TREE RADICALS IN PHOTOSYNTHESIS 233 



the viologens, whose potentials (independent of pH) are as high as 

 + 0.44 volt (in this case, the redudants are free radicals). 



7. Free Radicals in Photosynthesis and Chemosynthesis 



The realization of the wide range of stability of free radicals has 

 several consequences for the mechanism of photosynthesis. In the first 

 place, it shows that there is no need to avoid free radicals as intermediates 

 in setting up reaction mechanisms (as was once thought by Franck, 

 Herzfeld, and Stoll), provided they have a structure which permits an 

 adequate resonance stabilization (one of the consequences of the forma- 

 tion of the complex, iCOo}, may be such an improvement in the stability 

 of the intermediate radicals). 



In the second place, if free radicals are formed in the primary photo- 

 chemical process (and the intermediate reductant, HX, as well as the 

 intermediary oxidant, Z, postulated in 7.10a, probably are free radicals, 

 because they are formed by the action of single light quanta), their 

 energy will not necessarily be lost by conversion into saturated com- 

 pounds. Let us consider, as an illustration, the thionine-ferrous iron 

 reaction (of. Eqs. 7.9). The first product of the light reaction is the 

 radical, semithionine. At pH 3, the free energy of reduction of thionine 

 by ferrous ions to semithionine is about +15 kcal, whereas that of 

 dismutation is approximately — 3 kcal (estimated from potentiometric 

 data; cf. Michaelis, Schubert, and Granick 1940; Granick, Michaehs, 

 and Schubert 1940; and Michaelis and Granick 1941). Thus, in the 

 formation of one molecule of leuco thionine, by the cooperation of two 

 light quanta, according to the mechanism (7.9), as much as 80% of the 

 energy accumulated in the primary reaction is retained in the valence- 

 saturated product. 



It is thus possible that the first transformation of the primary photo- 

 chemical reduction product, HX, is a dismutation into H2X and X, and 

 that only a comparatively small amount of energy is lost in this process, 

 so that the reducing power of the saturated intermediate, H2X, is not 

 much less than that of the radical, HX. 



However, what we were after in introducing free radicals into the 

 chemistry of photosynthesis was a reductant with a potential higher than 

 that of any valence-saturated system, and by assuming dismutation as 

 the fate of the primary product, HX, we have lost this advantage. To 

 regain it, one may assume that the radical, HX, is used directhj for the re- 

 duction of {CO2} without preUminary dismutation, but more interesting 

 appears to be a mechanism in which a second catalytic system, Y-H2Y, is 

 inserted between the systems Z-HZ and X-H2X (c/. Scheme 7.1) because 

 this mechanism opens a way for the utilization of the energy of two 

 quanta for the formation of one radical, and thus for a new explanation of 



