Radiationless relaxation in a synthetic analogue of the green fluorescent protein chromophore

The fluorescence and ultrafast ground-state recovery times of the isolated chromophore of the green fluorescent protein have been studied in basic alcohol solutions. The fluorescence quantum yield increases more than 10(3) times between 295 and 77 K. The major part of the increase occurs in the supercooled liquid range, and continues below the glass transition. The ground-state recovery at 295 K is essentially (95%) complete in under 5 ps, is nonexponential, and only weakly dependent on solvent viscosity. These results are inconsistent with a viscosity-controlled radiationless process involving large scale intramolecular reorganization. If intramolecular motion is involved it must be of small scale. Alternative mechanisms are discussed. A thermally activated radiationless decay process is consistent with the present data, but the mechanism is unclear. For either mechanism the high quantum yield in the intact protein must arise through protein-chromophore interactions which effectively suppress the radiationless channel.