UV Response of the Green Fluorescent Protein Chromophore: Insights from Ab Initio Nonadiabatic Simulations

The green fluorescent protein (GFP) is widely used in imaging organisms at the subcellular level. However, upon exposure to UV or intense visible light, GFP undergoes irreversible reactions, altering its photocycle, which are believed to precede via photooxidation of the chromophore. The mechanism of this process is not well understood, even in the gas phase, with competing interpretations of photoelectron experiments on the isolated chromophore arguing either for nonadiabatic decay or intramolecular vibrational energy redistribution (IVR) and autoionization from an initially populated S₃ shape resonance. To address the controversy, we simulated the excited-state dynamics and time-resolved photoelectron spectroscopy (TRPES) of the GFP chromophore with ab initio multiple spawning and an on-the-fly multiconfigurational electronic structure using our dynamically weighted complete active space self-consistent field method. Our simulations show excellent agreement with experimental TRPES and reveal that S₃-S₂ nonadiabatic transitions do occur on an ultrafast time scale that can compete with autoionization; however, the conversion between the shape and Feshbach states occurs primarily adiabatically on the S₂ state. Furthermore, the threshold energies of the shape and Feshbach resonances are very similar, with a low barrier separating these regions of the PES, leading to both states being populated and reversibly interconverting within 50 fs of the initial photoexcitation. As a result, rapid autoionization still precedes via the shape resonance. The picture that emerges thus reconciles the two competing views of the GFP chromophore's UV response: both internal conversion and IVR from the shape resonance state are operative. Our findings of the involvement of the Feshbach state suggest new strategies to engineer FP chromophores with tailored photostabilities.