CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1519 



with a normal potential close to —0.8 volt, to a system with a normal po- 

 tential > 0.3 volt, perhaps with the assistance of high energy phosphate 

 produced by partial recombination of the primary oxidation and reduction 

 products (Ruben, Kok, van der Veen, Warburg and Burk). In any case, 

 the relation between the "Krasnovsky reaction" of chlorophyll in solution, 

 the "Hill reaction" of chlorophyll in chloroplast suspensions, and photo- 

 synthesis is a most interesting photochemical problem. 



Of seven fatty acids tested, in a study by Krasnovsky and Brin (1950), 

 only 10 ~^ M malic acid produced marked acceleration of reoxidation. 

 Temperature had little effect on the rate — a result taken as confirmation 

 of the hypothesis that the pink reduced form of chlorophyll is a semiqui- 

 none, and, as such, able to react wdth a very small activation energy. (If 

 this is the case, however, the low absolute rate of reoxidation becomes 

 puzzling.) 



As long as Krasnovsky's assumption of a reversible oxidation-reduction 

 of chlorophyll rests only on the observation of a (sometimes far-reaching, 

 but never complete) restoration of the extinction coefficient in the peak of 

 the red band, some doubt remains whether this assumption is correct and 

 whether the reaction does not leave an irreversible change in the chloro- 

 phyll molecule (in which case it could not serve as the basis for catalytic 

 activity). It is of some interest, therefore, that Holt (1952) in checking 

 Krasnovsky's experiments with ascorbic acid and quinone, was able to re- 

 peat the bleaching and recoloration cycle three or four times with the same 

 sample. True, the restored red band became weaker with each cycle; 

 nevertheless, the result seems incompatible with the assumption that after 

 a cycle all chlorophyll molecules that took part in it are left with a perma- 

 nent change in their structure. It seems more likely that the reaction is 

 basically reversible, but that a certain proportion of chlorophyll molecules 

 that take part in it undergo irreversible side reactions which lead to the loss 

 of absorption. It would be important to find a way to suppress these ir- 

 reversible reactions in vitro as effectively as they appear to be suppressed 

 in vivo. 



Krasnovsky and Voynovskaya (1952) compared the sensitization of oxidoreduction 

 by chlorophyll, bacteriochlorophyll, and their pheophytins (in 10~* M solutions), using 

 ascorbic acid and sodium sulfide (about 10 "^ M) as reductants, riboflavin or safranin T 

 (about 10 ~* M) as oxidant, in 85% aqueous pyridine (since sodium sulfide is more 

 soluble in aqueous solvents). No changes were observed in the dark in binary systems 

 pigment-reductant in pyridine (a slow pheophytinization occurred in alcohol in the pres- 

 ence of ascorbic acid); the pigment-oxidant binary systems and the mixtures oxidant- 

 ascorbic acid also were stable, but sodium sulfide reduced safranin T and riboflavin in 

 the dark, particularly in pyridine. Light had no effect on binary systems without the 

 sensitizer. The binary system sensitizer-reductant reacted in pyridine in light as de- 

 scribed in section 4. 



