Marcus-type driving force correlations reveal the mechanism of proton-coupled electron transfer for phenols and [Ru(bpy)3]3+ in water at low pH

Janne Soetbeer, Prateek Dongare, Leif Hammarström

Research output: Contribution to journalArticle

12 Citations (Scopus)

Abstract

Proton-coupled electron transfer (PCET) from tyrosine and other phenol derivatives in water is an important elementary reaction in chemistry and biology. We examined PCET between a series of phenol derivatives and photogenerated [Ru(bpy)3]3+ in low pH (≤4) water using the laser flash-quench technique. From an analysis of the kinetic data using a Marcus-type free energy relationship, we propose that our model system follows a stepwise electron transfer-proton transfer (ETPT) pathway with a pH independent rate constant at low pH in water. This is in contrast to the concerted or proton-first (PTET) mechanisms that often dominate at higher pH and/or with buffers as primary proton acceptors. The stepwise mechanism remains competitive despite a significant change in the pKa and redox potential of the phenols which leads to a span of rate constants from 1 × 105 to 2 × 109 M-1 s-1. These results support our previous studies which revealed separate mechanistic regions for PCET reactions and also assigned phenol oxidation by [Ru(bpy)3]3+ at low pH to a stepwise PCET mechanism.

Original languageEnglish
Pages (from-to)4607-4612
Number of pages6
JournalChemical Science
Volume7
Issue number7
DOIs
Publication statusPublished - 2016

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Phenols
Protons
Electrons
Water
Phenol
Rate constants
Derivatives
Proton transfer
Free energy
Tyrosine
tris(2,2'-bipyridine)ruthenium II
Buffers
Oxidation
Kinetics
Lasers

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Marcus-type driving force correlations reveal the mechanism of proton-coupled electron transfer for phenols and [Ru(bpy)3]3+ in water at low pH. / Soetbeer, Janne; Dongare, Prateek; Hammarström, Leif.

In: Chemical Science, Vol. 7, No. 7, 2016, p. 4607-4612.

Research output: Contribution to journalArticle

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N2 - Proton-coupled electron transfer (PCET) from tyrosine and other phenol derivatives in water is an important elementary reaction in chemistry and biology. We examined PCET between a series of phenol derivatives and photogenerated [Ru(bpy)3]3+ in low pH (≤4) water using the laser flash-quench technique. From an analysis of the kinetic data using a Marcus-type free energy relationship, we propose that our model system follows a stepwise electron transfer-proton transfer (ETPT) pathway with a pH independent rate constant at low pH in water. This is in contrast to the concerted or proton-first (PTET) mechanisms that often dominate at higher pH and/or with buffers as primary proton acceptors. The stepwise mechanism remains competitive despite a significant change in the pKa and redox potential of the phenols which leads to a span of rate constants from 1 × 105 to 2 × 109 M-1 s-1. These results support our previous studies which revealed separate mechanistic regions for PCET reactions and also assigned phenol oxidation by [Ru(bpy)3]3+ at low pH to a stepwise PCET mechanism.

AB - Proton-coupled electron transfer (PCET) from tyrosine and other phenol derivatives in water is an important elementary reaction in chemistry and biology. We examined PCET between a series of phenol derivatives and photogenerated [Ru(bpy)3]3+ in low pH (≤4) water using the laser flash-quench technique. From an analysis of the kinetic data using a Marcus-type free energy relationship, we propose that our model system follows a stepwise electron transfer-proton transfer (ETPT) pathway with a pH independent rate constant at low pH in water. This is in contrast to the concerted or proton-first (PTET) mechanisms that often dominate at higher pH and/or with buffers as primary proton acceptors. The stepwise mechanism remains competitive despite a significant change in the pKa and redox potential of the phenols which leads to a span of rate constants from 1 × 105 to 2 × 109 M-1 s-1. These results support our previous studies which revealed separate mechanistic regions for PCET reactions and also assigned phenol oxidation by [Ru(bpy)3]3+ at low pH to a stepwise PCET mechanism.

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