Mode of Action of the Photocatalytic System m Organisms 613 



It is thus probable that porphyrin pigments were the primary participants in 

 the phototransfer of electrons ; subsequent evolution gave rise to the phycobihn 

 pigments transferring the energy of hght quanta to the primarily formed sensi- 

 tizing porphyrins (or possibly to biocatalytic systems). 



DEVELOPMENT OF CONJUGATION OF PIGMENT 

 AND ENZYMIC SYSTEM 



The excited pigment molecule can pass into the metastable state; in the 

 absence of chemical interaction, the excitation energy of the molecule is rapidly 

 degraded, finally to heat. If an elementary oxidation-reduction act is involved, 

 the photoproducts thus formed (a pair of ion-radicals) can also undergo a back- 

 reaction, with wastage of energy. It is important that the active photoproducts 

 can be utilized in the metaboHc system before they become deprived of their 

 energy in one way or another. It is therefore not enough for the organism to be 

 endowed with a sensitizing pigment participating in the 'uphill' transfer of 

 electrons; in addition to this conjugated biochemical systems must be available 

 which utihze the active high-energy photoproducts. This is made clear by the 

 following example. The introduction of porphyrins into organisms results in a 

 photodynamic effect : it causes sensitized random oxidation of important meta- 

 boHc hydrogen donors and finally causes severe disturbance of the vital functions. 

 On the other hand, a sensitizing pigment in an organized living system with 

 co-ordinated photochemical and enzymic processes has the opposite effect of 

 maintaining normal metaboHsm (chlorophyll in the plants). 



N. M. Sisakyan [24] has demonstrated that protoplasmatic structures con- 

 taining pigments (chloroplasts) are the site of a variety of biocatalytic functions. 



Just as in the case of sensitized reactions, here too there may be mechanisms 

 of coupHng based on the transfer of an electron and the transfer of energy from 

 the excited sensitizer-pigment molecule. 



Incorporation of Sensitizing Pigments into Systems of Biochemical Electron 

 {Hydrogen) Transfer 



In the course of reversible photochemical reduction the excited pigment 

 molecule accepts an electron from the donor molecule and, in the dark act of 

 the process, transfers this electron 'uphill', with the formation of a system pos- 

 sessing a higher reduction potential. Model experiments in our laboratory re- 

 vealed that the molecules of porphyrin pigments upon illumination are capable 

 of taking up electrons (hydrogen) from biochemically important compounds 

 such as ascorbic acid, cysteine, polyphenols, and probably cytochromes. 



Even the most ancient types of organisms probably possessed sulphur- 

 containing amino acids and iron porphyrins, and it is therefore Hkely that these 

 compounds interacted with pigment systems. Light energy 'storage' is possible 

 if the photochemically mobilized electron is accepted by systems with a higher 

 negative redox potential than that of the initial electron donor (up to Eq = 

 — 0-35 V). We have demonstrated in 1948-1949 in collaboration with G. P. 

 Brin, that flavins and pyridine-nucleotides can be used as such systems [25]; 



