A photoelectron imaging and quantum chemistry study of the deprotonated cyan fluorescent protein chromophore anion

Cyan fluorescent protein (CFP), a blue-shifted analogue of green fluorescent protein, is used widely as a fluorescence resonance energy transfer donor in live cell fluorescence imaging. Here, we use a combination of anion photoelectron spectroscopy experiments and quantum chemistry calculations to probe the electronic structure of the CFP chromophore in its deprotonated anionic form. The vertical detachment energy measured as the maximum in the photoelectron spectrum is $2.75\pm 0.02$ eV, which is an improvement on our earlier measurement. We see evidence for competing internal conversion following resonant excitation of electronically excited states of the anion lying in the detachment continuum. We find that the first electronically excited valence state of the anion lies very close to the detachment threshold, supporting the conclusions of gas-phase photodestruction action spectroscopy measurements, and that the second electronically excited valence state lies around 1.25 eV above the detachment threshold. These electronic states have shape resonance character and mixed excited shape and Feshbach resonance character, respectively.