SIDXEY F. VELICK 



113 



thermal dissipation ol the excitation energy. I'he eHiciency oi the 

 261) niyLi excitation declines even more rapidly because it involves two 

 temperature-dependent processes, conversion to fluorescent light with- 

 in the jjyritline ring and energy transfer from adenine to pyridine. 

 Weakening of the transfer interaction can arise both from increased 

 thermal quenching of the adenine and from thermal dissociation of 

 the inner complex. Jn so far as dissociation is a factor, the amount 

 of spatial separation or disorientation need not be very large to 

 jjrodiice a large effect on the transfer process. 



Dissociation by change of solvent. The effect of solvent is much 

 more pronounced than that of temperature (Fig. 4) . The lower curve 

 is the excitation spectrum of DPNH in water and the upper curve 

 is the excitation spectrum of the same concentration of DPNH in 

 ethylene glycol monomethyl ether. Notice that the emission maxi- 



280 320 360 400 440 480 

 Excifation ma Emission 



Fig. ."). Corrected fluorescence excitation and emission spectra of DPNH, desamino- 

 DI'NH (adenine replaced by hypoxanthine), and APNH (nicotinamide replaced by 

 3-acetyl pyridine). Measurements were made of dilute aqueous solutions the optical 

 densities of which were equal at the respective long-wavelength absorption maxima. 



