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

• Published in: The journal of physical chemistry. B Vol. 120; no. 35; pp. 9308 - 9321
• Main Authors: Markle, Todd F, Zhang, Ming-Tian, Santoni, Marie-Pierre, Johannissen, Linus O, Hammarström, Leif
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 08.09.2016
• Subjects: Correlation; Electron transfer; Isotope effect; Mathematical models; Pyridines; Reaction kinetics; Tunneling; Tyrosine
• ISSN: 1520-6106; 1520-5207; 1520-5207
• DOI: 10.1021/acs.jpcb.6b05885

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 = k H/k D), 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.