504 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



decolorization of both the dye and the chlorophyll. The interpretation 

 of the bleaching as a reduction (rather than oxidation) of the dye seems 

 arbitrary; but Bohi quoted as proof an experiment with Janus green, a 

 blue-green dye whose first reduction product is red, and the second one 

 colorless. In this case, the second reduction stage is reversible, and the 

 red intermediate product (the dye safranine) is re-formed in the presence 

 of air. Upon illumination of a solution of chlorophyll and Janus green 

 (in methanol), the solution first turns red. If oxidized chlorophyll is 

 restored, e. g., by reduction with phenylhydrazine, reduction of the dye 

 can be carried a stage further. Addition of water and ether to methanol 

 allows the separation of the reduced azo dye from chlorophyll and the 

 demonstration that the dye has become entirely colorless. The sequence 

 of color changes: 



in water + O2 



> colorless > red 



blue-green > red- 



> green 



in ether 



is looked upon by Bohi as a proof that Janus green is bleached by re- 

 duction and chlorophyll by oxidation, rather than vice versa. 



In the experiments with binary chlorophyll-dyestuff mixtures, Bohi 

 used about ten times more dye than chlorophyll. This indicates that 

 one chlorophyll molecule caused the reduction of several molecules of 

 the dye, i. e., that chlorophyll catalyzed the reduction of the dye by some 

 other reductant. The only available reductant (assuming that the 

 solution was free of impurities), was the solvent (methanol); and so we 

 must think of Bohi's reaction as partly a direct reduction of the dye by 

 chlorophyll, and partly a sensitized reduction of the dye by methanol (the 

 latter being probably oxidized to aldehyde). This conclusion reminds 

 one of observations of Rabinowitch and Weiss (1937) on the chlorophyll- 

 ferric iron system. They found that, when the yellow solution, obtained 

 by oxidation of chlorophyll with ferric ions in methanol was allowed to 

 stand until the green color re-appeared, most of the added Fe+++ was 

 converted into Fe++. It may be suggested that the yellow "oxy chloro- 

 phyll," oxidizes methanol; consequently, a part of the "oxychlorophyll" 

 formed by reaction with Fe+++ ions reverts to the reduced state by 

 reaction with the solvent, and the net result is a chlorophyll-catalyzed 

 oxidation of methanol by ferric ions. 



If this interpretation of the results of Bohi and Rabinowitch and 

 Weiss is correct, chlorophyll can serve as an oxidation-reduction catalyst, 

 even in nonphotochemical reactions. According to Bohi, the chlorophyll- 

 Janus green reaction is accelerated by the provision of a specific reductant 

 for oxidized chlorophyll, e. g., turpentine oil, pinene, piperidine or 

 phenylhydrazine. These substances play the part which was assigned 



