Rates of deacttvation processes of indole derivatives in water-organic solvent mixtures - application to tryptophyl fluorescence of proteins

Abstract

The fluorescence quantum yield and the fluorescence decay of indole, 3-methy lindole, 1-methylindole and N-acetyltiyptophanamide have been measured in different water-dioxane mixtures. For the first three derivatives, the fluorescence decays were found independent of the emission wavelength and were analyzed as single exponential functions. In the case of N-methylindole the rate of the non radiative deactivation processes k nr increased linearly with the molar fraction of dioxane whereas for indole and scatole the variation of k nr was biphasic. This behaviour can be explained by two excited state deactivations of these non N-methylated compounds in water and high water content mixtures; one of these deactivation processes occuring through an hydrogen bond between the N-H group and a water molecule. The rate of non radiative deactivation of N-acetyltryptophanamide was dominated by the internal quenching involving the intramolecular carbonyl. The rate of the radiative deactivation process k F of these four compounds increased linearly with the wavenumber of the fluorescence spectrum maximum. Data relative to the three non N-methylated derivatives fell practically on the same straight line. Data from other works have been gathered in order to check if a similar relation between the intrinsic k F and XXX F values can exist for the tryptophyl fluorescence emission of proteins.