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

The fluorescence quantum yield and the fluorescence decay of indole, 3-methylindole, 1-methylindole and N-acetyltryptophanamide 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 knr increased linearly with the molar fraction of dioxane whereas for indole and scatole the variation of knr 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 kF 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 kF and vF values can exist for the tryptophyl fluorescence emission of proteins.