Photofragmentation of and electron photodetachment from a GFP model chromophore in a quadrupole ion trap

Abstract

Although green fluorescent protein (GFP) is widely used in the biological sciences, the photophysics controlling GFP fluorescence versus non-radiative decay pathways are not well understood. In previous work (Forbes and Jockusch, J. Am. Chem. Soc. 2009), we reported that a gaseous anionic model chromophore of GFP, p-hydroxybenzylidene-2,3-dimethylimidazolone (HBDI −), deactivates via electron photodetachment ( ePD) as well as photofragmentation. Distinct electronic action spectra were measured for these two pathways upon activation of HBDI − in a quadrupole ion trap (QIT) mass spectrometer. Here, we explore the mechanisms of HBDI − dissociation following photoexcitation at two characteristic wavelengths: 410 nm, at which the dominant pathway is ePD and at 480 nm, at which the branching ratio between ePD and fragmentation depends strongly on the experimental conditions employed. The results indicate that ePD from HBDI − at 410 nm is a single photon process with a rate that is unaffected by changing the pressure of the helium bath gas. This is consistent with a prompt electron detachment process at 410 nm. At 480 nm, photofragmentation in the QIT requires absorption of more than one photon and is suppressed by collisions, while ePD results from both single and multiple photon absorption. A model that accounts for all these observations is discussed. ePD and effective absorption cross sections are estimated as is the rate of collisional cooling within the QIT. This work explores the connection between absorption and action spectroscopy as applied to the photophysics of a biologically important chromophore.