REVERSIBLE PHOTOREDUCTION OF CHLOROPHYLL 1505 



slower than the corresponding Mg-free compounds (but were photautoxi- 

 dized faster than the latter in methanolic solution). 



Krasnovsky and Voynovskaya (1952) found that chlorophyll can be re- 

 duced photochemically in pyridine also by sodium sulfide, but the reduction 

 did not proceed beyond 25% (measured by the decline in the intensity of 

 the red band). 



Krasnovsky and Voynovskaya (1949) found that reversible reduction 

 can be obtained also with protochlorophijll (from pumpkin seeds). A re- 

 versible chemical reduction of this compound was described (c/. chapter 

 37B) by Godnev and Kalishevich, who used Timiriazev's reagent (zinc + 

 organic acid in pyridine). (For incomplete reversibility of this reaction, 

 see chap. 18, p. 457, and chap. 37B, p. 1779.) Photochemical reduction (to 

 a brown solution, with an absorption band at 470 m/x) was now obtained 

 by illumination in the presence of ascorbic acid, at 8° C. The brown prod- 

 uct was reoxidized after several hours in air, with (approximate) restora- 

 tion of the original spectrum. If the illumination was prolonged (30 min.) 

 and no red filter was used, a band at G75 mju appeared after reoxidation, 

 indicating partial conversion of the porphyrin, protochlorophyll, to a 

 chlorin (chlorophyll ?) ; with chlorophyll a, no conversion to a bacterio- 

 chlorin was noted (which would have produced a band at 780 m/i). 



Krasnovsky and Voynovskaya (1951) made similar reduction studies 

 with bacteriochlorophyll. As with chlorophyll, the reaction goes best in 

 pyridine, but was clearly observable also in alcohol. The reaction is rapidly 

 reversed in the dark, even without air. Bacteriopheophytin is reduced 

 faster and further than bacteriochlorophyll. The unstable reduction 

 product of bacteriochlorophyll is green, with an absorption band at 640 

 m^. No photoreduction w^as noted when malic, succinic, citric, or pyruvic 

 acid, thiosinamine, or sodium thiosulfate was added instead of ascorbate. 



The possibility of reversible hydrogenation of bacteriochlorophyll indi- 

 cates that the photoreduction of chlorophyll (or protochlorophyll) does not 

 occur in an isolated double bond in ring II (or II and IV); Krasnovsky 

 suggested that in both pigments hydrogenation disrupts one of the con- 

 jugated double bonds, creating a free radical. 



Krasnovsky and Gavrilova (1951) compared the photoreduction of 

 chlorophyll (a + h) by ascorbic acid and other organic acids in different 

 solvents. No significant reaction occurred in acetone or ethanol; in pyri- 

 dine, ascorbic acid alone gave rapid photoreduction (as described before). 

 In dioxane, too, ascorbic acid was the only one of the tested compounds to 

 react with chlorophyll, but in this solvent the reaction was irreversible; it 

 occurred even in the dark, but was accelerated by light ; the product showed 

 no characteristic absorption band at 525 m^i. The reaction did not occur 



