CHLOROPHYLL-SENSITIZED REACTIONS in VltrO 23 



rule to be reduced already present. Thus, for photoohemical reactions of chloro- 

 phyll in vitro, Dr. Franck postulates adsorption of the oxidant by chlorophyll in 

 the metastable triplet state, as shown in equation (3A), followed by excitation of 

 the chlorophyll complex to the excited triplet state tr* by sensitized fluorescence 

 and break up of the complex to form a reduced oxidant radical and a chlorophyll 

 radical, as shown in reaction sequence (3B). The chlorophyll radical is presumed 

 to lose an OH and rearrange to form a chlorophyll molecule. In photosynthesis 

 in vivo, Dr. Franck expects that the relatively long-lived triplet state chlorophyll 

 would adsorb (or become associated with) an oxidant, which would eventually be 

 reduced by the H on do, and with an enzyme, possibly a cytochrome, which would 

 carry off one of the OH's on Cg. This process would produce H-Ox- and HO- 

 Enz- radicals and leave the chlorophyll molecule in the ground state. 



One might expect that a chlorophyll molecule in the excited triplet state tr* 

 would transfer the H-atom directly to an oxidant during a collision, as shown in 

 reaction (4). The state tr* has sufficient energy to reduce quinone, ferricyanide, 

 oxalate, PGA, DPN, etc., by photochemical H-atom transfer. For the usual con- 

 centrations of the molecules, however, the yield would be very low. One has the 

 same problem with the excited triplet state tr* as he has with any higher excited 

 state, namely, if the excited molecule does not collide and react with another 

 molecule within ca. lO^^^ sec, the excitation energy will be dissipated as heat or 

 light and the yield of the photochemical reaction will become small. Dr. Franck 

 does not intend to endow the excited triplet state of chlorophyll tr* with an es- 

 pecially long Ufetime, which is what a good yield by reaction (4) would require. 

 He postulates the reaction sequence (3A) and (3B) in which a reaction complex is 

 formed by the long-lived metastable triplet state tr, thus not only obviating the 

 need for a collision with the short-Uving excited molecule tr* but even increasing 

 the probability of exciting the chlorophyll to the tr* state. 



Duysens : It seems to me that if you illuminate with weak light, you have a 

 low concentration of triplet; at higher light intensity you would have more triplet. 

 So you would think, if the transfer takes place through induced resonance, that 

 the efficiency of transfer from the excited singlet would be greater at higher 

 light intensity than at a lower light intensity and that would mean the fluo- 

 rescence yield of the chlorophyll would decrease at the high light intensity. Ex- 

 periments indicate, however, that the fluorescence yield is essentially constant. 

 I wonder how this difficulty can be resolved? 



Brugger : At each light intensity, one should come to a steady state in which 

 the ratio of molecules in the first excited state to those in the ground metastable 

 state is constant. The fluorescence yield is also roughly constant. I expect that the 

 rate of populating the excited metastable state should be proportional to the ir- 

 radiation intensity. It would seem that the percentages of the molecules in the 

 first excited singlet state which (1) fluoresce, (^) populate the ground metastable 

 state, or (3) excite the metastable state by sensitized fluorescence should be 

 roughly the same at all except the lowest light intensities. I do not feel that higher 

 intensities would populate the metastable state disproportionately heavily nor 

 that the efficiency of sensitized fluorescence would be much greater nor that fluo- 

 rescence would be markedly quenched. 



Rosenberg : May I rephrase Dr. Duysens' question to see whether I understand 



