Controlling Light-Induced Proton Transfer from the GFP Chromophore

Green Fluorescent Protein (GFP) is known to undergo excited‐state proton transfer (ESPT). Formation of a short H‐bond favors ultrafast ESPT in GFP‐like proteins, such as the GFP S65T/H148D mutant, but the detailed mechanism and its quantum nature remain to be resolved. Here we study in vacuo, light‐induced proton transfer from the GFP chromophore in hydrogen‐bonded complexes with two anionic proton acceptors, I⁻ and deprotonated trichloroacetic acid (TCA⁻). We address the role of the strong H‐bond and the quantum mechanical proton‐density distribution in the excited state, which determines the proton‐transfer probability. Our study shows that chemical modifications to the molecular network drastically change the proton‐transfer probability and it can become strongly wavelength dependent. The proton‐transfer branching ratio is found to be 60 % for the TCA complex and 10 % for the iodide complex, being highly dependent on the photon energy in the latter case. Using high‐level ab initio calculations, we show that light‐induced proton transfer takes place in S₁, revealing intrinsic photoacid properties of the isolated GFP chromophore in strongly bound H‐bonded complexes. ESPT is found to be very sensitive to the topography of the highly anharmonic potential in S₁, depending on the quantum‐density distribution upon vibrational excitation. We also show that the S₁ potential‐energy surface, and hence excited‐state proton transfer, can be controlled by altering the chromophore microenvironment.