BLEACHING OF CHLOROPHYLL 495 



in the presence of excess oxygen (c/. Chapter 19, pages 531 et seq.); this 

 makes it probable that the bleaching is caused by a photoxidation of the 

 pigment. 



Dead tissues and colloidal pigment extracts are more sensitive to 

 light than living plants; and solutions of chlorophyll in organic solvents are 

 much less photostable than the aqueous colloids — some of them are com- 

 pletely decolorized in a few hours by direct sunlight or strong artificial light. 



It was known to Senebier as early as 1788 that leaf extracts in ether and alcohol 

 are not lightproof. The bleaching was later investigated by Jodin (1864), Sachs (1864), 

 Timiriazev (1869), Gerland (1871), Wiesner (1874), and Reinke (1885), among others; 

 Jorgensen and Kidd (1916), Wurmser (1921), and Gaffron (1933) extended the study 

 to preparations of pure chlorophyll. 



Bohi (1929) and Weber (1936) studied the influence of organic 

 "accelerators" and "inhibitors." Reinke (1885), Dangeard (1910), and 

 AVager (1914) found that solid chlorophyll (in the form of a thin layer 

 on paper, or imbedded in collodion) bleaches even more rapidly than 

 chlorophyll in solution. Knorr and Albers (1935) and Albers and Knorr 

 (1935) recorded the gradual fading of chlorophyll fluorescence in different 

 solvents, an effect which probably represents another aspect of the photo- 

 chemical decomposition of the pigment. 



Vermeulen, AVassink, and Reman (1937) found that haderiochlorophyll 

 solutions are even more sensitive to light and air than are solutions of 

 ordinary chlorophyll. (This pigment, too, is much more stable in 

 colloidal aqueous extracts, and particularly in n\'ing purple bacteria; cf. 

 Katz and Wassink 1939.) 



The nature of the products of the bleaching of chlorophyll is unknown; no competent 

 worker in the field of chlorophyll chemistry has attempted to isolate and analyze them. 

 Scattered hints as to their properties can be found in the papers by Wager (1914) and 

 Ewart (1915), in the book of Willstatter and Stoll (1918), and in Rothemund's remarks 

 to the paper of Albers and Knorr (1935). Some conclusions were disputed, for example. 

 Wager's observation of the intermediate formation of a peroxide able to oxidize hydro- 

 iodic acid. The hypothesis of Ewart (1915) that, in the presence of carbon dioxide and 

 air, illuminated chlorophyll is transformed into xanthophyll (which Ewart supposed to 

 be formed by the oxidation of phytol), and into a waxy colorless substance also appears 

 improbable. 



A much discussed question was that of the occurrence of formaldehyde among the 

 products of photodecomposition of chlorophyll. It was often thought that the photo- 

 chemical formation of formaldehyde from chlorophyll might provide a clue to photo- 

 synthesis. Reduction of a carboxyl group in chlorophyll to a carbinol group, sphtting 

 off of formaldehyde and recarboxylation presented itself as a possible mechanism of 

 photosynthesis (RCOOH = chlorophyll): 



(18.19) RCOOH '^ ^ ) RCH2OH > RH + {CH2O 1 + CO2 



We described in chapter 4, while dealing with "artificial photosynthesis," the experiments 

 of Usher and Priestley (1911) on the alleged formation of formaldehyde by a photo- 

 chemical reduction of chlorophyll in the presence of carbon dioxide, and the criticisms 



