Characterization of proton coupled electron transfer in a biomimetic oxomanganese complex: Evaluation of the DFT B3LYP level of theory

Ting Wang, Gary W Brudvig, Victor S. Batista

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Abstract

The capabilities and limitations of the Becke-3-Lee-Yang-Parr (B3LYP) density functional theory (DFT) for modeling proton coupled electron transfer (PCET) in the mixedvalence oxomanganese complex [(bpy)2 Mn III (μ-O)2 MnIV (bpy)2] 3+ (1; bpy ) 2,2'-bipyridyl) are analyzed. Complex 1 serves as a prototypical synthetic model for studies of redox processes analogous to those responsible for water oxidation in the oxygen-evolving complex (OEC) of photosystem II (PSII). DFT B3LYP free energy calculations of redox potentials and pKas are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pKas of the oxo-ligands depend strongly on the oxidation states of the complex, changing by approximately 10 pH units (i.e., from pH ~ ∼ 2to pH ~∼ 12) upon III,IV ⇁ III,III reduction of complex 1. These computational results are consistent with the experimental pKas determined by solution magnetic susceptibility and near-IR spectroscopy as well as with the pH dependence of the redox potential reported by cyclic voltammogram measurements, suggesting that the III,IV ⇁ III,III reduction of complex 1 is coupled to protonation of the di-μ-oxo bridge as follows: [(bpy) 2 MnIII (μ-O)2 MnIV (bpy) 2]3+ + H+ + e- ⇁ [(bpy) 2 MnIII (μ-O)(μ-OH)MnIII (bpy) 2]3+. It is thus natural to expect that analogous redox processes might strongly modulate the pKas of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.

Original languageEnglish
Pages (from-to)755-760
Number of pages6
JournalJournal of Chemical Theory and Computation
Volume6
Issue number3
DOIs
Publication statusPublished - Mar 9 2010

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biomimetics
Biomimetics
Density functional theory
Protons
electron transfer
density functional theory
Photosystem II Protein Complex
protons
Electrons
evaluation
Oxygen
Oxidation
oxidation
oxygen
Ligands
thermodynamic cycles
2,2'-Dipyridyl
Deprotonation
ligands
Water

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Computer Science Applications

Cite this

Characterization of proton coupled electron transfer in a biomimetic oxomanganese complex : Evaluation of the DFT B3LYP level of theory. / Wang, Ting; Brudvig, Gary W; Batista, Victor S.

In: Journal of Chemical Theory and Computation, Vol. 6, No. 3, 09.03.2010, p. 755-760.

Research output: Contribution to journalArticle

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abstract = "The capabilities and limitations of the Becke-3-Lee-Yang-Parr (B3LYP) density functional theory (DFT) for modeling proton coupled electron transfer (PCET) in the mixedvalence oxomanganese complex [(bpy)2 Mn III (μ-O)2 MnIV (bpy)2] 3+ (1; bpy ) 2,2'-bipyridyl) are analyzed. Complex 1 serves as a prototypical synthetic model for studies of redox processes analogous to those responsible for water oxidation in the oxygen-evolving complex (OEC) of photosystem II (PSII). DFT B3LYP free energy calculations of redox potentials and pKas are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pKas of the oxo-ligands depend strongly on the oxidation states of the complex, changing by approximately 10 pH units (i.e., from pH ~ ∼ 2to pH ~∼ 12) upon III,IV ⇁ III,III reduction of complex 1. These computational results are consistent with the experimental pKas determined by solution magnetic susceptibility and near-IR spectroscopy as well as with the pH dependence of the redox potential reported by cyclic voltammogram measurements, suggesting that the III,IV ⇁ III,III reduction of complex 1 is coupled to protonation of the di-μ-oxo bridge as follows: [(bpy) 2 MnIII (μ-O)2 MnIV (bpy) 2]3+ + H+ + e- ⇁ [(bpy) 2 MnIII (μ-O)(μ-OH)MnIII (bpy) 2]3+. It is thus natural to expect that analogous redox processes might strongly modulate the pKas of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.",
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AB - The capabilities and limitations of the Becke-3-Lee-Yang-Parr (B3LYP) density functional theory (DFT) for modeling proton coupled electron transfer (PCET) in the mixedvalence oxomanganese complex [(bpy)2 Mn III (μ-O)2 MnIV (bpy)2] 3+ (1; bpy ) 2,2'-bipyridyl) are analyzed. Complex 1 serves as a prototypical synthetic model for studies of redox processes analogous to those responsible for water oxidation in the oxygen-evolving complex (OEC) of photosystem II (PSII). DFT B3LYP free energy calculations of redox potentials and pKas are obtained according to the thermodynamic cycle formalism applied in conjunction with a continuum solvation model. We find that the pKas of the oxo-ligands depend strongly on the oxidation states of the complex, changing by approximately 10 pH units (i.e., from pH ~ ∼ 2to pH ~∼ 12) upon III,IV ⇁ III,III reduction of complex 1. These computational results are consistent with the experimental pKas determined by solution magnetic susceptibility and near-IR spectroscopy as well as with the pH dependence of the redox potential reported by cyclic voltammogram measurements, suggesting that the III,IV ⇁ III,III reduction of complex 1 is coupled to protonation of the di-μ-oxo bridge as follows: [(bpy) 2 MnIII (μ-O)2 MnIV (bpy) 2]3+ + H+ + e- ⇁ [(bpy) 2 MnIII (μ-O)(μ-OH)MnIII (bpy) 2]3+. It is thus natural to expect that analogous redox processes might strongly modulate the pKas of oxo and hydroxo/water ligands in the OEC of PSII, leading to deprotonation of the OEC upon oxidation state transitions.

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