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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 p<i>K</i><sub>a</sub> values of participating groups and their electrostatic
interactions with their environment. For end groups that have acidic
or basic p<i>K</i><sub>a</sub> 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