Photoabsorption studies of neutral green fluorescent protein model chromophores in vacuo

We report on gas-phase experimental and theoretical studies on the neutral form of the green-fluorescent protein (GFP) chromophore using six different models, each carrying a spectator positive charge. Theoretical studies were carried out to quantify the effect of the spectator charge on the absorption maximum of the true neutral. The study also includes models having the possibility of forming intra-molecular hydrogen bonds, and their effect on the absorption profile is analyzed. The charge redistribution caused by a strong intra-molecular hydrogen bond was found to give rise to a red shift in going from non-hydrogen bonded to hydrogen bonded models. For the non-hydrogen bonded models, the length of the side chain as well as the group carrying the spectator charge, was varied to explore the possibility of shifts in absorption maximum due to these variations. No shifts were observed. The implications of these results in tuning the absorption maximum of the neutral form of the GFP chromophores are discussed.