1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Department of Physics and Astronomy, Science and Technology, Aarhus University
This chapter combines recent advances in understanding the photophysics of the chromophore anion of the Green Fluorescent Protein (GFP) from the jellyfish Aequorea Victoria. GFP and its homologues are widely used for in vivo labeling in biology through their remarkable fluorescent properties. Besides long-timescale light emission, the GFP proteins also show an unusual diversity in terms of their non-radiative excited-state decay channels, including ultrafast conical intersection dynamics and light-driven electron transfer, where GFP acts as an electron donor in photochemical reactions. Knowledge of intrinsic properties of the GFP photoabsorbing molecular unit is a prerequisite in understanding the atomic-scale interactions that play a key role for the diverse functioning of these proteins. Here, we show how recent developments in action and photoelectron spectroscopy combined with state-of-the-art electronic structure theory provide valuable insights into photo-initiated quantum dynamics and enable to disclose mechanisms of multiple intrinsic excited-state decay channels in the bare GFP chromophore anion. When taken out of the protein, the deprotonated chromophore exhibits the ultrafast excited state dynamics, where non-radiative decay occurs on a (sub)picosecond timescale. Deactivation includes resonant electron emission and fast internal conversion followed by slow statistical decay in the vibrationally hot ground state. Remarkably, both electronic and nuclear excited-state decay channels may here efficiently compete with each other in spite of their inherently different intrinsic timescales. The reason behind this is an efficient coupling between the nuclear and electronic motion in the photo-initiated dynamics, where the energy may be transferred from nuclei to electrons and from electrons to nuclei mediated by specific vibrational modes. Prompt photodetachment occurs indirectly through vibrational autodetachment out of the first excited state within the energy range of the corresponding absorption band. Time-resolved transient photoelectron spectroscopy confirms the ultrafast decay of the excited-state population through internal conversion, which here proceeds through a conical intersection seam. We discuss the ways, by which the GFP proteins may use such efficient electron-to-nuclei coupling revealed in the intrinsic excited-state decay of their chromophore, to guide the photochemistry and photophysics upon which their functioning is based.