SIDXEY l\ \ ElACK 



13:5 



80 



FAD in CH30(CH2)20H 

 °-°FMNinH20 



FAD in H,0 



-o- 



10 20 30 40 ^50 



Temperature, C 



60 



Fig. 16. 1 he relati\e Hiiorescciue quantum yields of flavin niononnclcolide and 

 dinucleotide as a function of temperature in water and in methyl tarbitol. The 

 cin\e for FMN in methyl carhitol coincides with that in water and is not siiown. 



In methyl carbitol, the FAD emission is enhanced approximately ten- 

 fold and now coincides with that of FMN with respect both to 

 quantimi yield and to temperature coefficient. Thus the inner com- 

 plex of FAD behaves like that of DPNH. An explanation of the 

 temperature-independence of FAD emission is provided by an exami- 

 nation of the excitation spectrimi. 



The excitation spectrinn of FAD. Superimposed on the absorption 

 spectrum of FAD in Fig. 17 is the fluorescence excitation spectrum. 

 Within the limits of error of the light source calibration, the two 

 curves coincide in the two long wavelength bands, but the quantum 

 yield is lower when the excitation is in the ultraviolet. Unlike re- 

 duced nicotinamide, the contribution of the flavin to absorption in 

 the 260 niyu, region is about twice that of the adenine. Consequently 

 excitation in this region arises chiefly from direct absorption of 

 photons in the isoalloxazine ring structure. There is in fact no 

 fluorescence excitation by absorption in the adenine. The mono- 

 nucleotide is excited more efficiently in this region than is the di- 



