Excited-state dynamics in the green fluorescent protein chromophore

The remarkable suppression of radiationless decay by the green fluorescent protein (GFP) is investigated through ultrafast fluorescence spectroscopy of its isolated chromophore in solution. Decay data are measured by fluorescence up-conversion as a function of solvent and wavelength for both neutral and anionic forms of the chromophore. All fluorescence decays are found to be well described by two exponentially decaying components. The effect of medium viscosity is slight, suggesting that the intramolecular motion promoting radiationless decay is a volume-conserving one. A minor effect of solvent polarity and H-bonding ability on the decay times is observed. The two decay constants are independent of emission wavelength, but their relative weights are not. Time-resolved fluorescence spectroscopy shows that the Stokes shift is complete in <100 fs, and that subsequent spectral evolution is limited to a small spectral narrowing. These data are discussed in terms of a two-state two-mode model, originally proposed to describe isomerization in bacteriorhodopsin (Gonzalez-Luque et al. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 9379). It is suggested that modification to the displacement and curvature of the excited-state potential energy surface of the chromophore by the protein may be sufficient to account for the dramatic enhancement of chromophore fluorescence in GFP.