First-principle characterization of the energy landscape and optical spectra of the Green Fluorescent Protein along A-I-B proton transfer route
Structures and optical spectra of the green fluorescent protein (GFP) forms along the proton transfer route A-I-B are characterized by first-principle calculations. We show that in the ground electronic state the structure representing the wild type GFP with the neutral chromophore (form A) is lowest in energy, whereas the systems with the anionic chromophore (B and I forms) are about 1 kcal/mol higher. In the S65T mutant, the structures with the anionic chromophore are significantly lower in energy than systems with the neutral chromophore. The role of the nearby amino acid residues in the chromophore-containing pocket is re-examined. The calculations reveal that the structural differences between the I and B forms (the former has a slightly red-shifted absorption relative to the latter) are not based on the Thr203 orientation, but on the Glu222 position. In the case of wtGFP, the hydrogen bond between the chromophore and the His148 residue stabilizes the structures with the deprotonated phenolic ring in I and B forms. In the S65T mutant, concerted contributions from the His148 and Thr203 residues are responsible for a considerable energy gap between the lowest energy structure of the B type with the anionic chromophore from other structures.