On Mechanism of Enhanced Fluorescence in Green Fluorescent Protein

In spite of the numerous experimental and theoretical studies on green fluorescent protein and its modifications, there is still no definitive answer to the central question: why such systems exhibit enhanced fluorescence. Based upon detailed quantum-chemical estimations, we advocate the following hypothesis. In the green fluorescent protein ground electronic state, the protein surrounding strains the chromophore with respect to its native intramolecular conformational preference in vacuo or in solution. Absorbing a photon of the proper wavelength not only causes a joint proton–electron transfer in and around the chromophore, but also increases the intrinsic strain of the latter. Since conformational relaxation of such a structure will not require any additional energy input, the energy gained by the chromophore cannot be dissipated into the chromophore\u27s internal non-radiative degrees of freedom, and thus it returns as a fluorescence emission