Low-lying electronic excitations of the green fluorescent protein chromophore

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 proto nation states of the chromophore. The rich photophysical behaviour GFP exhi bits and the equilibrium between its protonation forms is influenced by bot h internal and external factors. Here, we characterize the structure and el ectronic spectra of the neutral and anionic forms of the chromophore in vac uo by restricted and unrestricted Hartree-Fock, by single excitation CI, an d by MCSCF/PT calculations. The calculated chromophore structure is in good accord with recently obtained crystallographic data, whereas the electroni c spectra agree with recent absorption and optical hole-burning studies. Th e low-lying singlet state excitations are solely due to pi-->pi* transition s and include a strong HOMO --> LUMO coupling in particular (oscillator str ength 1.54 for neutral chromphore and 2.19 for anionic chromophore). Vertic al excitation does not induce a significant charge transfer between both ri ngs but rather leads to a charge transfer between the two ring systems and the bridging group in both neutral and anionic chromophores. Furthermore, g eometry relaxation of the S-1-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 it s S-1 excited state. (C) 2000 Elsevier Science B.V. All rights reserved.