498 



PHOTOCHEMISTRY OP PIGMENTS IN VITRO 



CHAP. 18 



orange fluorescence in the photoxidation of chlorophyll dissolved in benzene or acetone 

 (Fig. 55). Ferguson and Webb (1941) noticed a gradual increase in absorption in 

 the green and infrared in illuminated leaf extracts. 



650 600 550 500 

 Wave length; mju 



450 



Fig. 55. — Absorption spectrum of the decomposition products of chlorophyll a 

 (unbroken curve) and chlorophyll b (broken curve) in acetone (after Aronoff and Mac- 

 Kinney 1943). 



2. Photoxidation as the Cause of Bleaching 



As mentioned above, the chemical nature of the bleaching process is 

 unknown and is probably complex. Most authors assumed that bleach- 

 ing is caused by photoxidation; but the possibility of pliotor eduction shall 

 not be overlooked, particularly in easily oxidizable solvents, or in the 

 presence of oxidizable impurities. Elimination of magnesium may be an 

 intermediary step, causing a temporary replacement of the pure green 

 color of chlorophyll by the olive color of pheophytin. According to 

 Jorgensen and Kidd (1916) and Aronoff and Mackinney (1943) bleaching 

 takes this path in all acid solutions; while no intermediate pheophytin 

 formation can be observed in neutral or alkaline media. 



Arguments in favor of autoxidation as the cause of bleaching of 

 chlorophyll are twofold. In the first place, Jorgensen and Kidd (1916) 

 and Wurmser (1921) found that chlorophyll solutions do not bleach in 

 the absence of oxygen, e. g., in a nitrogen atmosphere, and Warner 

 (1914) and Wager (1914) made the same observation with solid chloro- 

 phyll in collodion films. In the second place, an absorption of oxygen has 

 been observed to occur during the bleaching. 



However, the quantitative results of the latter experiments indicate 

 that most of the absorbed oxygen was utilized for the sensitized autoxi- 

 dation of the solvent, or impurities, and not for the oxidation of chloro- 

 phyll itself. Jodin (1864) found, for example, in the first study of this 

 problem, that 0.7 gram of oxygen was taken up in a month by 1 gram 



