An ultrafast polarisation spectroscopy study of internal conversion and orientational relaxation of the chromophore of the green fluorescent protein

Ultrafast polarisation spectroscopy is employed to investigate excited state relaxation in the green fluorescent protein (GFP) chromophore in anion and neutral forms. The dominant relaxation channel is internal conversion (IC). The rate of IC is larger in the neutral than the anion. In both the relaxation time is a few picoseconds and independent of viscosity. This rules out IC induced by large scale intramolecular reorganisation. Alternative mechanisms are discussed. How the protein so effectively suppress this radiationless channel remains unclear. A second slower component in the ground state recovery indicates the existence of a bottleneck. This component arises through diffusive reorientation.