Mode of Action of the Photocatalytic System in Organisms 609 



phyll as the 'main' pigment. The high concentration of chlorophyll in the 

 granules of the plasties of present-day organisms ensures sufficiently complete 

 absorption in the visible region of the solar spectrum. 



The question arises as to what accounts for the widespread occurrence of 

 magnesium complexes (chlorophyll), considering that metal-free porphyrins 

 also have a photosensitizing action ? Evidently we must take account of the 

 following circumstances. The presence of a central magnesium atom possessing 

 two co-ordination 'vacancies' enables the pigment to bind polar molecules, 

 including molecules of water [lo], and possibly accounts for the Hntdng of the 

 pigment with basic groups (histidine?) of a specific protein [ii]. Apart from 

 this, the magnesiimi atom influences the photochemical properties of the entire 

 molecule, determining the higher activity of the pigment. Comparative investi- 

 gation of the photochemical properties of chlorophyll and phaeophytin has 

 shown that the photoreduced form of chlorophyll is more active than the cor- 

 responding form of phaeophytin [12]. 



The most ancient forms of photosynthetic organisms possess not only por- 

 phyrin pigments, but also a set of carotenoids and phycobilins. Degradation of 

 porphyrins results in the formation of bile pigments ; these, on combining with 

 proteins, could have given rise to fluorescent phycobilins. 



The route of carotenoid synthesis is quite different. The question as to 

 whether the primary organisms could have possessed this type of pigment is 

 still rather obscure. The study of the mechanism of its action seems to indicate 

 that the porphyrins were an older type of sensitizers, moreover capable of photo- 

 chemical electron transfer. In animal organisms, on the other hand, compounds 

 of carotenoids with proteins became the prevailing photoreceptors. 



MECHANISM OF PHOTOCATALYTIC ACTION 



The hght absorbed by pigments in photosynthetic organisms is used for an 

 oxidation-reduction process opposing the thermodynamic potential gradient 

 with 'storage' of energy in the reaction products. Studies from our laboratory 

 (see [13]) revealed the mechanism of such processes in model systems; the 

 photochemistry of pigments of photosynthetic organisms was studied in these 

 experiments in its phylogenetic and comparative chemical aspects. Chlorophylls 

 a and b, bacteriochlorophyll, protochlorophyll, porphyrins, phycobilins, phae- 

 ophytins, etc., as well as the synthetic analogues of these pigments, phthalo- 

 cyanins, were studied. 



Electron Transfer 



When iron is replaced by magnesium, the porphyrins lose the ability to 

 catalyse a dark electron transfer, but, upon illumination, acquire photocatalytic 

 activity — the capacity for reversible photochemical acceptance of hydrogen 

 (electrons) from donor molecules. This ability of chlorophyll and its analogues 

 to undergo a reaction of reversible photoreduction was discovered by the author 

 of the present paper in 1948 [13-14]. 



It is an essential feature of this reaction that part of the light quantum energy 



39 



