Unravelling the Turn-On Fluorescence Mechanism of a Fluorescein-Based Probe in GABA Receptors.

GABA (γ-aminobutyric acid type A) receptors are ligand-gated ion channels mediating fast inhibitory transmission in the mammalian brain. Here we report the molecular and electronic mechanism governing the turn-on emission of a fluorescein-based imaging probe able to target the human GABA receptor. Multiscale calculations evidence a drastic conformational change of the probe from folded in solution to extended upon binding to the receptor. Intramolecular ππ-stacking interactions present in the folded probe are responsible for quenching fluorescence in solution. In contrast, unfolding within the GABA receptor changes the nature of the bright excited state triggering emission. Remarkably, this turn-on effect only manifests for the dianionic prototropic form of the imaging probe, which is found to be the strongest binder to the GABA receptor. This study is expected to assist the design of new photoactivatable screening tools for allosteric modulators of the GABA receptor.