Proton transfer dependence on hydrogen-bonding of solvent to the water wire: a theoretical study

The mechanism and dynamics of double proton transfer dependence on hydrogen-bonding of solvent molecules to the bridging water in a water wire were studied by a direct ab initio dynamics approach with variational transition-state theory including multidimensional tunneling. Long-range proton transfers in solution and within enzymes may have very different mechanisms depending on the pK(a) values of participating groups and their electrostatic interactions with their environment. For end groups that have acidic or basic pK(a) values, proton transfers by the classical Grotthuss and "proton-hole" transfer mechanisms, respectively, are energetically favorable. This study shows that these processes are facilitated by hydrogen-bond accepting and donating solvent molecule interactions with the water wire in the transition state (TS), respectively. Tunneling also depends very much on the hydrogen bonding to the water wire. All molecules hydrogen bonded to the water wire, even if they raised and narrowed energy barriers, reduced the tunneling coefficients of double proton transfer, which was attributed to the increased effective mass of transferring protons near the TS. The theoretical HH/DD KIE, including tunneling, was in good agreement with experimental KIE values. These results suggest that the classical Grotthuss and proton-hole transfer mechanisms require quite different solvent (or protein) environments near the TS for the most efficient processes.