1848 SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



No convincing spectroscopic evidence of a doublet structure of the red 

 absorption band in mature plants has been published. Duysens (un- 

 published) has suggested, however, that the greater width of the absorption 

 band of chlorophyll in vivo (compared to that of the same pigment in vitro), 

 may be interpreted as due to the superposition of two bands, with peaks 

 close enough to merge into a single band. The situation in green plants 

 would then be similar to that in purple bacteria, where two (or three) 

 infrared bands, present in vivo, are replaced by a single band after the 

 destruction of the pigment complex. An alternative explanation of the 

 broadening could be based, however, on enhanced coupling of electronic 

 excitation Avith vibrations in the pigment-lipoid-protein complex (c/. the 

 explanation of the greater width of the bands in chlorophyll crystals in 

 section 3 above). 



To sum up, from the point of view of absorption and fluorescence 

 spectra, the hypotheses of Seybold and Egle, and of Krasnovsky and Brin, 

 must be considered as unproved, even if not impossible. 



The second pastulate of Krasnovsky and Brin — that the bulk of chloro- 

 phyll is photochemically inactive — is quite implausible. The high quan- 

 tum yield of photosynthesis seems incompatible with the assumption that 

 the bulk of chlorophyll in the cell is photochemically inactive. The fluores- 

 cence experiments of Duysens, and of French and Young (chap. 24, sec. 7; 

 this chapter, sec. 7), and action spectra of photosynthesis of green, brown 

 and red algae, give convincing evidence that light quanta absorbed by 

 all forms of chlorophyll are transferred, with high efficiency, to chloro- 

 phyll a, and utihzed there for photosynthesis. Krasnovsky 's ''chlorophyll 

 reserve," with an absorption band overlapping the fluorescence band of 

 "active" chlorophyll would be a "sink" into which all excitation energy 

 would disappear without the possibility of it ever being used for photo- 

 synthesis (assuming this "reserve" itself is photochemically inert, as 

 suggested by Krasnovsky). 



In a more recent publication, Krasnovsky, Kosobutskaya and Voynovskaya (1953) 

 suggested that the "inactive, polymerized" Chi 678 contributes to photosynthesis by 

 resonance energy transfer to the "active, monomeric" Chi 670. However, energy 

 transfer in this direction should be negligible compared to that in the opposite direction, 

 from Chi 670 to Chi 678, because of the relative position of the absorption and fluo- 

 rescence bands of the two forms. We will see in section (c) below that Krasnovsky's 

 analogous explanation of the several absorption peaks of bacteriochlorophyll in vivo 

 runs into even greater difficulties. 



The assumption of photochemical activity of "chlorophyll 670" and 

 photochemical inertness of "chlorophyll 678" was based on the observation 

 of Krasnovsky and Kosobutskaya (described in part B of this chapter, 

 section 1) that, when plastid fragments from etiolated leaves, or these 



