| Abstract
| - The remarkable suppression of radiationless decay by the green fluorescent protein (GFP) is investigatedthrough ultrafast fluorescence spectroscopy of its isolated chromophore in solution. Decay data are measuredby fluorescence up-conversion as a function of solvent and wavelength for both neutral and anionic forms ofthe chromophore. All fluorescence decays are found to be well described by two exponentially decayingcomponents. The effect of medium viscosity is slight, suggesting that the intramolecular motion promotingradiationless decay is a volume-conserving one. A minor effect of solvent polarity and H-bonding ability onthe decay times is observed. The two decay constants are independent of emission wavelength, but theirrelative 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 arediscussed in terms of a two-state two-mode model, originally proposed to describe isomerization inbacteriorhodopsin (Gonzalez-Luque et al. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 9379). It is suggested thatmodification to the displacement and curvature of the excited-state potential energy surface of the chromophoreby the protein may be sufficient to account for the dramatic enhancement of chromophore fluorescence inGFP.
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