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Abstract

Green Fluorescent Protein (GFP) is a spontaneously fluorescent protein due to its p- hydroxylbenzylideneimidazolidinone chromophore. It has absorbance maxima at two different wavelengths that are attributed to different protonation states of the chromophore. The rich photophysical behaviour GFP exhibits and the equilibrium between its protonation forms is influenced by both internal and external factors. Here, we characterize the structure and electronic spectra of the neutral and anionic forms of the chromophore in vacuo by restricted and unrestricted Hartree–Fock, by single excitation CI, and by MCSCF/PT calculations. The calculated chromophore structure is in good accord with recently obtained crystallographic data, whereas the electronic spectra agree with recent absorption and optical hole-burning studies. The low-lying singlet state excitations are solely due to π→π∗ transitions and include a strong HOMO→LUMO coupling in particular (oscillator strength 1.54 for neutral chromphore and 2.19 for anionic chromophore). Vertical excitation does not induce a significant charge transfer between both rings but rather leads to a charge transfer between the two ring systems and the bridging group in both neutral and anionic chromophores. Furthermore, geometry relaxation of the S1-states employing planar symmetry constraints completely alters the bonding pattern of the two ring-bridging bonds, which reflects the intrinsic tendency of the chromophore for isomerization in its S1 excited state.