Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical

Comparing the Redox Properties of W32 and Y32 Generated Inside the Structurally Characterized α3W and α3Y Proteins

Starla D. Glover, Robin Tyburski, Li Liang, Cecilia Tommos, Leif Hammarström

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4 Citations (Scopus)

Abstract

Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°′) with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y (Ravichandran, K. R.; Zong, A. B.; Taguchi, A. T.; Nocera, D. G.; Stubbe, J.; Tommos, C. J. Am. Chem. Soc. 2017, 139, 2994-3004). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°′(W32(N•+/NH)) of 1293 mV, and a E°′(W32(N/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N/NH) is 109 ± 4 mV more oxidizing than Y32(O/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32 is presented, and analysis of decay kinetics show that W32 persists ∼104 times longer than aqueous W due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.

Original languageEnglish
Pages (from-to)185-192
Number of pages8
JournalJournal of the American Chemical Society
Volume140
Issue number1
DOIs
Publication statusPublished - Jan 10 2018

Fingerprint

Tryptophan
Oxidation-Reduction
Protons
Electrons
Proteins
Kinetics
Thermodynamics
Voltammetry
Proton transfer
Water
Absorption spectroscopy
Tyrosine
Absorption spectra
Hydrogen
Spectrum Analysis
Electrodes
Stabilization
Oxidation

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

@article{19a4a542e96147b9884f5d407d2e378c,
title = "Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W32 • and Y32 • Generated Inside the Structurally Characterized α3W and α3Y Proteins",
abstract = "Protein-based {"}hole{"} hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°′) with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y (Ravichandran, K. R.; Zong, A. B.; Taguchi, A. T.; Nocera, D. G.; Stubbe, J.; Tommos, C. J. Am. Chem. Soc. 2017, 139, 2994-3004). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°′(W32(N•+/NH)) of 1293 mV, and a E°′(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32 • is presented, and analysis of decay kinetics show that W32 • persists ∼104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.",
author = "Glover, {Starla D.} and Robin Tyburski and Li Liang and Cecilia Tommos and Leif Hammarstr{\"o}m",
year = "2018",
month = "1",
day = "10",
doi = "10.1021/jacs.7b08032",
language = "English",
volume = "140",
pages = "185--192",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
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TY - JOUR

T1 - Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical

T2 - Comparing the Redox Properties of W32 • and Y32 • Generated Inside the Structurally Characterized α3W and α3Y Proteins

AU - Glover, Starla D.

AU - Tyburski, Robin

AU - Liang, Li

AU - Tommos, Cecilia

AU - Hammarström, Leif

PY - 2018/1/10

Y1 - 2018/1/10

N2 - Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°′) with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y (Ravichandran, K. R.; Zong, A. B.; Taguchi, A. T.; Nocera, D. G.; Stubbe, J.; Tommos, C. J. Am. Chem. Soc. 2017, 139, 2994-3004). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°′(W32(N•+/NH)) of 1293 mV, and a E°′(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32 • is presented, and analysis of decay kinetics show that W32 • persists ∼104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.

AB - Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°′) with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y (Ravichandran, K. R.; Zong, A. B.; Taguchi, A. T.; Nocera, D. G.; Stubbe, J.; Tommos, C. J. Am. Chem. Soc. 2017, 139, 2994-3004). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°′(W32(N•+/NH)) of 1293 mV, and a E°′(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32 • is presented, and analysis of decay kinetics show that W32 • persists ∼104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.

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