ENERGY MIGRATION AND THE PHOTOSYNTHETIC UNIT 1285 



conclusion should also be applicable to depolarization, which occurs at con- 

 centrations ten or a hundred times lower than that required for quenching. 

 (It seems unlikely that complete dissipation of excitation energy, implied in 

 quenching, should occur by remote interaction while the much "gentler" 

 depolarizing interaction required direct encounters.) 



The concentration depolarization and concentration quenching of dyestuff solutions 

 were again studied experimentally by Sevchenko (1944), and found to be in agreement 

 with the predictions of the theory of "molecular induction" (which is another term for 

 resonance exchange). Pekerman (1947) found that, when dyestuff solutions are taken 

 up by sintered glass, whose pore diameters are such as to cause dye molecules to form 

 one-dimensional trains, self-depolarization and self-quenching are decreased— probably 

 because, under these conditions, resonance transfer is possible in one direction only, and 

 is therefore less effective than in bulk solution. 



According to this concept, concentration depolarization indicates (a) 

 that a considerable proportion of the fluorescent light is emitted, not by the 

 primary excited pigment molecules, but by molecules to which the excita- 

 tion energy has been transferred between excitation and emission, and (6) 

 that this transfer takes place without actual encounters of the emitting 

 molecules with the primarily excited molecules. 



(It may be useful to point out that the assumed mechanism of energy 

 exchange is different from absorption and secondary re-emission of fluores- 

 cence, a phenomenon that also is possible in concentrated solutions. Es- 

 timates indicate that, because of the displacement of the fluorescence 

 band toward the longer waves compared with the absorption band, the 

 probability of "secondary fluorescence" of this type is much too small to 

 account for the observed depolarization.) 



If the energy transfer does not require (or prefer) parallel orientation of 

 the molecules (an admittedly extreme assumption), the "self -sensitized" 

 fluorescence will be completely unpolarized. In this case, the decrease of 

 polarization with increased concentration will provide a direct measure of 

 the average number of energy transfers during the excitation period. 

 (Equal distribution of the probability of re-emission over the primarily 

 excited and one secondarily excited molecule will produce 50% relative de- 

 polarization; distribution over three molecules, 67% depolarization, and 

 so on.) In a IQ-^ M solution of fluorescein in glycerol, the polarization is 

 only 20% of that in dilute solution. If remote energy transfer is the only 

 mechanism of depolarization, this figure indicates the distribution of the 

 excitation probability over five pigment molecules. 



At the same concentration, the fluorescence yield is reduced by about 

 one half compared with dilute solutions. If this "self-quenching" effect 

 also is ascribed to remote energy transfer, the two phenomena together indi- 

 cate an energy exchange over an average of ten molecules. Perrin sug- 



