109 



James Franck and J. L. Rosenberg 



As opposed to singlet excitation, metastatle excitation 

 cannot participate in the migration at the super--unit level. As 

 a result, each Cyto^ formed during a period of irradiation with 

 700 m[x only must wait a much longer period to receive a quantim 

 of excitation for sensitizing Step (2). The Cyto^ "^H "thus 

 have a greater chance to he destroyed by a competing dark 

 reaction, such as one with AH. Our objection to this possi- 

 bility is that it has no connection with the bleaching of 

 crystalline chlorophyll, which plays such a great role in 

 observed difference spectra. 



b) Photo- oxidation of exposed chlorophyll. 



Althovigh photo-oxidation may occur with small probability 

 with any type of irradiation, there are two important differ- 

 ences when the oxidized chlorophyll is at the reaction center of 

 a crystal -containing unit. The first is that the res\alting 

 Chl+ captures an electron from a neighboring crystalline chloro- 

 phyll because the ionization potential for the latter is lower. 

 This in itself would slow the recovery reaction (5). The second 

 difference is that half of the Cyt is oxidized at the steady 

 state and is unavailable for the recovery reaction (3). This 

 follows from the non-migration of metastable energy among 

 exposed molecules and is in agreement with the observed increase 

 in oxidized cytochrome dirring long wave-length irradiation. 

 A progressive accumiAlation of units containing Chl+ within the 

 layers woiild lead to low efficiency if the trapped hole impedes 

 the flow of excitation energy outward. Our objection to this 

 possibility is that it is in contradiction to the finding by 

 Gorindjee and Rabinowitch of the long wave-length decline and 

 enhancement for the Hill reaction in the absence of oxygen^l9J. 



c) Alternative electron transfer in competition with Step (2), 

 We propose that triplet excitation can be used to sensitize 

 Reaction (h), which is a reversal of the recovery reaction (3), 



Cyt + Chi* ,^ Cyt + Chl"^ {h) 



"^ ox met 



while singlet excitation cannot be so used. In the first 

 instance, such an event would resiolt in the deto\xr of two 

 quanta from photosynthetic purposes. Secondarily, it would 

 produce much greater losses because the resultant bleached Chl 

 would transfer its oxidation to crystalline chlorophyll. As in 

 (b ) above, the trapped Chl+ within the layer would impede the 

 outward flow of excitation until it has imdergone recovery by a 

 dark recapture of an electron from the Cytred. The basic idea 



