Protonation effects on the chromophore of green fluorescent protein. Quantum chemical study of the absorption spectrum

The absorption spectrum of the chromophore of the green fluorescent protein (GFP) was investigated by INDO/S-CI model calculations. Geometrical variations of the hydrogen bonds between the chromophore and its environment were studied in detail. Based on the good agreement between experimental and calculated data, the spectrum of GFP is assigned. The absorptions at 397 and 477 nm are due to the chromophore cation and zwitterion, respectively. The phenolic oxygen of the chromophore is deprotonated in the excited state, but can be protonated or deprotonated in the ground state. The heterocyclic nitrogen remains protonated after excitation.