SENSITIZED REDUCTION OF CARBON DIOXIDE 



91 



blue-leuco methylene blue. This conclusion — which, if correct, would 

 be of extraordinary importance for the theory of photosynthesis, and for 

 the imitation of this process in vitro — was based on the observation that 

 formaldehyde can be detected in water in which collodion films impreg- 

 nated with alcoholic solutions of the two dyestuffs have been exposed 

 to light. We mentioned on page 68 the controversy concerning the 

 formaldehyde formation by gelatin films containing only chlorophyll. 

 Baur found no formaldehyde after the illumination of pure chloro- 

 phyll-collodion films, but obtained positive results with chlorophyll- 

 methylene blue films. One possible explanation of these results is the 

 photoxidation of methyl groups in methylene blue by chlorophyll (or of 

 methyl groups of chlorophyll by methylene blue); but Baur suggested 

 a more complicated mechanism, described by the series of equation 

 (4.27) in which excited chlorophyll molecules are alternatively "depolar- 

 ized" by hydrogen and hydroxyl ions, and whose final result is the 

 oxidation of water by the carboxyl group of chlorophyll, with methylene 

 blue playing the part of a catalyst (XCOOR is chlorophyll, MB is 

 methylene blue and MB — is leuco methylene blue) : 



(4.27a) 



s light 



C=0 + 2 OH- > 



RO 



X o 



\ / 



c 



RO O 



+ H2O 



(4.27b) 



X 



^RO 

 X 



o 



o 



o 



+ MB 



RO 



+ MB- 



O 



o 



(4.27c) 



(4.27d) 

 (4.27) 



C 



RO 



+ 2H+ 



light 



^ XR++ + H2CO + O2 



O 



XR++ + MB- 



-> XR + MB 



XCOOR + H2O > XR + H2CO + O2 



The scheme is obviously a very arbitrary one. Baur, however, used 

 it as a starting point for a whole series of experiments. First, he showed 

 (Baur and Fricker 1937) that chlorophyll can be replaced by eosin and 

 that other reversibly reducible dyestuffs or inorganic redox systems can 

 be substituted for methylene blue. Instead of collodion films, he found 

 colophony (rosin) suspensions in water more suitable. The "sensitizer" 

 (chlorophyll or eosin) was contained in the sol particles, the "auxiliary" 

 redox pair in the aqueous phase. The auxiliary systems included 

 thionine, malachite green, safranine, quercetin, Janus red, neutral red, 

 phenosafranine, gallocyanin, hydroquinone, Nile blue and ferric chloride, 



