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Shallow Distance Dependence for Proton-Coupled Tyrosine Oxidation in Oligoproline Peptides
journal contribution
posted on 2020-06-29, 18:10 authored by Brian Koronkiewicz, John Swierk, Kevin Regan, James M. MayerWe
have explored the kinetic effect of increasing electron transfer
(ET) distance in a biomimetic, proton-coupled electron-transfer (PCET)
system. Biological ET often occurs simultaneously with proton transfer
(PT) in order to avoid the high-energy, charged intermediates resulting
from the stepwise transfer of protons and electrons. These concerted
proton–electron-transfer (CPET) reactions are implicated in
numerous biological ET pathways. In many cases, PT is coupled to long-range
ET. While many studies have shown that the rate of ET is sensitive
to the distance between the electron donor and acceptor, extensions
to biological CPET reactions are sparse. The possibility of a unique
ET distance dependence for CPET reactions deserves further exploration,
as this could have implications for how we understand biological ET.
We therefore explored the ET distance dependence for the CPET oxidation
of tyrosine in a model system. We prepared a series of metallopeptides
with a tyrosine separated from a Ru(bpy)32+ complex
by an oligoproline bridge of increasing length. Rate constants
for intramolecular tyrosine oxidation were measured using the
flash-quench transient absorption technique in aqueous solutions.
The rate constants for tyrosine oxidation decreased by 125-fold with
three added proline residues between tyrosine and the oxidant. By
comparison, related intramolecular ET rate constants in very
similar constructs were reported to decrease by 4–5 orders
of magnitude over the same number of prolines. The observed shallow
distance dependence for tyrosine oxidation is proposed to originate
in part from the requirement for stronger oxidants, leading to a smaller
hole-transfer effective tunneling barrier height. The shallow distance
dependence observed here and extensions to distance-dependent CPET
reactions have potential implications for long-range charge transfers.