Quantum Mechanical Enhancement of Rate Constants and Kinetic Isotope Effects for Water-Mediated Proton Transfer in a Model Biological System

Biological
systems have been shown to shuttle excess protons long
distances by taking advantage of tightly organized hydrogen-bonded
water bridges in hydrophobic protein cavities, and similar effects
have been observed in carbon nanotubes. In this theoretical study
we investigate how quantum effects of proton motion impact the rate
constants for charge transfer in a model system consisting of a donor
and acceptor molecule separated by a single-molecule water bridge.
We calculate quantum and classical rate constants for the transfer
of an excess proton over two possible paths, one with an H3O+ intermediate, and one with an OH– intermediate. Quantum effects are included through ring polymer
molecular dynamics (RPMD) calculations. We observe a 4-fold enhancement
of reaction rate constants due to proton tunneling at temperatures
between 280 and 320 K, as shown by transmission coefficient calculations.
Deuteration of the donor and acceptor proton are shown to decrease
the reaction rate constant by a factor of 50, and this is another
indicator that tunneling plays an important role in this proton transfer
mechanism.