1504 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



If the interpretation of Krasnovsky's results as proving the possibiHty 

 of a reverHihh photochemical reduction of chlorophyll is confirmed, as well as 

 the conclusion, based on observations of Rabinowitch, Porret, Weiss, 

 Livingston, and Linschitz that chlorophyll can undergo reversible photochem- 

 ical oxidation, the green plant pigment emerges as a compound capable of 

 both reversible reduction and reversible oxidation in light, a combination 

 which could be important for its function in photosynthesis, particularly if 

 the latter involves two sets of photochemical reactions — "photoxidations" 

 and "photoreductions," with chlorophyll as an intermediate (c/. chapter 7). 



It appears that reversible oxidation of chlorophyll may require the pres- 

 ence of polar molectdes (such as alcohol or water), while reversible reduction 

 requires the presence of basic molecules (such as pj^ridine). According to 

 Evstigneev and Gavrilova (1950), presence of Mg enhances the tendency for 

 photoxidation, reduces that for photoreduction. 



Krasnovsky and Brin (1949) suggested that chlorophyll attaches itself 

 to pyridine in the same way as does hemochromogen to a protein^ — by a 

 link between the metal atom and a nitrogen atom in an imidazole ring; 

 a type of complexing known to affect the redox potential of hemins. 



Krasnovsky, Brin and Voynovskaya (1949) investigated more systemati- 

 cally the influence of solvent on the photoreduction of chlorophyll a and b 

 by ascorbic acid. They found that in pyridine the height of the red ab- 

 sorption peak was reduced in light by 90%, and that 80% of this bleaching 

 was reversed in the dark. Dilution of pyridine with water decreased both 

 the extent and the reversibility of bleaching. In aniline the red peak was 

 reduced in light only by 22%, and only half of this bleaching was reversed 

 in the dark. In ethanol the corresponding figures were 20 and 14%; in 

 acetone, 29 and 19%. Addition of as little as 0.3% pyridine, imidazole, or 

 histidine, to alcohol or acetone, increased the bleaching to 80%, with one- 

 half of it being reversed in the dark. Twenty-six organic and two inorganic 

 compounds were tried out as reducing agents in pyridine. Positive results 

 were obtained with ascorbate, dihydroxymaleate, cysteine, phenylhydra- 

 zine, and hydrogen sulfide; pyruvic acid (previously mentioned by Kras- 

 novsky as giving results similar to those produced by ascorbic acid) was 

 now listed among nonreactive compounds. 



Renewed investigation of chlorophyll h showed that its reduction by 

 ascorbic acid in light, and reoxidation in dark, gives a product with absorp- 

 tion bands at 693 and 432 mju, which is neither chlorophyll a, nor pheophy- 

 tin a or b. (Conversion of chlorophyll b to chlorophyll a in this reaction 

 was first suggested as a possibility, cj. above.) 



Evstigneev and Gavrilova (1950) noted that chlorophyll and Mg- 

 phthalocyanine were reduced by ascorbic acid in light in pyridine solution 



