458 



S. S. Brody and M. Brody 



B. The Aggregate in Living Systems 



1 . Evidence Based on Emission: On the basis of the affects of heating and 

 drying on fluorescence of green leaves, Seybold and Engle (49) proposed the 

 existence of two forms of chlorophyll. They visualized most of the chloro- 

 phyll as being in a nonfluorescent, protein-bound state with a relatively small 

 fraction dissolved in a lipid phase. Franck (25), founding his considerations 

 on still other experimental work, presented a more detailed picture in which 

 he suggested a high population of nonfluorescent, "protected chlorophyll" not 

 directly connected with the photochemistry of photosynthesis, and a smaller 

 population of fluorescent, "unprotected chlorophyll" in contact with water and 

 capable of sensitizing photochemistry in vivo. 



Support for the existence of more than one species of chlorophyll in vivo 

 also comes from observations of fluorescence during the greening process in 

 a wide variety of organisms, both at room temperature (27) and at 77°K 

 (12,31,44,48). 



The discrepancy between the fluorescence yield.s, determined directly (41), 

 and from measurements of fluorescence lifetimes (13), was also interpreted 

 as evidence for the existence of fluorescent and nonfluorescent forms of 

 chlorophyll in vivo ( 9 ). Brody and Brody ( 9 ) suggested, however, that the 

 fluorescent form (emitting at 685 m|jL) corresponds to a monomer, and that 

 the'hon-fluorescent'^fori-n (not contributing to emission at 685 my.) corresponds 

 to an aggregate. 



The many similarities between low tenriperatur e emission spectra of photo- 

 synthetic organisms and concentrated solutions of chlorophyll have prompted 

 the suggestion that chlorophyll aggregates are present in vivo (4, 9 ), e. g. , 

 Brody and Brody ( 9 ) suggested that the 720 mfjL emission reported at room 

 temperature by various workers (19, 3C), corresponds to aggregated chloro- 

 phyll. 



Lavorel (42) has shown that during the period of fluorescence induction in 

 whole cells, there are changes in shape of the fluorescence spectrum. He has 

 been able to resolve two fluorescence components - one constant and the 

 other variable - the variable component has a maximum at 720 mfi (at room 

 temperature),' this corresponds exactly to the maximum of emission of the 

 chlorophyll aggregate ( 4 ). 



2. Evidence Based on Absorption Spectra: The origin of the concept of 

 more than one form of chlorophyll in vivo is often associated with the attempt 

 to interpret the absorption properties of purple bacteria. (19, 39, 5^. The 

 single absorption band with maximum at '^ 770 m|j. - which is characteristic 

 of bacteriochlorophyll in vitro - is the only form one obtains upon extrac- 

 tion of bacteriochlorophyll from organisms which exhibit one, two or three 

 absorption bands in the living state. The bands, which differ in relative 

 intensity, have maxima at about 800, 850 and 890 nifji. They have been 



