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

Abstract

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.

GRAPHICAL ABSTRACT

Keywords:

• Anion photoelectron spectroscopy
• autodetachment
• fluorescent proteins
• photochemistry
• photophysics

Acknowledgments

The corresponding author is dedicating this paper to Professor Tim Softley to thank him for his encouragement and support and for providing her with the best possible start to her career. We are grateful to Dr Richard Fitzmaurice for synthesising the CFP chromophore, Professor Graham Worth for advice on the computational chemistry calculations and Dr Frank Otto for computational support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) [EP/L005646/1] and Royal Society and Leverhulme Trust [SRF/R1/180079].