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 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.