title: 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.
authors.affiliation: Department of Chemistry - Ångström Laboratory, Uppsala University , P.O. Box 523, S-75120 Uppsala, Sweden.
authors.forename: Todd F
authors.initials: TF
authors.lastname: Markle
authors.affiliation: Department of Chemistry - Ångström Laboratory, Uppsala University , P.O. Box 523, S-75120 Uppsala, Sweden.
authors.forename: Ming-Tian
authors.initials: MT
authors.lastname: Zhang
authors.affiliation: Department of Chemistry - Ångström Laboratory, Uppsala University , P.O. Box 523, S-75120 Uppsala, Sweden.
authors.forename: Marie-Pierre
authors.initials: MP
authors.lastname: Santoni
authors.affiliation: Department of Chemistry - Ångström Laboratory, Uppsala University , P.O. Box 523, S-75120 Uppsala, Sweden.
authors.forename: Linus O
authors.initials: LO
authors.lastname: Johannissen
authors.affiliation: Department of Chemistry - Ångström Laboratory, Uppsala University , P.O. Box 523, S-75120 Uppsala, Sweden.
authors.forename: Leif
authors.initials: L
authors.lastname: Hammarström