320 LIGHT AND LIFE 



that is, between the stage of dihydroporphyrin and porphyrin, as a 

 transferring agent of the hydrogen from water to something else. 



Tliis was a very early notion, and the earliest experiments which 

 were devised to test it, such as doing photosynthesis in deuterated 

 water to see if the two hydrogens that were picked up on the chloro- 

 phyll macrocycle were deuterium, failed of positive results. The 

 first experiment of this kind was done by Ruben back before the 

 war (53) and a second one was done in our own laboratory (6) , 

 using tritium in the hope we could detect smaller amounts of photo- 

 sensitized exchange; this also failed to show a tritium incorporation 

 into chlorophyll very much greater than that of new synthesis of 

 the entire molecule. 



These unsuccessful results then led to the next notion, namely, that 

 the chlorophyll might be functioning not between the level of photo- 

 chlorophyll and chlorophyll (between the level of porphyrin and 

 dihydroporphyrin) but between the level of dihydroporphyrin and 

 tetrahydroporphyrin, as represented by bacteriochlorophyll. In that 

 case, any study of deuterium exchange in chlorophyll would fail. 

 If the hydrogen transfer involved first the photochemical reduction 

 of the chlorophyll (dihydroporphyrin) to tetrahydroporphyrin, and 

 if this, then, w^as transferring its hydrogen to the acceptor which 

 ultimately reduced CO2, and w^e analyzed only for the dihydro- 

 porphyrin, we would not, of course, find any isotope in the dihydro- 

 form. It would have been passed on to the ultimate reducing agent. 



This would require that in the green plant there should be traces 

 of dihydrochlorophyll (tetrahydroporphyrin) , although the steady 

 state, or equilibrium, amount of dihydrochlorophyll might be mi- 

 nutely small and hard to discover. We have yet to perform an ex- 

 periment in which we seek to find dihydrochlorophyll (or something 

 close to it) in the green plant and to determine whether or not it 

 undergoes a photosensitized isotope exchange. A similar experiment 

 might very well be done in photosynthetic bacteria in which, pre- 

 sumably, the steady state, or equilibrium, amount of tetrahydro- 

 porphyrin (or dihydrochlorin) is large and the dihydroporphyrin 

 (or chlorin) is small. This should show, if this type of transformation 

 is the way in which the reaction is proceeding, a large and easily 

 detectable photosensitized deuterium, or tritium, exchange. As far 

 as I know, this experiment has not yet been done. 



There is, however, one type of photosensitized deuterium ex- 

 change experiment which has been successful, and this is the ex- 

 periment of Vishniac (75) in which he has shown what appears 



