MELVIN CALVIN 



321 



to be the exchange of a labile proton, presumably on chlorophyll, 

 which is photocatalyzed. He believed it to be the relatively labile 

 isocyclic hydrogen which is exchangeable, since it is enolizable. Vish- 

 niac also believes to have shown a photosensitized, or photoaccelerated, 

 exchange ol some proton, on other compounds not identical with 

 chlorophyll. Perhaps some of this could possibly be the dihydro- 

 chlorophyll mentioned earlier. Some of it might also be in the form 

 of the next hydrogen carriers (see later) . It remains to be seen what 

 the exact nature of this exchange reaction is and whether it has any 

 connection with the photosynthetic process. 



Photochemical Hydrogen Transfer — Model Systems 



\Vith this background it seems worthwhile to examine some model 

 systems for photochemical hydrogen transfer. The model systems 

 chosen (long before the aforementioned exchange experiments with 

 chlorophyll were done) were those which did not have the side- 

 chains on them which made the compound considerably more labile 

 with respect to incidental transformations. Chlorophyll itself was 

 relatively difficult to obtain in completely pure form, so we under- 

 took to synthesize a model substance which would not be subject to 

 the above-mentioned difficulties and which would have only the 

 porphyrin nucleus and the dihydro- and tetrahydroporphyrin possi- 

 bilities. Such a molecule is the simple tetraphenylporphyrin whose 

 structure is shown in Fig. 2 and in which the four phenyl groups are 

 on the bridging carbon atom and which contains a simple porphyrin 

 nucleus. This material is relatively easy to synthesize. It is made 

 simply by heating benzaldehyde with pyrrole in the presence of zinc 



H (t) H 



Zn TETRAPHENYLPORPHIN Zn TETRAPHENYLCHLORIN 



Fig. 2. Structural formula of zinc tetraphenylporphin and zinc tetraphenylchlorin. 



