REVERSIBLE PHOTOREDUCTION OF CHLOROPHYLL 1501 



no relation to the quenching of fluorescence {cf. scheme 19.11 on page 54G; 

 also page 788, and scheme 23.11). 



Calvin and Dorough (1948) and Huennekens and Calvin (1949) de- 

 scribed the photochemical oxidation of certain chlorins to porphins by 

 quinone, and reduction back to chlorins by phenylhydrazine. 



Evstigneev and Gavrilova (1953^) found that irreversible auioxidalion of 

 chlorophyll a (in toluene) is accelerated by water and other polar molecules 

 and slowed down by pyridine and other basic molecules (which favor the 

 ^hoioreduction of chlorophyll). 



3. Reversible Photoxidation of Bacteriochlorophyll 



Krasnovsky and Vojnovskaja (1951) noted that dissolved bacterio- 

 chlorophyll photoxidizes in air; the oxidation is faster in alcohol than in 

 pyridine. The alcoholic solution gives, after oxidation, a strong peroxide 

 test with ferrous thiocyanate. Adding ascorbic acid, or hydrogen sulfide, 

 regenerates the pigment; after 10 hours standing in the oxidized state the 

 greater part of the pigment could still be regenerated in this way. Oxi- 

 dants other than molecular oxygen (p-quinone, nitrite, nitrate, hematin) 

 and reductants other than ascorbate or hydrogen sulfide (malic, pyruvic, 

 succinic acids; thiosinamine) were ineffective. o-Quinone oxidized bac- 

 teriochlorophyll, even in the dark, to a compound with a chlorin-type 

 spectrum (with a strong band at 680 m/x), as mentioned by Schneider in 

 1934. Bacteriopheophytin is less easily oxidizable than bacteriochloro- 

 phyll; its photoxidation, too, is reversed by ascorbic acid. 



The autoxidation of bacteriochlorophyll was interpreted by Krasnovsky 

 as addition of Jxygen to a photochemically produced biradical. As with 

 chlorophyll, iiu reversible dehydrogenation could be observed (the reaction 

 with o-quinone is irreversible). 



4. Reversible Photoreduction of Chlorophyll and Its Derivatives 

 (Addendum to Chapter 18, Section A5) 



In chapter 16 (page 457) and 18 (page 505) we have discussed attempts 

 to achieve reversible reduction of chlorophyll to a "leuco-chlorophyll." 

 The conclusion was that no truly reversible reduction has as yet been 

 achieved, and that chlorophjdl, as it is known in vitro, appears to be more 

 inclined to undergo reversible oxidation, than reversible reduction. The 

 above-described experiments of Livingston and co-workers support (or, at 

 least, do not contradict) the hypothesis that the reversible bleaching of 

 chlorophjdl is the result of reversible oxidation. Krasnovsky (1948M, on 

 the other hand, has described experiments indicating a reversible photo- 

 chemical reduction of chlorophyll, apparently to an unstable pink radical, 

 by ascorbic acid. The reaction was observed in dry pyridine as solvent; 



