ABSORPTION SPECTRUM CHANGES 99 



Strehler: I am quite certain it is emitted from chlorophyll, but that sa3's noth- 

 ing about whether the energy for exciting it comes from a reaction involving the 

 chlorophyll molecule. 



Frenkel: Then you could have a recombination of two molecules other than 

 chlorophyll? 



Strehler: I didn't say that both of them were other than chlorophyll, did I? 



Duysens: I maj^ perhaps make a suggestion. If reduced riboflavin in the 

 chloroplast tries to emit light, it will be unable to emit it because the chlorophyll 

 traps the excitation energy and you will get chlorophyll a luminescence. So it is 

 quite possible there is luminescence of riboflavin. 



Strehler: Riboflavin is known to be involved in other luminescences as the 

 emitting molecule. Light emitted by flavin could certainly be trapped by an}^ 

 other absorber present, and what is a more likely absorber in green plants than 

 chlorophyll? I hope j'ou don't get the impression that I have evidence for this 

 hypothesis. I am simply saying that this is another possibility. 



Rabinowitch : It sounds to me like why make it simple when it can be made 

 complicated. 



Strehler: In support of the hypothesis it has been shown with firefly extracts 

 that added fluorescent dyes in a relatively low concentration change completely 

 the color of the light emitted by firefly extracts. If one chemiluminescent system 

 can do it, certainly another can. 



Becker: You can talk about this until you are blue in the face, but I think it is 

 important that you realize you cannot say anything about this energy transfer 

 unless you realize that the molecules that are going to receive it have energy 

 values of the same comparable value. 



Duysens: They are there. The fluorescence spectrum of riboflavin overlaps 

 the continuous absorption spectrum of chlorophyll. 



Strehler: The riboflavin emission band is quite broad. It extends from about 

 485 to 620 m;u, and, although this region does not encompass a major band of 

 chlorophyll, there certainlj^ is enough absorption there, particularly if the mole- 

 cules are in intimate association, to permit energy transfer. 



References 



1. Strehler, B., and Arnold, W., J. Gen. Physiol, 34, 809 (1951). 



2. Chance, B., /. Biol. Chem., 202, 397 (1953). 



3. Duysens, L., Science, 120, 353 (1954). 



4. Lundeg&rdh, H., P/i?/sio/.PtoMtomm, 7, 375 (1954). 



5. Witt, H., Nalurwiss., 3, 72 (1955). 



6. French, C, Carnegie Institution of Washington Year Book, 53, 182 (1954). 



7. Strehler, B. L., and Lynch, V. H., Arch. Biochem. and Biophys., 1957, in press. 



