Ultrafast excited and ground-state dynamics of the green fluorescent protein chromophore in solution

Ultrafast dispersed pump-dump-probe spectroscopy was applied to HBDI (4'-hydroxybenzylidene-2,3-dimethyl-imidazolinone), a model green fluorescent protein (GFP) chromophore in solution with different protonation states. The measured three-dimensional data was analyzed using a global analysis method that enables the spectral and temporal characterization of overlapping photoinduced transient states. A unified phenomenological model is presented to describe the observed data. Two excitation pathways are identified: a 1-photon excited-state twisting and a 2-photon ionization process. The ionization pathway results in the generation of solvated electrons and HBDI radicals. The twisting dynamics was resolved on both electronic states with slower twisting on the ground state than the excited state. This is ascribed to the multidimensional hula-twist mechanism. A weak viscosity dependence was observed when the aqueous solution data were contrasted with the signals collected in a 66% glycerol/water solution.