Electronic Excitations of Green Fluorescent Proteins:  Modeling Solvatochromatic Shifts of Red Fluorescent Protein Chromophore Model Compound in Aqueous Solutions

While green fluorescent proteins (GFPs) have been widely used as tools in biochemistry, cell biology, and molecular genetics, novel red fluorescent proteins (RFPs) with red fluorescence emission have also been identified, as complements to the existing GFP technology. The unusual spectrophotometric and fluorescence properties of GFPs and RFPs are controlled by the protonation states and possibly cis/trans isomerization within their chromophores. In this work, we have investigated the electronic structures, liquid structures, and solvent shifts of the possible neutral and anionic protonated states and the cis/trans isomerization of a RFP chromophore model compound HBMPI in aqueous solutions. The calculations reproduced the experimental absorption solvatochromatic shifts of dilute HBMPI in water under neutral and anionic conditions. Unlike the GFP chromophore, the RFP chromophore model compound HBMPI in basic solution can only adopt a conformation where the CC bond between the bridge group and the imidazolinone ring and the C−C bond between the imidazolinone and ethylene groups exist in cis and trans conformations, respectively. Moreover, the solvent−solute hydrogen-bonding interactions are found to contribute significantly to the total solvent shifts of π−π* excitations of aqueous HBMPI solutions, signifying the importance of protein environment in the determination of the conformation of the chromophores in red fluorescent proteins.