(;. WEBER 



87 



thai tlicsc molecules also exhibit, in common with indole, large 

 shifts in the Ihioiescence spectiuni upon change of solvent. Thus in 

 either case the contribution of ionic structures to the excited state 

 is very important. It reaches nearly 50 per cent for some naphthy- 

 lamine derivatives (W^eber, unpublished) and is only slightly smaller 

 in the case of indole. In these ionic structures the forms 



/\ 



+ 



1 

 H 



•v^ 



N^ 



H 



H 



must make the greatest contribution in either case, and the reaction 

 with the hydroxy! ion in the excited state receives a satisfactory 

 explanation. 



The interaction of the excited ring with proton carrier groups like 

 COOH and NH3+ receives also confirmation from experiments with 

 glycyl peptides of both tyrosine and tryptophan. In these it would be 

 expected that the short-range interaction between the relatively far 

 away COOH or NH3+ and the excited ring wall require a rotational 

 diffusion of the side-chain to bring the appropriate group into con- 

 tact with the ring. Both glycyltryptophan and glycyltyrosine have 

 low quantum yields (in the neighborhood of 0.04) , which are in- 

 creased to almost the normal value of 0.2 in a medium of high 

 viscosity such as propylene glycol at low temperature (Teale, 

 unpublished) . 



Fluorescence Excitation Spectrum 

 This is coincident with the absorption spectrum over the range 

 of wavelengths that has been investigated (210-310 m/i.) . 



Lifetime of the Excitation 

 Direct measurements of this quantity are not yet available. By 

 measurements of oscillator strength and application of the well-knowm 

 radiation equation, the natural oscillator lifetime of tyrosine (and 

 also of cresol and phenol) is found to be nearly 40 ni/xsec, while that 

 of indole tryptophan is 12 myutsec. With quantum yields of 20 per 

 cent the respective lifetimes in water would be 8 and 2.5 m^nsec re- 



