Fluorescence Lifetimes of Tryptophan: Structural Origin and Relation with S_o → ¹L_b and S_o → ¹L_a Transitions

Abstract

We measured fluorescence lifetimes of L-Tryptophan dissolved in de-ionized water and in ethanol in the absence and the presence of high progesterone concentrations. The hormone absorbs between 220 and 280 with a peak around 250 nm, while its absorption is equal to zero beyond 280 nm. Tryptophan excitation spectrum recorded in presence of progesterone shows that the S_o → ¹L_a transition is completely abolished while the S_o → ¹L_b transition is not affected. Emission of L-tryptophan in water occurs with two fluorescence lifetimes, 0.40 and 2.8 ns. In ethanol, three fluorescence lifetimes equal to around 0.2, 1.8 and 4.8 ns were observed. Addition of progesterone to the medium does not affect any of the fluorescence lifetimes indicating clearly that both transitions could induce tryptophan excitation and that recorded fluorescence lifetimes could be assigned to sub-structures generated in the excited state.