612 A. A. KRASNOVSKIÏ 



It should be pointed out that in recent spectroscopic investigations of living 

 organisms it proved possible to detect the appearance of intermediate photo- 

 products with an absorption maximum corresponding to the photo-reduced 

 form of chlorophyll [17]. 



Unlike biocatalysts or enzymes, the chlorophyll pigments thus became able 

 to effectuate a 'photochemical' electron transfer. This property appeared in 

 organisms already endowed with a rather elaborate metabolic system. For the 

 result of the photochemical process to be metabolically utilized it was necessary 

 that this process be incorporated into the system of enzymic reactions; in this 

 way could the wastage of light quantum energy by ineffective backreactions of 

 the active photoproducts be prevented. 



Energy Transfer 



On absorbing a quantum of light, the pigment molecule passes into an excited 

 state. A necessary condition for effective 'resonance' transfer of energy^ from an 

 excited energy-donating molecule to an energy-acceptor molecule is an 'over- 

 lapping' of the fluorescence spectrum of the donor molecule with the absorption 

 spectrum of the acceptor molecule [18-19]. 



Since the fluorescence maximum of porphyrin pigments is situated in the 

 red region of the spectrum, effective transfer of energy should be possible to a 

 compound possessing an absorption band in the far-red region; in organisms, 

 however, we know of no intermediate systems satisfying these requirements. 

 In the course of evolution of the pigment system of organisms there could 

 have arisen pigments whose fluorescence spectrum might have 'overlapped' the 

 absorption region of porphyrin pigments. These requirements are satisfied by 

 the phycobilins — phycocyanin and phycoerythrin — which have absorption peaks 

 at 620 and 560-490 m/i, respectively. The work of several investigators on blue- 

 green and red algae revealed the phenomenon of sensitized fluorescence of chloro- 

 phyll upon excitation in the region of the absorption maximum of the phyco- 

 bilins [20]. The light absorbed by these pigments is also used for photosynthesis. 

 Similar phenomena were likewise observed with respect to the carotenoids of 

 diatomaccous algae [21]. 



Energy can thus be transferred from the phycobiUns and carotenoids to chloro- 

 phyll, which is the final acceptor of energy and uses it 'chemically' by partici- 

 pating in processes of photochemical electron transfer. 



These conclusions are in accordance with the findings of our laboratory' [22] 

 concerning the photochemical properties of phycoerythrin isolated from the 

 red algae Calliihaumion. It was found that phycoerythrin, as distinct from 

 chlorophyll, has a high photochemical stability, is incapable of reversible 

 photorcduction, and does not have a photosensitizing effect in oxidation- 

 reduction reactions. 



A. N. Terenin and V. L. Ermolaev [23] discovered the phenomenon of sensi- 

 tized phosphorescence, which indicates transfer of energy from the biradical 

 state of the donor molecule, with concomitant transition of the acceptor mole- 

 cule as well into a long-lived biradical state. Such a mechanism of energy 

 transfer is also possible in photosynthctic organisms. 



