EUGENE RAIilNOWITCH AND GOVINDJEE 385 



studies are worth being repeated using more precise photochemical 

 methods. 



The cnh;iiucinciu ol the quantum yield in the far red by auxiliary 

 light suggests that the "inactive" form, Chi a 690, is not entirely in- 

 active, but requires a balanced co-excitation of an "active" form 

 to contribute fully to photosynthesis. It seems natural to think in 

 this connection of the possibility that the primary photochemical 

 process in photosynthesis might consist of two steps. Excited Chi a 

 690 may be able to bring about only one of these steps, while ex- 

 cited Chi a 670 may be able to sensitize both of them. More speci- 

 fically, Franck (15) suggested that the Chi a 690 excitation leads 

 practically entirely to the triplet state (because it occurs in chloro- 

 phyll associated only with lipides, in which the singlet-triplet con- 

 version is accelerated by the existence of an intermediate n,Tr state) ; 

 while the Chi a 670 excitation leads to a singlet excited state long- 

 lived enough to emit fluorescence (because it occurs in chlorophyll a 

 associated with water, in which the n,Tr state lies above the singlet 

 excited 7r,7r* state) . Why the cooperation of a chlorophyll a molecule, 

 in an excited singlet state, with a chlorophyll a molecule in the triplet 

 state is needed to bring about photosynthesis, is a separate question. 



Franck's hypothesis is, of course, not the only possible explanation 

 of the different photochemical capacities of Chi a 670 and Chi a 690 

 and of their cooperation in photosynthesis. Another alternative would 

 be, for example, to postulate a limited photochemical capacity of a 

 dimeric or polymeric form of chlorophyll a, and more versatile 

 capacities of the monomeric form. 



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