ENERGY TRANSFER BETWEEN DIFFERENT PIGMENT MOLECULES 1301 



In chapter 24, we mentioned the fluorescence experiments of Van Nor- 

 man, French and Macdowall (1948) which made it plausible (but did not 

 prove definitely) that a transfer of excitation energy from phycocyanin to 

 chlorophyll actually does occur in the red alga, Gigartina harveyana. Sub- 

 sequently, French and Young (1952) developed a double-monochromator 

 permitting the determination of the fluorescence spectra of algae with 

 monochromatic excitation. These experiments will be described in Chapter 

 37C (section 7). They indicate that the energy absorbed by chlorophyll in 

 the blue-violet band, is not transferred to the phycobilins (probably because 

 the chlorophyll molecule in the upper state of the blue-violet band is 

 changed practically instantaneously, by internal conversion, into the ex- 

 cited state reached directly by absorption in the red band, leaving the mole- 

 cule with a quantum too small to be acceptable to phycoerythrin or phyco- 

 cyanin). The quanta absorbed by phycoerythrin, on the other hand, are 

 transferred to both phycocyanin and chlorophyll a, causing simultaneous 

 cule with a quantum too small to be acceptable to phycoerylthrin or phyco- 

 fluorescence of all three pigments (figs. 37C.41-47). The fluorescence spec- 

 trum excited by 453.5 m^ (a frequency absorbed mostly by phycoerythrin), 

 undergoes a characteristic change upon heating: the phycoerythrin band 

 increases in intensity relatively to the chlorophyll band indicating that the 

 resonance coupling, responsible for the phycoerythrin-sensitized fluores- 

 cence of chlorophyll, has been destroyed or weakened by heating. 



Systematic studies of sensitized fluorescence in algae and bacteria have 

 been carried out by Duysens (1951, 1952); these, too, will be described in 

 chapter 37C. They prove an effective transfer of excitation energy from 

 all pigments present in photosynthesizing cells to the one pigment with the 

 lowest excitation level {i. e., one whose absorption band is located furthest 

 towards the infrared). In purple bacteria, this ultimate energy acceptor 

 is one of the several bacteriochlorophylls (BChl "890"); in green plants 

 and algae, chlorophyll a; in red algae, it may be either chlorophyll a or a 

 minor pigment with an absorption band located (in vivo) at about 700 ran — 

 perhaps chlorophyll d. 



The efficiency of energy transfer from phycobilins or carotenoids to 

 chlorophyll a (or to bacteriochlorophyll "890"), evidenced by sensitized 

 fluorescence, parallels closely the contribution of these pigments to photo- 

 synthesis. This provides a strong support for the hypothesis that the 

 quanta absorbed by the "accessory" pigments are utilized in photosynthesis 

 by being first transferred to the main "photocatalytic" pigment — chloro- 

 phyll a. The observations of French and Young (chapter 37C, section 7) 

 that in red algae only chlorophyll a fluorescence shows induction phenomena 

 and a dependence of the yield on light-intensity (i. e., indirectly, on the 

 rate of photosynthesis) also support this hypothesis. 



The following estimates of the efficiency of energy transfer in vivo were 

 made by Duysens for purple bacteria: 



