Competition between proton and H-atom transfer: The role of the chromophore environment in the green fluorescent protein

Excited state ab initio calculations were performed on clusters mimicking the green fluorescent protein (GFP) to understand the competition between excited-state proton/H-atom transfer (ESPT/ESHAT) from the OH group of the chromophore to the hydrogen bonded wire. A crossing between the ππ∗ and a πσ∗ states occurs when the OH bond length is stretched leading to ESHAT for the chromophore hydrogen bonded to the water molecule. This crossing is a property of the chromophore itself and can be modulated by further solvation of the chromophore hydroxyl group which can then induce an ESPT/ESHAT competition.