Proton-Coupled Electron Transfer in a Series of Ruthenium-Linked Tyrosines with Internal Bases: Evaluation of a Tunneling Model for Experimental Temperature-Dependent Kinetics

Abstract

Photoinitiated proton-coupled electron transfer (PCET) kinetics has been investigated in a series of four modified tyrosines linked to a ruthenium photosensitizer in acetonitrile, with each tyrosine bearing an internal hydrogen bond to a covalently linked pyridine or benzimidazole base. After correcting for differences in driving force, it is found that the intrinsic PCET rate constant still varies by 2 orders of magnitude. The differences in rates, as well as the magnitude of the kinetic isotope effect (KIE = kH/kD), both generally correlate with DFT calculated proton donor-acceptor distances. An Arrhenius analysis of temperature dependent data shows that the difference in reactivity arises primarily from differences in activation energies. We use this kinetic data to evaluate a commonly employed theoretical model for proton tunneling which includes a harmonic distribution of proton donor-acceptor distances due to vibrational motions of the molecule. Applying this model to the experimental data yields the conclusion that donor-acceptor compression is more facile in the compounds with shorter PT distance; however, this is contrary to independent calculations for the same compounds. This discrepancy is likely because the assumption in the model of Morse-shaped proton potential energy surfaces is inappropriate for (strongly) hydrogen-bonded systems. These results question the general applicability of this model. The results also suggest that a correlation of rate vs proton tunneling distance for the series of compounds is complicated by a concomitant variation of other relevant parameters.

Shallow Distance Dependence for Proton-Coupled Tyrosine Oxidation in Oligoproline Peptides

“…This approach follows that developed for similar systems without prolines by the Hammarström laboratory. 26,29,[47][48][49][50][51][52][53][54] Oligoprolines were chosen as the bridge in our system because of the rich precedent for using this motif as a semi-rigid spacer in water, where they are known to adopt polyproline II (PPII) helical structures. [55][56][57][58][59][60][61][62][63][64] Furthermore, there are numerous studies of ET distance dependence utilizing oligoproline bridges, (Figure 2A), which consistently found β ≥ 1.2 Å -1 .…”

Activationless Multiple-Site Concerted Proton–Electron Tunneling

“…The phenol-pyridine unit has the typical planar structure with a short OH⋯N hydrogen bond [ d (O⋯N) = 2.566(5) Å]. 12,14…”

Reconciling Imbalanced Transition State Effects with Nonadiabatic CPET Reactions

“…Several formulations for this coupling exist, 47,85-87 but it is commonly taken as the product of an electronic coupling and the Franck-Condon overlap between proton wavefunctions. 7,40,88,89 Thus, instead of representing proton transfer with a single potential energy surface characterized by an activation barrier to climb, this model envisions two separate potential energy wells which represent the restoring force holding the proton on either the reactant or the product. The vibronic overlap of the proton's wavefunctions in these two wells, along with the with the electronic coupling between them, takes the place of the barrier height in determining the rate of proton transfer.…”

“…Model systems for nonadiabatic CPET typically use either parabolic or Morse potentials which are parameterized with the stretching frequency and bond dissociation energy of the relevant X-H bond. 7,40,88,89 However, it has been noted these potential energy wells do not adequately capture the anharmonicity seen in DFT-calculated potential wells of specific systems. [89][90][91][92][93] This increased anharmonicity results in qualitatively wider potential wells, with a shallower rise away from the minima.…”