BLEACHING OF CHLOROPHYLL 



497 



absorbs about once in a second) is several hours, 7 must be less than 

 10^1 Porret and Rabinowitch (1937) estimated that for ethyl chloro- 

 phyllide a in methanol, the quantum yield of irreversible bleaching is of 

 the order of 10~® only; while Livingston (1941) found y c^ 5 X 10~^ in 

 methanol, and a three times larger value in acetone. Aronoff and 

 Mackinney (1943) gave quantum yields of about 5 X lO""^ for chlorophyll 

 solutions in acetone and benzene. 



All these estimates are based on the assumption that the reaction products do not 

 absorb any red light at all. It is not certain, however, that the color disappears in 

 the very first step of the photochemical transformation; to the contrary, the first reaction 

 products may still be green and the photometric determination of the quantum yield 

 may thus refer to a secondary decomposition step. According to Knorr and Albers 

 (1935) and Albers and Knorr (1935), photochemical transformations often reveal them- 

 selves, in chlorophyll solutions, by changes in the fluorescence spectrum, without equally 

 conspicuous changes in color. Obviously, bleaching should be studied by repeated 

 determinations of the whole extinction curve, rather than by colorimetry or photometry 

 in monochromatic light. Figure 54 shows the successive changes in the extinction 



700 600 500 



Wave length, m^ 



400 A 



Fig. 54.- — Changes in the absorption spec- 

 trum of illuminated chlorophyll solution in ace- 

 tone (after Wurmser, 1921). 



curve of an illuminated chlorophyll solution in acetone. The red band disappears 

 completely after several days. The violet band is much more persistent, showing 

 that the porphin structure is maintained in the first stages of the photodecomposition. 

 Aronoff and Mackinney (1943) observed the formation of pink intermediates with 



