REVERSIBLE PHOTOREDUCTION OF CHLOROPHYLL 1503 



action (curve ^) . The slightly increased absorption in the green may indi- 

 cate the formation of a small amount of pheophytin. Curve 3 is the ap- 

 proximate extinction curve of the unstable pink product. Complete ab- 

 sorption curves of reduced chlorophyll (cf. fig. 37C.10) were determined 

 by Evstigneev and Gavrilova (1953^). They indicated two reduced species 

 (perhaps an ion and a neutral semiquinone molecule). 



In the discussion, Krasnovsky pointed out that the reversible reduction 

 of chlorophyll apparently requires the presence of a basic compound (such 

 as pyridine) ; he attributed this tentatively to the greater stability of an 

 ionic form of the free radical "mono-dehydrochlorophyll" (which is 

 the probable immediate product of the reaction between light-excited 

 chlorophyll and ascorbic acid). He suggested that this semiquinone is 

 formed by hydrogenation of one of the conjugated double bonds in the 

 "aromatic" porphin system. 



The following scheme of reversible reduction was suggested : 



(35.13) ^0=0/ + hi' 



(Formation of a "biradical" by light absorption: the C=C bond is as- 

 sumed to be part of the conjugated system.) 



(35.14) 



N. . . HA + ^C— c/ > NH+ + A + \c— C"/ 



(Ascorbic acid, bound by a hydrogen bond to pyridine, cedes a proton to 

 pyridine and an electron to the biradical of chlorophyll, thus forming mono- 

 dehydroascorbic acid, and an anion of the monohydrochlorophyll radical.) 



In a follow-up reaction, the anion could combine with an H+ ion and the 

 resulting neutral semiquinone dismutates into a quinone (chlorophyll) and 

 a hydroquinone (didehydrochlorophyll = stable reduced product?); but 

 in a basic solvent, such as pyridine, the H+ concentration may be so low 

 as to make these steps improbable, and to limit the reversible reduction 

 to the radical stage. 



The absence of reversible reduction in alcoholic solution was attributed 

 by Krasnovsky to the rapid reoxidation of the neutral form of the semiqui- 



t H 



none, yC — C\^ (which can be formed because of the presence of H+ ions 



in ethanol). 



Krasnovsky suggested that reversible photochemical reduction of 

 chlorophyll by ascorbic acid (a compound that is present in all green 

 plants) may be a step in photosynthesis. If this were true, however, 

 monodehydroascorbic acid should be able to liberate oxygen from water. 



