154 LIGHT AND LIFE 



TABLE 3 



Effect of Solvent on Absorption Properties of 



3-Aminopyridine Derivatives 



Water Propylene Glycol 



3-Aminopyridine methiodide 323 3440 336 3440 



3-Aminopyridine analogue of DPN 332 5350 341 5930 



Although there are very distinct differences between the absorption 

 properties of the analogue and the methiodide, the most dramatic 

 changes are observed in the fluorescence properties of the analogue. 

 When an aqueous solution of the analogue is excited at its absorption 

 maximum, fluorescence is observed maximally at 420 m^u, with an 

 intensity which is one-twenty-fifth that of an equimolar solution of 

 aminopyridine methiodide. On the other hand, the fluorescence in- 

 tensity of the analogue is only one-half that of methiodide in propylene 

 glycol as shown in Table 4. A comparison of the fluorescence spectra 

 of the analogue in water and propylene glycol is shown in Fig. 9. 



The ten per cent greater molar absorption of the analogue in 

 propylene glycol as compared with that in water cannot account for 

 the fluorescence changes observed in the two solvents. It appears that 

 adenine-quenching of the aminopyridine fluorescence is comparable 

 to the quenching of the isoalloxazine moiety of flavin adenine di- 

 nucleotide (FAD) by the aminopurine base, as reported by Weber 

 (23). 



It should be pointed out that we do not mean to imply that the 

 aminopyridine analogue serves as a model for oxidized DPN. The 

 amino group of aminopyridine is an electron-donating group, while 

 the carboxamide of DPN is an efficient electron withdrawing group. 

 These have opposite effects on the electron density of the pyridinium 

 ring and should also affect the nature of adenine-pyridine interactions. 



TABLE 4 



Effect ok a Solvent on Fluorescence Intensities of 



3-Aminopyridine Derivatives 



