Mode of Action of the Photocatalytic System in Organisms 611 



bonds with atoms of hydrogen, magnesium or zinc in the centre of the molecule. 

 Iron complexes display little activity in reactions of this type. Detailed investi- 

 gation of the mechanism of the reaction, carried out in our laboratory, revealed 

 that the primary photo-act is an electron transfer from the donor molecule to the 

 pigment molecule with the formation of a pair of ion radicals : • X • + AH -> 

 • X- + • AH+. According to A. N. Terenin [15], it is most probable that the 

 pigment molecule reacts in the biradical state. The properties of the primarily 

 arising photoreduced form of chlorophyll and phaeophytin were extensively 

 studied by V. B. Evstigneev & B. A. Gavrilova [12]. 



The subsequent uptake of a proton • X~ + ' AH+ -^ • XH + • A results in 

 formation of more stable reduced types, the absorption spectra of which are 

 shown for different pigments, in Fig. i. 



Investigations carried out in our laboratory have demonstrated that the 

 sensitizing action of chlorophyll and its analogues is based on the reaction of 



-B + BH2 



hv 

 Fig. 2. — Electron transfer i Proton transfer. 



reversible photoreduction; the reduced form of pigment arising in the photo- 

 act reacts in the dark stage of the reaction with the ultimate electron acceptor. 

 (See [16 and 13].) The reversible transformations of the pigment molecule 

 results in an electron transfer from the donor molecule to the acceptor molecule, 

 with eventual regeneration of the molecule of the sensitizing pigment, as shown 

 in simplified form in Fig. 2. 



Work done in our laboratory has revealed that ascorbic acid, dihydroxymaleic 

 acid, cysteine, hydrogen sulphide, hydroquinone, pyrocatechol and probably 

 cytochrome (Fe++) can act as hydrogen donors in reactions of this type; oxygen, 

 quinones, cytochrome (Fe+++), riboflavin, pyridine-nucleotides, and several 

 dyes (thionine, safranine, phenol-indophenol, etc. [13]) can be used as acceptors 

 of the electron (hydrogen) from the photo-reduced form of chlorophyll. 



Porphyrin pigments are thus capable of transferring an electron from the 

 donor molecule to the acceptor molecule either with energy storage (+ AF), 

 or thermodynamically 'downhill' (— AF). In those cases where the final acceptor 

 is atmospheric oxygen, the process results in sensitized oxidation of the hydrogen 

 donor. 



