457 



S. S. Brody and M. Brody 



500 600 



WAVELENGTH, m/l 



700 



Fig. I. Difference in absorption between dilute (10"^M) and concentrated 

 (8 X 10 '^M) ethanolic solutions of chlorophyll a, at room temperature. 



low temperatures, but they conclude that the chlorophyll dimer is formed upon 

 codling, although they point out that some of their data indicate the presence 

 of dimer at room temperature also. (Another possibility is that upon cooling 

 the dimer band undergoes a greater shift than the monomer band. ) 



3. Evidence Based on Fluorescence Excitation Spectra : From their 

 studies of the action spectra for sensitization of chlorophyll fluorescence in 

 highly concentrated solutions, Lavorel (43), and Weber and Teale (59) con- 

 cluded that dimers of chlorophyll are present at room temperature. They 

 attributed the "long wavelength decline" in efficiency of sensitization of 

 fluorescence to an absorption band of a nonfluorescent chlorophyll dimer. 



Low temperature (77 K) fluorescence excitation spectra have been deter- 

 mined with concentrated solutions of chlorophyll ( 7 ). That there is a marked 

 difference between the spectrum for exciting emission at 720, and for excit- 

 ing emission at 680 m|j., suggests that emission at 720 arises from a speices 

 other than monomeric chlorophyll; in concentrated solution an aggregate 

 becomes suspect. (To date, these excitation spectra have not been analysed 

 to yield their various components.) 



4. Evidence Based on Data from Other Sources: The formation of chloro- 

 phyll aggregates in acetone at concentrations in excess of 2 x 10" M was 

 reported by Rodrigo (47). From measurements of light scattering, he 

 determined the size of the aggregate as a function of concentration. At 

 9 X 10"^ M, the aggregate seemed to reach its average maximum size of 

 three chlorophyll molecules. 



Aronoff (3 ) has studied the coUigative properties of highly concentrated 

 solutions of chlorophyll in benezene and has shown that aggregation results 

 in the formation of dimers, and that larger aggregates are of negligible 

 concentration. It remains to be seen whether there is an unique size of aggre- 

 gate formed in different solvents, e. g. , dimers in benzene, trimers in ace- 

 tone. 



