132 



LIGHT AND LIFE 



Flavin Nucleotides 

 Flavin adenine diniicleotide conformation. The absorption spectra 

 ot the constituent mononucleotides of FAD are not additive in the 

 diniicleotide, and the fluorescence quantum yield of the dinucleotide 

 is about one-tenth that of the flavin mononucleotide. It was pro- 

 posed, on these grounds, that FAD also occurs as a folded inner com- 

 plex (38) . One of the ):)ossible folded forms is illustrated in Fig. 

 15. The chief jjoints to be noticed are occurrence of an open chain 

 ribitol instead of a ribose on the isoalloxazine and the congruent 

 structures of the adenine and isoalloxazine Avhich would favor a face- 

 to-face contact. 



N 



O^^N 



N 



^\ 



Fig. 1"). A skeletal model of fla\in adenine dinucleotide in a folded conforma- 

 tion. The dashed portion of the figiue is the adenylic acid moiety of the dinucleo- 

 tide and is located on a plane below that of the Havin mononucleotide. 



Effects of teinprrature and sohient. The emissions of fia\in mono- 

 luicleotide (FMiN) and of FAD are maximal at about 520 n\fx and 

 extentl out into the red. Fig. Ki shows the relative fluorescence 

 quantum yields ol the two nucleotides as a function of temperature 

 in two tlilfereiit solvents. The strong emission of FMN, with a 

 (juantum yield of about 25 per cent, shows a typical negative tem- 

 perature coefficient and is identical in water and in methyl carbitol. 

 In water, the emission of F.\D is virtually indejjendent of temperature. 



