Photodetachment Spectra of Deprotonated Fluorescent Protein Chromophore Anions

Isolated model anion chromophores of the green and cyan fluorescent proteins were generated in an electrospray ion source, and their photodetachment spectra were recorded using photoelectron imaging. Vertical photodetachment energies of 2.85(10) and 4.08(10) eV have been measured for the model green fluorescent protein chromophore anion, corresponding to photodetachment from the ground electronic state of the anion to the ground and first excited electronic states of the radical, respectively. For the model cyan fluorescent protein chromophore anion, vertical photodetachment energies of 2.88(10) and 3.96(10) eV have been measured, corresponding to detachment from the ground electronic state of the anion to the ground and first excited electronic states of the neutral radical, respectively. We also find evidence suggesting that autoionization of electronically excited states of the chromophore anions competes with direct photodetachment. For comparison and to benchmark our measurements, the vertical photodetachment energies of deprotonated phenol and indole anions have also been recorded and presented. Quantum chemistry calculations support our assignments. We discuss our results in the context of the isolated protein chromophore anions acting as electron donors, one of their potential biological functions.