Kinetics of switchable proton escape from a proton-wire within green fluorescence protein

The emission from the acidic form of the green fluorescence protein (GFP) changes with increasing time and temperature from t^-1/2 to t ^-3/2 asymptotics. It is shown that a model of proton diffusion along a one-dimensional hydrogen-bond network within the protein, with a switch (Thr203) allowing for proton escape, explains the data quantitatively. From a comparison of the model with experiment, we obtain the rate parameters for proton dissociation from the chromophore (showing an inverse temperature effect), the ratio of the proton association constant squared to its diffusion constant (exhibiting no temperature effect), and the time constant for switch opening (with a significant Arrhenius dependence). Thus, proton dissociation has a small negative activation energy (assigned to a complex of the anionic chromophore with H₃O⁺), whereas the switch has a large positive activation energy (assigned to Thr203 side-chain rotation). Proton migration is possibly the outcome of the concerted motion of several protons within GFP.