Structure and rotation barriers for ground and excited states of the isolated chromophore of the green fluorescent protein

Semiempirical CISD calculations were carried out on models of differently protonated forms of the isolated chromophore active in the green fluorescent protein (GFP). Electronic excitation (S₀→S₁) is found to considerably alter the equilibrium conformation of the chromophore. Low activation barriers to rotation about the exocyclic bond C_γ-C_β adjacent to the phenol ring are calculated for the cationic form in both states, S₀ and S₁, for the neutral chromophore in the S₀, and for the zwitterion in the S₁ state. It is of interest that only in the protonated state the energy gap between the two potential energy surfaces is small enough to allow internal conversion of the isolated chromophore. We propose that the fast internal conversion observed in GFP mutants is also associated with an avoided crossing of the S₀ and S₁ surfaces of the cation during the rotation around the bond C_β-C_α.