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LIGHT AND LIFE 



the transfer to occur with the observed efficiency the adenine and 

 pyridine rings must be in close juxtaposition. In order to observe 

 the adenine excited state fluoronietrically at room temperatiue it 

 is necessary to work in strong acid solution. The emission band may 

 be observed in neutral solutions at liquid nitrogen temperature. The 

 adenine emission band overlaps the absorption band of the reduced 

 pyridinium ring, and conditions are thus satisfied for excitation energy 

 transfer by the coupled oscillator mechanism. However, the Forster 

 equation (9) for interactions of this type applies to dipoles that are 

 separated by several molecular diameters and mtist therefore be modi- 

 fied for interactions at short distances or within an inner complex. 



Effect of temperature. If free DPNH occurs in solution as a folded 

 inner complex the complex shoidd be dissociable as a function of 

 temperature, solvent, and structmal modification. Fig. 3 shows the 

 effect of temperatme on the fluorescence excitation spectrum. You 

 Avill notice that the relative quantum yield of excitation in the 340 

 m^ band declines with increasing temperature as a result of increased 



o 



c 



o 



260 300 340 380 

 Excitation, mu 



420 460 500 

 Emission, ny< 



Fig. I. Corrected fluorescence excitation and emission spectra of DPNH at 

 the same concentrations in water and in methyl carbitol. 



