The mechanism of proton transfer between adjacent sites on the molecular surface

The surface of a protein, or a membrane, is spotted with a multitude of proton binding sites, some of which are only few Å apart. When a proton is released from one site, it propagates through the water by a random walk under the bias of the local electrostatic potential determined by the distribution of the charges on the protein. Eventually, the released protons are dispersed in the bulk, but during the first few nanoseconds after the dissociation, the protons can be trapped by encounter with nearby acceptor sites. While the study of this reaction on the surface of a protein suffers from experimental and theoretical difficulties, it can be investigated with simple model compounds like derivatives of fluorescein. In the present study, we evaluate the mechanism of proton transfer reactions that proceed, preferentially, inside the Coulomb cage of the dye molecules. Kinetic analysis of the measured dynamics reveals the role of the dimension of the Coulomb cage on the efficiency of the reaction and how the ordering of the water molecules by the dye affects the kinetic isotope effect.