Taking the green fluorescence out of the protein: dynamics of the isolated GFP chromophore anion

The green fluorescent protein (GFP) is employed extensively as a marker in biology and the life sciences as a result of its spectacular fluorescence properties. Here, we employ femtosecond time-resolved photoelectron spectroscopy to investigate the ultrafast excited state dynamics of the isolated GFP chromophore anion. Excited state population is found to decay bi-exponentially, with characteristic lifetimes of 330 fs and 1.4 ps. Distinct photoelectron spectra can be assigned to each of these timescales and point to the presence of a transient intermediate along the decay coordinate. Guided by ab initio calculations, we assign these observations to twisting about the C-C-C bridge followed by internal conversion to the anion ground state. The dynamics in vacuo are very similar to those observed in solution, despite the difference in absorption spectra between the two media. This is consistent with the protein environment restricting rotation about the C-C-C bond in order to prevent ultrafast internal conversion and preserve the fluorescence.