BENTLEY GLASS 859 



from a icducing ageiii (siuh as ascorbic acid) lo an oxidized substance 

 (such as an azo dye). M. Calvin, in his contribution, is concerned 

 ^\ith the conversion of electromagnetic energy into chemical energy, 

 which does not occur in such processes as these, but does occui in 

 photosynthesis itself. 



Chlorophyll is a porphyrin with an isocyclic ring and two extra 

 hydrogen atoms on one of the pyrrole rings; in other words, it is a 

 diliydroporphyrin. Bacteriochlorophyll has two additional atoms on 

 a second pyrrole ring. Protochlorophyll, which is formed in green 

 plants growing in the absence of light, lacks the two hydrogen atoms 

 and instead possesses a double bond in the pyrrole ring. When light 

 is supplied to etiolated plants, the double bond is replaced by the 

 two hydrogen atoms, that is, protochlorophyll is reduced to the ordi- 

 nary dihydro form of chlorophyll. These relations, as well as more 

 general considerations of the nature of photosynthesis, have suggested 

 to Calvin and to others that the function of chlorophyll may be to 

 serve as a hydrogen carrier between water and the ultimate reducing 

 agents in COo fixation, among which is TPNH. Early experiments 

 seemed to rule out the possibility that the shift from protochloro- 

 phyll was the level involved in photosynthesis, so attention was 

 shifted to the change between the dihydro- and tetrahydro-porphyrins. 

 The critical experiments to determine whether there is any trace of 

 the latter (bacteriochlorophyll) in green plants have not yet been per- 

 formed. 



Meanwhile, model systems for studying photocliemical hydrogen 

 transfer have been studied. Calvin's group has utilized tetraphenyl- 

 porphin for this purpose. Easy to synthesize as a zinc porphyrin, it 

 contains traces of the zinc chlorin (or dihydro) form; and the spectral 

 differences of the two porphyrins are easy to distinguish. With a 

 hydrogen acceptor such as a quinone, it is easy to demonstrate a light- 

 induced transfer of hydrogen from zinc chlorin to the acceptor; but 

 the reaction does not involve any storage of chemical energy, and the 

 light serves only to supply the activation energy which is needed. 

 However, the kinetics of the reaction do reveal that a long-lived 

 excited state of the chlorin is involved, perhaps the triplet state. The 

 reverse reaction, which would involve an increase in potential chemi- 

 cal energy, was also successfidly achieved by reducing the zinc tetra- 

 phenylporphyrin with benzoin (an ene-diol) in light, but the quantum 

 yield was very low — indeed, much less than in the opposite direction. 

 It is noteworthy that these photoinduced reductions of zinc porphyrin 

 do not stop at the chlorin (diliydroporphyrin) stage, but go on to a 



