Abstract
Indole is an important molecular motif in many biological molecules and exists in its deprotonated anionic form in the cyan fluorescent protein, an analogue of green fluorescent protein. However, the electronic structure of the deprotonated indole anion has been relatively unexplored. Here, we use a combination of anion photoelectron velocity-map imaging measurements and quantum chemistry calculations to probe the electronic structure of the deprotonated indole anion. We report vertical detachment energies (VDEs) of 2.45 ± 0.05 eV and 3.20 ± 0.05 eV, respectively. The value for D0 is in agreement with recent high-resolution measurements whereas the value for D1 is a new measurement. We find that the first electronically excited singlet state of the anion, S1(ππ*), lies above the VDE and has shape resonance character with respect to the D0 detachment continuum and Feshbach resonance character with respect to the D1 continuum.
Highlights
Indole, a prototypical polycyclic aromatic nitrogen heterocycle, plays an important role in defining the ultraviolet (UV) absorption and fluorescence spectra of tryptophan
The lowest electronically excited singlet state of the deprotonated indole anion to arise from a HOMO - p* transition that is delocalised across the whole anion
Improving our understanding of the electronic structure of indole and other small molecular building blocks that make up the chromophores of photoactive proteins is important for first principles design of new photoactive protein chromophores with specific characteristics
Summary
A prototypical polycyclic aromatic nitrogen heterocycle, plays an important role in defining the ultraviolet (UV) absorption and fluorescence spectra of tryptophan. The fluorescence from CFP is considerably less bright than that from GFP, suggesting that competing nonradiative decay processes play an important role in the excitedstate dynamics of the CFP protein. In some ways, this is quite remarkable considering that the only difference between CFP and GFP is the substitution of a para-phenol building block in the GFP chromophore by an indole building block in the CFP chromophore
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