A Theory of the Photochemical Part of 

 Photosynthesis 



JAMES FRANCK, * University of Chicago {Fels Fund) , Chicago, Illinois 



The fluorescence yield of chlorophyll a in green plants is low and, 

 very roughly speaking, constant over the range of illumination in- 

 tensities from that which compensates respiration to the one which 

 produces saturation. When secondary processes involving molecular 

 oxygen are avoided, the fluorescence yield remains unaltered well 

 into the region of saturation. This indicates that the processes 

 principally responsible for the limitation of the fluorescence yield are 

 those associated with the excitation of the chlorophyll molecules 

 rather than with the utilization of the excitation energy for 

 photochemical purposes. The transition of the excited chlorophyll 

 molecule into the metastable triplet state is the principal cause 

 of the low fluorescence yield. Since the energy of this state is 

 not sufficient to transfer one hydrogen atom from water to the usual 

 photosynthetic oxidant at a sufficiently great rate (from a donor 

 capable after dehydrogenation to initiate a series of reactions leading 

 to oxygen evolution), we conclude that the cooperation of two absorp- 

 tion acts is needed. For several reasons, we are convinced that phos- 

 phoglyceric acid itself is the primary H acceptor. The hydroxyl of the 

 water will, according to our picture, be transferred to an enzyme in- 

 volved in photosynthetic oxygen production. Since chlorophyll hy- 

 drates easily, we introduce the assumption that the oxygen on Cg 

 in the keto form of chlorophyll a will be hydrated. Then the steps 

 by which an H atom is transferred to the PGA are : 



1. Excitation of the chlorophyll molecule to its first excited singlet 

 state. 



2. Transition to the metastable triplet state. 



3. Adsorption of PGA and of the enzyme which acts as OH acceptor 

 by chlorophyll in its long-lived metastable state. This process 

 occmrs because the metastable state of chlorophyll has qualities simi- 

 lar to those of a bi-radical. 



* .\s presented by J. E. Brugj^er. 



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