The effect of environment on the dynamics of proton dissociation in water

The effect of the local ordering of water molecules, adjacent to the molecular surface, on the dynamics of excited state proton transfer to bulk was monitored with the pyranine-γ-CD inclusion complex as a model. The bound pyranine (a commonly used photoacid) exhibits slower dissociation dynamics, with activation energy of the proton dissociation reaction that is significantly higher than that of the reaction of the free pyranine. To understand the source of these modulations of the rate constants, the interaction of the pyranine with the water was investigated by molecular dynamics calculations. The solvation patterns of both; the pyranine and γ-CD, differ in the complex from that of the free compounds. In the case of the pyranine's hydroxyl, the inclusion in the torus of the γ-CD reduces the number of water molecules in its immediate vicinity and their ordering, thus accounting for the variation in the rates of the proton transfer reactions. On increasing the temperature, the water of the pyranine's hydroxyl third solvation shell, but not those of the first and the second shell, lose their strict orientation, thus facilitating the dissociation of the pyranine.