Deactivation Pathways of an Isolated Green Fluorescent Protein Model Chromophore Studied by Electronic Action Spectroscopy

The mechanism of fluorescence and fluorescence quenching of the green fluorescent protein (GFP) is not well-understood. To gain insight into the effect of the surrounding protein on the chromophore buried at its center, the intrinsic electronic absorption and deactivation pathways of a gaseous model chromophore, p-hydroxybenzylidene-2,3-dimethylimidazolone (HBDI) were investigated. No fluorescence from photoactivated gaseous HBDI− was detected in the range 480−1100 nm, in line with the ultrafast rate of internal conversion of HBDI− in solution. Two different gas-phase deactivation pathways were found: photofragmentation and electron photodetachment. Electronic action spectra for each deactivation pathway were constructed by monitoring the disappearance of HBDI− and appearance of product ions as a function of excitation wavelength. The action spectra measured for each pathway are distinct, with electron photodetachment being strongly favored at higher photon energies. The combined (total) gas-phase action spectrum has a band origin at 482.5 nm (23340 cm−1) and covers a broad spectral range, 390−510 nm. This extended gas-phase action spectrum exhibits vibronic activity that matches well with the results of previous cold condensed-phase experiments and high-level in vacuo computations, with features evident at +550, +1500, and +2800 cm−1 with respect to the band origin.