832 LIGHT AND LIFE 



A / that may be calculated from the dielectric constant and refrac- 

 tive index of the medium. In aqueous solution the quantum yields 

 for the three species of molecule are 0.04 for phenylalanine, 0.21 for 

 tyrosine, and 0.20 for tryptophan. The influence of pH upon the 

 efficiency shows that protons (including those derived from the 

 — COOH and — NH3+ groups of the amino acid) can quench the 

 fluorescence of both tyrosine and tryptophan (A. White) . Both amino 

 acids are half-quenched at pH 2.3, which is the pK of the carboxyl 

 group; whereas esters and decarboxylated derivatives are quenched 

 only at pH —0.55 in the phenol series and pH -f-0.5 in the indole 

 series. In the alkaline range, members of the phenol series lose all 

 fluorescence upon ionizing to the phenolate ion. Tryptophan and its 

 derivatives exhibit a conspicuous increase in fluorescence at the pH 

 at which the — NH3+ groups are discharged. The — NHo form of 

 the molecule has fluorescence efficiencies up to 0.6 which signifies 

 that about half the time an excited indole ring can attract a proton 

 from the — NH3+ group on the side-chain. Substances such as acetic 

 acid and hydroxylamine, which can directly donate protons to the 

 excited ring, consequently quench the fluorescence of molecules of 

 this series. The fluorescence of indole and tryptophan is also quenched 

 in alkaline solution, probably through a reaction of the excited mole- 

 cule with OH- to ionize the N of the indole ring, followed by a non- 

 radiative transition to the ground state. Substitution of a methyl 

 group for H on the indole N atom blocks this quenching effect. The 

 contribution of the ionic structure to the excited state is thus very 

 large, and in certain naphthylamine derivatives may reach 50 per 

 cent. Further evidence for the importance of proton carrier groups 

 comes from studies of glycyltyrosine and glycyltryptophan, in which 

 the —COOH and — NH3+ groups are farther removed from the 

 excited ring than in the simple aromatic amino acids. In water these 

 have very low quantum yields (ca. 0.04) , which rise to about normal 

 values of 0.2 in a medium of high viscosity (Teale) . 



No direct measurements of the lifetime of the excited states in the 

 aromatic amino acids have been made. From the oscillator strength 

 of the molecule and lifetime together with the quantum yield for 

 the fluorescence, the lifetime of excited tyrosine is calculated to be 

 about 8 ni/xsec, and that of tryptophan 2.5 n^sec. 



The polarization of the emitted fluorescence, measured in viscous 

 media, enables one to gain important information on two points: (1) 

 l)y making measurements at vanishing concentration and with dif- 



