110 



James Franck and J. L, Rosenberg 



imderlying this possibility is not so much an ad hoc hypothesis 

 as it might seem. It is not unplausible to assTune that in an 

 excited Chl-Cyt complex more charge vould tend to be transferred 

 from the Chi to the Cyt for triplet excitation than for singlet 

 under the influence of the mutual repulsion of two electrons 

 with parallel spins. This explanation provides an interpretation 

 of the Emerson effect, the enhancement by short-wave irradiation^ 

 and the 7OO bleaching. 



Finally, we have to discuss the possible role of the 7OO m\x 

 levels as a sink for shorter wave-length excitation. In our 

 model, singlet excitation of amorphous chlorophyll molecules 

 within a crystal -containing unit may be degraded to the 7OO level 

 during its passage through the crystal on the way to the reaction 

 center. The converse is not true, however. Singlet excitation 

 of exposed chlorophylls migrating through the super-unit might 

 sensitize a crystalline moleciile bordering on some reaction 

 center, but would do so by preferentially sensitizing the 

 stronger 68O level rather than the 7OO, with which the resonance 

 overlap is poorer. As discussed previously, 68O excitation near 

 the reaction center can be used with high probability for 

 recreating singlet excitation at the exposed site. Thus, an 

 overall degradation of singlet to triplet excitation at the 

 super-unit level can occur with only a small probability. 



Red and Blue -Green Algae 



These algae can be incorporated into our general scheme if we 

 postulate the following pict-ure of the photosynthetic apparatus. 

 Most of the chlorophyll is contained in crystal-containing units, 

 so that even 67O m^ absorption has the effect of sending mostly 

 triplet excitation to the reaction centers . The phycobilins are 

 contained in units devoid of chlorophyll, but these units still 

 have chlorophyll a as the exposed pigment at the reaction center. 

 These phycobilinous units can provide interaction with the 

 chlorophyllous units by way of super-unit energy migration in 

 such a manner as to divide the photochemical chores in part 

 between the two kinds of unit. Absorption in the chlorophyll 

 units will be used predominantly for Step (l ) while absorption 

 in the phycobilin units will be used more than half the time in 

 sensitizing Step (2). 



