Theoretical Study of the Substituent Effect on the Electronic Excited States of Chromophore in Cyan Fluorescent Proteins

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We performed ab initio molecular orbital and density functional theory calculations for enhanced cyan fluorescent protein, gold fluorescent protein, and (4-Me)-tryptophan enhanced cyan fluorescent protein to elucidate the substituent effect on the electronic structure of the chromophore in the fluorescent proteins. The electron correlation effect on the electronic structures is investigated as well. Our calculated excitation and deexcitation energies reasonably reproduce the corresponding experimental values. The qualitative Stokes shifts for the three fluorescent proteins were realized taking the electronic correlation into account. We found that the amino group of the chromophore in gold fluorescent protein enhances charge transfer among the ground and excited states, while the substituent effect of the methyl group, which is a weak electron donor, in (4-Me)-tryptophan enhanced cyan fluorescent protein was small. The charge transfer is the origin of the Stokes shift found in gold fluorescent protein. Thus, we succeeded in elucidating the substituent effect on the electronic structure of the chromophore in three fluorescent proteins.