608 A. A. KRASNOVSKIÏ 



functions necessary between biocatalysts (enzymes) and photosensitizers 

 (pigments). 



DEVELOPMENT OF THE PIGMENT SYSTEM 



Unlike the biocatalysts, which act in very small amounts, pigments must be 

 present in autotrophic organisms in much greater quantities so as to ensure 

 sufficiently complete absorption of solar radiation by the organism, and the 

 storage of light energy in the form of potential chemical energy of the reaction 

 products. 



A small amount of sensitizing pigment, comparable to the amount of enzymes, 

 would suffice only if the sensitizing pigment initiated a thermodjuamically 

 spontaneous chain process, with the multiple utilization of light quantum energy 

 to overcome potential barriers; however, we know of no types of metabolism 

 utilizing chain processes: in all known types of metabohsm multistage con- 

 jugated reactions are predominant. 



The possible absorption of small quantities of porphyrins of abiogenic origin 

 would not ensure any appreciable absorption of Ught by the organism. For the 

 transition to autotrophy the organism would have to be able to synthesize pig- 

 ments on a big scale. The vigorous biosynthesis of porphyrins, in turn, calls 

 for an elaborate system of metabolism. These considerations are in accord with 

 A. I. Oparin's ideas about photoautotrophy having arisen on the basis of a fully 

 developed heterotrophic system of metabolism [i]. 



The primary photosensitizers could have been porphyrins formed in the une 

 of biosynthesis of the iron-porphyrin biocatalysts. For instance, the most ancient 

 type of photoautotrophs, the photosynthcsizing bacteria, have been found to 

 contain considerable amounts of free porphyrins [8] in addition to bacterio- 

 chlorophyll and iron porphyrins [7]. 



However, the simple porphyrins have an absorption spectrvmi which does not 

 ensure sufficiently full utilization of the entire visible region of solar radiation. 

 The long- wave absorption maximum of the porphyrins is at about 620 m/n ; the 

 value of the molar extinction coefficient (K,„) in this maximum, which determines 

 the 'dyeing powers', i.e., the probability of absorption of hght quanta, is low as 

 compared with the Km in the Soret band, situated at about 400 m/i. In the 

 reducing medium of the ancient biosphere still devoid of free oxygen, there 

 existed primordial photosynthetic bacteria. It is not surprising, therefore, that 

 these organisms contain the most reduced form of porphyrin pigment, bac- 

 teriochlorophyll, with both the 'semi-isolated' double bonds hydrogenated. The 

 absorption maximum of the bacteriochlorophyll thus formed hes in the near 

 infrared region of the spectrum. 



The possibility of such photoreduction of porphyrin is demonstrated by our 

 model experiments in collaboration with K. K. Voïnovskaya. It has been found 

 that the porphyrins of bacteria [8] and haematoporphyrin [9], upon illumination 

 without air in the presence of reducing agents, are indeed capable of forming 

 photoproducts possessing the absorption spectrum of bacteriochlorin (with a 

 maximum at about 750 m/t). 



The subsequent evolution of pigments took the course of formation of chloro- 



