Unraveling the Photoionization Dynamics of Indole in Aqueous and Ethanol Solutions

Gaurav Kumar, Michael Kellogg, Shivalee Dey, Thomas A A Oliver*, Stephen E Bradforth*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review


The photoionization dynamics of indole, the Ultraviolet-B chromophore of tryptophan, were explored in water and ethanol using ultrafast transient absorption spectroscopy with 292, 268 and 200 nm excitation. By studying the femtosecond-to-nanosecond dynamics of indole in two different solvents a new photophysical model has been generated that explains many previously unsolved facets of indole’s complex solution phase photochemistry. Photoionization is only an active pathway for indole in aqueous solution, leading to a reduction in the fluorescence quantum yield in water rich environments, which is frequently used in biophysical experiments as a key signature of protein folded state. Photoionization of indole in aqueous solution was observed for all three pump wavelengths but via two different mechanisms. For 200 nm excitation, electrons are ballistically ejected directly into bulk solvent. Conversely, 292 and 268 nm excitation populates an admixture of two 1ππ* states, which form a dynamic equilibrium with a tightly bound indole cation and electron ion-pair. The ion-pair dissociates on a nanosecond timescale generating separated solvated electrons and indole cations. The charged species serve as important precursors to triplet indole production and greatly enhance the overall intersystem crossing rate. Our proposed photophysical model for indole in aqueous solution is the most appropriate for describing photoinduced dynamics of tryptophan in a polypeptide sequences – tryptophan in aqueous pH 7 solution is zwitterionic, unlike in peptides, and resultantly has a competitive excited state proton transfer pathway that quenches the tryptophan fluorescence.
Original languageEnglish
Pages (from-to)4158–4170
Number of pages13
JournalJournal of Physical Chemistry B
Issue number17
Early online date24 Apr 2024
Publication statusPublished - 2 May 2024

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© 2024 The Authors. Published by American Chemical Society


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