Cation-enhanced deprotonation of water by a strong photobase

We have used picosecond fluorescence spectroscopy to study the proton-dissociation dynamics of bulk water and H²O molecules solvating Mg²⁺ ions in aqueous solutions. We have analyzed the photo-initiated proton-transfer reaction to a photobase 6-aminoquinoline by the Collins-Kimball approach and have modeled the ensuing bimolecular reaction dynamics by the Smoluchowski equation with radiation boundary conditions. We have found the on-contact proton transfer rate to follow the Marcus free-energy relation for proton transfer and estimate by this rate-equilibrium correlation the considerable enhancement in the acidity of the water molecules solvating the Mg²⁺ ion. Our findings may be used in the study of metalloenzymes such as carbonic anhydrases (CAs), which catalyze the reversible addition reaction of OH^- to CO² by increasing the reactivity of the zinc-bound water molecules by means of stabilizing the product of water dissociation, the OH^- anion.