FLASH-ILLUMINATED ("HLOROPLASTS 87 



a very rapid one. This substrate, it should be empliasized, is not a very 

 diffusil)Ie substance since it is present in well-washed chloroplast 

 preparations. Perhaps it is some portion of a protein or a carotenoid 

 molecule bound closely to chlorophyll. 



2. Another possibility which seems to be equally consistent with 

 our results is that only few chlorophyll molecules, or perhaps none, 

 ever have an opportunity to undergo internal conversion into the 

 metastable state. The electronic excitation energy is transferred by 

 induction resonance to a small population of distinctly different pig- 

 ment molecules, which may be some pigment different from chloro- 

 phyll or chlorophyll molecules so situated as to be able to act as 

 efficient traps for electronic quanta. We may imagine the mechanism 

 of these trapping centers to function along lines proposed by Franck 

 and Livingston (8) for self-quenching among dye molecules. The 

 electronic energy passes undegraded to these centers, which will 

 efficiently depopulate the excited singlet state of chlorophyll be- 

 cause they are associated with a substance which will induce internal 

 conversion out of the singlet state. The latter induction reaction 

 occurs because the centers are a different kind of dye molecule with 

 fluorescence emission farther to the red than chlorophyll, or because 

 they have extreme configurational distortion. A trapping center of a 

 different type than chlorophyll a is possible providing its concentra- 

 tion is truly so low as to be undetectable in the careful studies which 

 have been made of absorption and emission spectra in plants. Franck 

 has recently proposed that these trapping centers may be chlorophyll 

 in the metastable state and we cannot exclude such an interesting 

 possibility completely (2). We feel, however, that sufficient chloro- 

 phyll molecules should have remained after the flash in our experi- 

 ments if all chlorophyll molecules have an equal chance of becoming 

 trapping centers by reason of an eciual probability of passing into the 

 metastable state. Thus it seems necessary again to suppose that only a 

 small proportion of all chlorophyll molecules are properly situated to 

 pass rapidly into the metastable state. It may not be wise to main- 

 tain a preference in this matter at the present time, but we believe 

 the most attracti\'e picture to consist of a photosynthetic unit 

 established purely by geometry about certain chlorophyll molecules 

 closely associated with subsidiary reactants of photosynthesis and 

 electronically altered by them in such a way as to enhance the rate of 

 internal conversion out of the excited singlet state into a state which 



