MELVIN CALVIN 329 



CliiioKoi'iiM.L Photosensitized Transformations 



Hydrogen Transfer Reactions Catalyzed by Chlorophyll 



Let us return to the sensitized hydrogen transfer reaction, which is 

 what one of the overall reactions of photosynthesis is presumed to 

 be (hydrogen transfer from water to pyridine nucleotide) , and de- 

 termine which ones may be catalyzed by chlorophyll. The classic ex- 

 amples are the hydrogen transfers from materials such as ascorbic 

 acid and hydrazine to dyestuff-acceptors such as azo dyes (methyl 

 red). These reactions have been known for some time, and in the 

 last 15 years they have been studied a great deal, particularly in 

 the Soviet Union. One such reaction is called the Krasnovskii re- 

 action after the man who has spent a great deal of time studying it 



(42, 43, 44, 46, 47) . Krasnovskii used chlorophyll and porphyrin 

 model substances as sensitizers to transfer hydrogen from a variety 

 of donors (ascorbic acid, particularly) to methyl red and other azo 

 dyes. He did it under such conditions that he was able to show 

 two steps as separate events, that is, the transfer of hydrogen from 

 the hydrogen donor to chlorojDhyll to give some intermediate, fol- 

 lowed by the transfer of hydrogen from this intermediate to the hy- 

 drogen acceptor, giving back again the initial chlorophyll. By cool- 

 ing the reaction mixture, and performing the experiment in a basic 

 solvent such as pyridine, Krasnovskii was able to show that chloro- 

 phyll plus ascorbic acid, without the addition of a hydrogen ac- 

 ceptor, would go from a green color to a pink color. This pink color 

 was presumed to be some intermediate, not necessarily bacteriochloro- 

 phyll, since the spectrum did not correspond. The reaction re- 

 verses in the dark, and the "pink" intermediate is not a radical (49) . 

 The general result of all of these studies is shown in Fig. 8 (27, 42, 

 43, 44, 46, 47) , in which the whole series of transformations is sys- 

 tematized. The chlorophyll absorbs the light, the excited chloro- 

 phyll (probably in a triplet state, as the kinetics indicate that such 

 is the case) removes either a hydrogen atom, or an electron from 

 the donor (^Ho) to give what Krasnovskii believes to be a radical, 

 or a radical ion (Ch-) , (at low temperatures, he believes he has 

 caught this radical ion (45) ) , leaving behind a positive radical 



(^H2+) -^vhich then dissociates to give a proton. The free radical 

 or ion (AH or AH.^-^) can then go ahead and reduce another chloro- 

 phyll, and the free radical ion of chlorophyll (Ch-) can hand on 

 the hydrogen atom, or electron (or both) to the dyestuff (Dy) to give 

 back the chlorophyll starting material and the partly reduced semi- 



