We have explored the kinetic effect of increasing
electron transfer distance in a biomimetic, proton coupled electron transfer system
(PCET). Biological electron transfer is often simultaneous with proton transfer
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 electron
transfer pathways. In many cases, proton transfer is coupled to long-range
electron transfer. While many studies have shown that the rate of electron
transfer is sensitive to the distance between the electron donor and acceptor,
extensions to biological CPET reactions are sparse. The possibility of a unique
electron transfer distance dependence for CPET reactions deserves further
exploration, as this could have implications for how we understand biological
electron transfer. We therefore explored the electron transfer 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)<sub>3</sub><sup>2+</sup>
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 prolines 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, at least in part,
from the requirement for stronger oxidants, leading to a smaller hole transfer
tunneling barrier height. The shallow distance dependence observed here and
extensions to distance dependent CPET reactions have far-reaching implications
for long-range charge transfers
Distance dependence of long-range electron transfer through helical peptides
