78 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



by oxygen, thus leaving an equivalent quantity of oxidized water. In 

 other words, we could expect the occurrence of a dyestuff-sensitized 

 formation of hydrogen peroxide, according to equation (4.14), by the 

 mechanism which was contemplated above in the case of the zinc oxide 

 sensitization. Blum and Spealman (1933) have in fact claimed the 

 formation of hydrogen peroxide in illuminated fluorescein solutions, and 

 Yamafuji and coworkers (1938, 1939) have also obtained positive hydro- 

 gen peroxide tests with illuminated solutions of chlorophyll, eosin and 

 hematoporphyrin. 



While the dyestuff-sensitized " photautoxidation " of water, indicated 

 (but by no means proved) by the experiments of Blum and Spealman 

 and Yamafuji, appears explicable according to the hypothesis of "hidden" 

 photochemical oxidation-reduction reactions, the alleged formation of 

 hydrogen peroxide in oxygen-free dyestuff solutions is, if true, a much 

 more remarkable phenomenon. Yamafuji and coworkers (1938-1939) 

 have asserted that illuminated tissue extracts, dyestuff solutions, and 

 zinc oxide suspensions also produce hydrogen peroxide in the absence of 

 oxygen, although much less than under aerobic conditions. As discussed 

 on page 73, this result (if true) would indicate a sensitized decomposition 

 of water into hydrogen and hydrogen peroxide, according to equation 

 (4.4). In the case of zinc oxide, the energy of two ultraviolet quanta 

 (about 75 kcal per einstein) is sufl[icient to bring about reaction (4.4); 

 but two quanta of visible, particularly red light (60-40 kcal per einstein), 

 are insufficient for this purpose. This makes us doubt whether oxygen 

 (or other oxidants) have actually been eliminated in the experiments of 

 Yamafuji and coworkers. The problem is too important to allow the 

 acceptance of their results without further confirmation. Baur and Reb- 

 mann (1921) have attempted to achieve a sensitized photolysis of water in 

 visible light, using uranyl salts, quinine, eosin, rhodamine and other 

 sensitizers, but could obtain no traces of oxygen. 



C. The Chemical and Photochemical Reduction 

 OF Carbon Dioxide * 



If one primary photochemical process in photosynthesis is the hydro- 

 gen transfer from water to an intermediate acceptor, the reduction of 

 carbon dioxide may be brought about either by a second photochemical 

 reaction, or by a "dark" enzymatic reaction with the reduced primary 

 hydrogen acceptor (cf. Chapter 7). We are therefore interested, in the 

 present chapter, both in photochemical and nonphotochemical reduction 

 of carbon dioxide in vitro. 



* Bibliography, page 96. 



