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Title: Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction

Abstract

Proton-coupled electron-transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII) inspired [Ru(bpy) 2(phen-imidazole-Ph(OH)( tBu) 2)] 2+, in which Ru(III) generated by a flash-quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the Ru III/IIcouple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically favorable by both stepwise or concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with k ET = 2.7 × 10 7 s ₋1 and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λ DA = 1.16 eV and 280 < H DA < 540 cm ₋1 consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen evolving complex (OEC). In our PSII-inspired complex, the recombination reaction activation energy is < 2 kcal mol ₋1. In conclusion, the reaction is nonadiabatic (V PCET ~ 22 cm ₋1more » (H) and 49 cm ₋1 (D)), concerted, and exhibits an unexpected inverse KIE of 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.« less

Authors:
 [1];  [1];  [1]
  1. Brookhaven National Lab. (BNL), Upton, NY (United States). Chemistry Division
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1336211
Report Number(s):
BNL-113231-2016-JA
Journal ID: ISSN 0002-7863; R&D Project: CO026; KC0304030
Grant/Contract Number:
SC00112704
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 138; Journal Issue: 36; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Manbeck, Gerald F., Fujita, Etsuko, and Concepcion, Javier J. Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction. United States: N. p., 2016. Web. doi:10.1021/jacs.6b03506.
Manbeck, Gerald F., Fujita, Etsuko, & Concepcion, Javier J. Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction. United States. doi:10.1021/jacs.6b03506.
Manbeck, Gerald F., Fujita, Etsuko, and Concepcion, Javier J. 2016. "Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction". United States. doi:10.1021/jacs.6b03506. https://www.osti.gov/servlets/purl/1336211.
@article{osti_1336211,
title = {Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction},
author = {Manbeck, Gerald F. and Fujita, Etsuko and Concepcion, Javier J.},
abstractNote = {Proton-coupled electron-transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII) inspired [Ru(bpy)2(phen-imidazole-Ph(OH)(tBu)2)]2+, in which Ru(III) generated by a flash-quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the RuIII/IIcouple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically favorable by both stepwise or concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with kET = 2.7 × 107 s₋1 and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λDA = 1.16 eV and 280 < HDA < 540 cm₋1 consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen evolving complex (OEC). In our PSII-inspired complex, the recombination reaction activation energy is < 2 kcal mol₋1. In conclusion, the reaction is nonadiabatic (VPCET ~ 22 cm₋1 (H) and 49 cm₋1 (D)), concerted, and exhibits an unexpected inverse KIE of 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.},
doi = {10.1021/jacs.6b03506},
journal = {Journal of the American Chemical Society},
number = 36,
volume = 138,
place = {United States},
year = 2016,
month = 8
}

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  • Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole–phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. Furthermore, when the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Furthermore, theory predicts a decrease in themore » redox potential of the phenol by ~300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Our results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.« less
  • Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole–phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. Furthermore, when the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Furthermore, theory predicts a decrease in themore » redox potential of the phenol by ~300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Our results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.« less
  • Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays comprising different benzimidazole-phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electro-chemical oxidation of the phenol will take place. Also, theory predicts a reduction in the oxidation potentialmore » of the phenol by ~300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.« less
  • Structural studies of the intermediate S-states of the oxygen evolving complex (OEC) by X-ray absorption spectroscopy have suggested a structural motif for the tetranuclear manganese cluster consisting of a pair of bis oxo-bridged, high-valent Mn dimers [Mn{sup 2}({mu}-O){sup 2}]. In order to understand the possible roles for the Mn{sup 2}({mu}-O){sup 2} structural unit in the chemistry of the OEC, we have studied a series of model complexes that are also effective catalysts of H{sup 2}O{sup 2} disproportionation, reminiscent of the catalase activity in the S{sup 2}{yields}S{sup 0} cycle. Thus the [Mn(X-salpn)({mu}-O)]{sup 2} complexes are both structural and partial functional modelsmore » for the OEC. Here, we explore the energetics of proton-coupled electron transfer from protonated Mn{sup 2}({mu}-O){sup 2} units by studying the individual energy contributions of the oxidation and deprotonation steps for [Mn(X-salpn)({mu}-O)]{sup 2} with X = H, 3,5-di(Cl), and 3,5-di(NO{sup 2}). Providing a range of 8 pK{sup a} units and 800 mV for each oxidation or protonation state, these complexes allow quantitation of the critical role protons may play in the highly efficient oxidation of the OEC. 21 refs., 1 fig., 1 tab.« less