CHLOROPHYLLS a AND h 



613 



plete absorption of all visible light appears to be unnecessary, or even harm- 

 ful. As stated before (compare Volume I, chapter 19), chlorophyll prob- 

 ably was adopted as sensitizer for photosynthesis not because of a particu- 

 larly favorable absorption spectnim, l)ut because of its peculiar photosensi- 

 tizing properties. Once having found a suitable sensitizer, natiu'e then 

 made adjustments to improve the supply of light energy to species living 

 in unfavorable habitats — by increasing the quantity of chlorophyll b in 

 shade plants, and by providing brown algae with fucoxanthol, and red 

 algae with phycobilins. The presence of the latter makes the red algae 

 capable of growth even under a thick layer of blue-green sea water. 



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UJ 



on 



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UJ 



o 

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 u. 



UJ 



o 

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tr 

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Normal 



390 430 470 510 550 590 630 670 710 



WAVE LENGTH, m,i 



Fig. 21. 4A. Al)sorption spectrum of allomerized chlorophyll a in methanol 



(after Livingston 1948). 



A word must be said about the absorption spectrum of allomerized chloro- 

 phyll. When chlorophyll in alcoholic solution is permitted to stand in air, 

 it is "allomerized," ?'. e., according to Conant and Fischer, oxidized at the 

 r(10) iitom (cf. Vol. I, page 4(i()). This reaction is catalyzed, according to 

 Livingston and co-workers, by salts such as LaCls and BaCl2. Spectro- 

 scopic evidence indicates that a similar, or identical, oxidation occurs also 

 under the influence of iodine or bromine even in the absence of air: 

 Fischer's chemical observations indicate that quinone has the same effect. 

 According to Livingston (1948, 1949), allomerization of chlorophyll is 

 characterized by a spectral change, illustrated by figure 21. 4A. Tlu^ two 



