84 



LIGHT AND LllE 



tyrosine, and 0.20 for tryptophan. White (30) has studied the in- 

 fluence of pH upon the fluorescence efficiency of tyrosine and trypto- 

 phan. From her stucUes it appears that protons can quench the 

 fluorescence of both rings, and that the potential source of the 

 quencher protons must be sought also in the COOH and NH3+ 

 groups in the amino acid. Thus both tyrosine and tryptophan are 

 half quenched at pW 2.3, the pK of the carboxyl group, while the 

 esters and the decarboxylated derivatives are quenched only at pH 



ACID QUENCHING OF FLUORESCENCE 

 OF TYROSINE DERIVATIVES 



LJ 



O 



z 



UJ 



o 

 (/) 



LJ 



cr 

 o 



Ll. 

 O 



>- 

 \- 



z 



UJ 



I- 



A^-A 



-A— A 



RAMINE 

 PHENOL 



CYL TYROSYL 

 GLYCINE 



ROSINE Me ESTER 



POLYTYROSINE 



♦ 



TYROSINE 



./ 



/ 



GLYCYL 

 TYROSINE 



J 



J I \ L 



0.9 -0.3 0.3 



H 



12 3 4 5 6 7 



Fig. 1. Effect oi l)\\ u\Hm ilic lliioicscciuc of lyrosine and derivatives (acid range;. 



— 0.55 in the phenol series and at pW 0.5 in the indole series (Fig. 1) . 

 In the alkaline range the phenolic derivatives lose all fluorescence 

 upon dissociating to give the phenolate ion. Tryptoj)han and some 

 tryptophan derivatives, though not indole, show a conspicuous in- 

 crease in fluorescence in the pW region where the NH3+ groups are 

 discharged, and in the NH^ form can have absolute fluorescence 

 efficiencies of the order of 0.6 (Fig. 2) . Thus the excited indole ring 

 is able to attract a jiroton from the NH3+ in the side chain, in 

 about 50 per cent of the excitations. In agreement with these ideas 



