CHLOROPHYLL PARTICIPATION IN THE PRIMARY PROCESS 555 



Ruben, Frenkel, and Kamen (1942) intended to check the a ' & transformation 



theory by means of radioactive magnesium, basing their method on the more rapid 

 "pheophytization" of chlorophyll a, found in experiments in vitro {cf. page 467). They 

 hoped to be able to introduce Mg* selectively into chlorophyll a in vivo and then to 

 investigate the conversion of this "tagged" chlorophyll a into chlorophyll b during 

 photosjmthesis. However, no measurable exchange of magnesium in chlorophyll in 

 vivo with Mg*N03 could be obtained in the short time available, so that the purpose 

 of the investigation has remained unfulfilled. 



Most recent theories of photosynthesis, which assumed a photo- 

 chemical oxidation and photochemical reduction of chlorophyll, have 

 renounced any distinction between the functions of the chlorophylls a 

 and b. 



Stoll (1932) suggested that chlorophyll can lose reversibly two 

 hydrogen atoms, by a process analogous to allomerization (which Stoll 

 and Wiedemann thought they had succeeded in reversing, at least in 

 the case of chlorophyll b, cf. page 462). Since allomerized chlorophyll 

 has the same color as the nonallomerized product, a half-and-half distri- 

 bution of chlorophyll between the ordinary and allomerized form could 

 take place in vivo without becoming apparent to the eye. The oxidized 

 (allomerized) form was thought by Stoll to be able to oxidize water 

 photochemically, while the reduced (nonallomerized) form was assumed 

 to be able to reduce carbon dioxide, also by a photochemical reaction. 



Using suggestions made by Franck in 1935, Stoll (1936) later sub- 

 stituted for his first concept the theory of an hydrogen-hydroxyl exchange 

 in chlorophyll; in this theory, too, both the oxidation of chlorophyll to 

 an hydroxychlorophyll (perhaps by an exchange of the hydrogen atom 

 in position 10 for an hj^droxyl radical, leading to the 10-hydroxychloro- 

 phyll mentioned on page 461), and the reduction of the latter back to 

 ordinary chlorophyll were supposed to be photochemical processes. 



The most recent (third) theory of Franck and Herzfeld (1941), 

 described by equations (7.12) and scheme 7.VA (with X now standing for 

 oxidized chlorophyll, and HX for reduced chlorophyll) represented a 

 return to StoU's earlier picture. In this theory, chlorophyll was supposed 

 to oscillate during photosynthesis between the two green forms designated 

 by Franck as Chi and HChl, respectively, one of them being active in 

 the photochemical oxidation of water and the other, in the photochemical 

 reduction of carbon dioxide. 



The eight quanta mechanism (7.11), too, can be formulated with 

 chlorophyll as photocatalyst — by identifying Y with Chi and HY with 

 HChl. 



It is scarcely necessary to rewrite here equation systems (7.11) and 



(7.12) or schemes (7.V) and (7.VA) in order to illustrate these concepts. 



Of the three types of theories we have reviewed, those of type (b) 



are least plausible — since we have no experimental evidence indicating 



