G70 LIGHT AND LIFE 



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DISCUSSION 



Dr. French: I just wanted to ask Dr. Thimann if the action spectrum 

 of the red effect has been made. 



Dr. Thimann: No, it hasn't, I am sorry to say. It would be a very nice 

 thing to do. 



Dr. Galston: Dr. Thimann mentioned, and quite properly, that the action 

 spectra for phototropism do not at the moment permit us to discriminate 

 absolutely amongst the known pigments that present themselves as candidates 

 for photoreceptor. I think, however, that the manner of statement of the 

 problem led to the conclusion that it was impossible for riboflavin or one of 

 its close relatives to be the photoreceptor. I would like to counteract this 

 impression. 



Dr. Henry Harbury and his collaborators at Yale, to whom Dr. Thimann 

 referred, have shown that the solvent in which isoalloxazine-like molecules 

 fuid themselves can shift their spectrum very markedly from a single peak 

 (aqueous solvent) to a triple peak (benzene solvent) in the visible range. 

 Similar things can happen, in fact, with FMN. Therefore, I believe there is 

 every possibility tliat FMN, FAD or other naturally occurring molecules of 

 the isoalloxazine type could, in fact, be the photoreceptor if only they find 

 themselves in the proper medium, such as a lipoidal phase. 



In Harbury's actual experiment, 3-methyl lumiflavin was used, which in 

 benzene, shows 3 visible absorption peaks. If one takes FMN and puts it 

 in water, one gets a clear single peaked spectrum. If, however, one merely 

 adds pyridine in increasing quantities to this aqueous solution, one can find 

 increasing evidence of a triple peak. When one has a preponderance of 

 pyridine present, one does have a clearly triple-peaked spectrum in the 

 visible. My only plea would be, therefore, not to disqualify the flavins 

 because of their absorption spectra, because, depending on their state in 

 whatever cytoplasmic body we find them, they may have an absorption 

 spectrum which closely resembles that of the photoreceptor. 



Now, a question to Dp. Thimann. Dr. Thimann mentioned the analytical 

 results of my graduate student, Asomaning, on the flavins and carotenes in 

 illuminated barley f)ats. I would like to ask him what he thinks of other 

 data in the literature on the relative light sensitivity and carotene content 

 in Phycomyces, in albino corn and in albino sunflower. Many years ago, 



