chi* -^ Chi, 

 Chi, + Chi ^ Chi,, + Chi,,, 

 Chl„, + Chl„, -^ 2 Chi 



Many experiments have been made to prove that the excited chlorophyll 

 molecules are changed into long-lived forms to provide the starting point of 

 photosynthesis. Though some of the results obtained are of interest, there is 

 insufficient evidence to support this point of view. On the one hand, a 

 direct, photochemical action of chlorophyll with regeneration of the pigment 

 is assumed. On the other hand, the function of chlorophyll is thought to 

 consist in a transfer of excitation energy to appropriate acceptors, making 

 these ready to react chemically. 



Strehler and Arnold (49) found that after withdrawal of illumination green 

 plants emit for a few minutes — and even for as long as 200 minutes (48) — a 



