Mode of Action of the Photocatalytic System in Organisms 607 



isms, photosynthetic bacteria and the blue-green algae, possess a fully developed 

 pigment system containing chlorophylls, phycobilins and carotenoids. 



Complex compounds of porphyrins are widespread among biocatalysts and 

 pigments; the nature of the central metal atom is of great importance, deter- 

 mining many properties of the complex. 



The most abundant elements in the Earth's crust [2] are: O (49-4%), Si 

 (27-8%), Al (8-5%), Fe (5-0%), Ca (3-5%), K (2-5%), Na (2-6%), Mg (2%), and 

 Ti(o-6%). 



Porphyrins with Si, Al, or Ti in the centre of the molecule have not been 

 found in organisms. One of the reasons for this is probably the low solubiUty of 

 the compounds of these elements in the biosphere and, hence, the difficulty of 

 their utilization by organisms. 



Porphyrins containing potassium, sodium, or calcium undergo hydrolysis in 

 aqueous media, the metal atom being replaced by atoms of hydrogen [3] ; of the 

 metals listed above, there remain, therefore, only Fe and Mg, porphyrin com- 

 plexes of which are widespread in organisms. 



The iron complexes possess diverse catalytic properties and a low photo- 

 chemical activity; they do not exhibit fluorescence. Magnesium, hydrogen or 

 zinc in the centre of the molecule give rise to complexes that are inactive cata- 

 lytically, but extremely active photochemically, and exhibit bright fluorescence. 

 Is there, however, really such a sharp distinction between the catalytically and 

 photochemically active complexes ? Probably there could have been transition 

 types in the process of evolution from 'dark' to 'Light' metabolism; this leads us 

 to the question whether the activity of biocatalysts is affected by light. In other 

 words, do biocatalytically active complexes also have a photosensitizing action ? 



Many biocatalysts are coloured compounds, i.e. they absorb light in the visible 

 region of the spectrum. This applies to the porphyrin complexes, to flavins, and 

 other compounds. The amount of biocatalysts in organisms is usually small, and 

 they absorb only a small portion of the solar radiation falling upon the organism. 



There is a vast but often contradictory hterature about the effects of visible 

 light on enzyme activity. Light has a particularly pronounced effect on the trans- 

 formations of free flavins, which possess a bright fluorescence and photosensi- 

 tizing action; however, the action of light on flavin-protein complexes in organ- 

 isms requires further elucidation. 



We have recently reported on the accelerating effect of visible light on the 

 oxidation-reduction changes of cytochrome [4]. Earlier we found that light 

 exerted an influence on reactions catalysed by crystalline analogues of the por- 

 phyrins, the phthalocyanins and their copper or magnesium complexes. These 

 compounds have a catalytic effect on the oxidation of ascorbic acid and the de- 

 composition of hydrogen peroxide in solution; illumination accelerates the 

 catalytic reaction [5]. Illumination also accelerates the decomposition of hydrogen 

 peroxide and the oxidation of fatty acids catalysed by Ti02 [6]. The known 

 biocatalysts are usually (with the exception of the flavins) rather poor photo- 

 sensitizers; in a number of cases Hght can have an influence on their action, but 

 there is no energy storage in the end products of the reaction. 



The further development of photocatalytic systems made a speciahzation of 



