1300 THE PIGMENT FACTOR CHAP. 32 



(e. g., between chlorophyll a and chlorophyll h, or between carotenoids, 

 phycobilins, and chlorophyll). It was repeatedly suggested in chapter 30 

 that the (by now well established) sensitizing action of so-called "acces- 

 sory" pigments in photosynthesis may be based on the transfer of the energy 

 quanta, absorbed by these pigments, to chlorophyll — perhaps, the only 

 pigment that can (or, at least, the only one that does) act as the chemical 

 "photocatalyst" in photosynthesis. 



Theoretically, the probability of the resonance transfer of excitation 

 energy between different molecules by the "slow" resonance mechanism is 

 determined (similarly to that between identical molecules), by the over- 

 lapping of the fluorescence band of the primary light absorber (energy 

 donor) and the absorption band of the energy acceptor. In a mixture of 

 several chlorophylls this overlapping may be so wide that the probability 

 of energy transfer between them can be of the same order of magnitude as 

 for the energy transfer between two molecules of a single component. 

 Forster (1947) estimated, for example, that the probability of energy migra- 

 tion from 6 to a is about one half that from one a to another a. The transfer 

 in the opposite direction — from a to b — is, however, about 300 times less 

 probable (Duysens 1952). 



Band overlapping is considerable also between the phj^cobilins and the 

 chlorophylls, since the fluorescence of the former pigments lies in the region 

 where the latter ones absorb strongly. It is generally less extensive be- 

 tween the carotenoids and the phycobilins or chlorophylls. 



Arnold and Oppenheimer (1950) first discussed the probability of reson- 

 ance transfer of energy (a process they called "internal conversion") be- 

 tween two different photosynthetic pigments, using phycocyanin and 

 chlorophyll in blue-green algae as an example. They estimated, from crude 

 observations, that 1 or 2% of the quanta absorbed by phycocyanin in 

 Chroococcus are re-emitted as phycocyanin fluorescence. Postulating that 

 the relative probabilities of fluorescence and energy dissipation in the 

 chromoproteid should be the same in vivo as in vitro, and noticing that in the 

 pigment extract this ratio is about 1:4 (i. e., about 80% of the absorbed 

 quanta are dissipated, and 20% re-emitted), they concluded that dissipa- 

 tion can account, in vivo, for only 4-8% of the absorbed quanta. This 

 leaves 90-95% unaccounted for by either fluorescence or dissipation — and 

 thus, they postulated, transferred to chlorophyll. Arnold and Oppen- 

 heimer estimated that the transfer probability ("internal conversion coef- 

 ficient") required for a transfer yield of 90-95%, lies within the theoretical 

 limits one can calculate from the classical model of two coupled oscillators, 

 by using the smallest and the largest plausible value, respectively, of the 

 average distance between the molecules of the two pigments in the Chro- 

 ococcus cell. 



