Dipole-bound states in the meta form of the green fluorescent protein chromophore observed by cryogenic action spectroscopy

Abstract

We report the observation of states arising from the interaction of an excess electron with the dipole moment of a biochromophore, the meta form of the green fluorescent protein (GFP) model chromophore p-hydroxybenzylidene-2,3-dimethylimidazolinone (mHBDI), using cryogenic ion action spectroscopy. Distinct spectral features appear in the 19 400–20 100 cm⁻¹ region near the electron detachment threshold of mHBDI. Dipole-bound (DBS) and dipole-resonance (DRS) states are identified below and above the detachment energy, respectively. The vertical detachment energy is determined to be 19 620 cm⁻¹. The DBS-band origin is at 19 444 cm⁻¹ with a binding energy of 176 cm⁻¹. Complementary DFT (ωB97X-D/aug-cc-pVTZ) calculations provide insight into the structure and vibrational resonances of the excess electron in this biologically relevant chromophore. The results extend the understanding of dipole-bound and resonance states in complex molecular systems. We find that one low-energy mode of the neutral radical is particularly Franck–Condon active. The same mode has previously been found to dominate absorption spectra of other GFP-chromophore derivatives. It is suggested that the DRSs are potential doorway states for capture of low energy electrons.