Long-range electron transfer with myoglobin immobilized at Au/mixed-SAM junctions: mechanistic impact of the strong protein confinement.
Abstract: Horse muscle myoglobin (Mb) was tightly immobilized at Au-deposited ~15-Å-thick mixed-type (1:1) alkanethiol SAMs, HS-(CH₂)₁₁-COOH/HS-(CH₂)₁₁-OH, and placed in contact with buffered H₂O or D₂O solutions. Fast-scan cyclic voltammetry (CV) and a Marcus-equation-based analysis were applied to determine unimolecular standard rate constants and reorganization free energies for electron transfer (ET), under variable-temperature (15-55 °C) and -pressure (0.01-150 MPa) conditions. The CV signal was surprisingly stable and reproducible even after multiple temperature and pressure cycles. The data analysis revealed the following values: standard rate constant, 33 s⁻¹ (25 °C, 0.01 MPa, H₂O); reorganization free energy, 0.5 ± 0.1 eV (throughout); activation enthalpy, 12 ± 3 kJ mol⁻¹; activation volume, -3.1 ± 0.2 cm³ mol⁻¹; and pH-dependent solvent kinetic isotope effect (k(H)⁰/k(D)⁰), 0.7-1.4. Furthermore, the values for the rate constant and reorganization free energy are very similar to those previously found for cytochrome c electrostatically immobilized at the monocomponent Au/HS-(CH₂)₁₁-COOH junction. In vivo, Mb apparently forms a natural electrostatic complex with cytochrome b₅ (cyt-b₅) through the "dynamic" (loose) docking pattern, allowing for a slow ET that is intrinsically coupled to the water's removal from the "defective" heme iron (altogether shaping the biological repair mechanism for Mb's "met" form). In contrary, our experiments rather mimic the case of a "simple" (tight) docking of the redesigned (mutant) Mb with cyt-b₅ (Nocek et al. J. Am. Chem. Soc. 2010, 132, 6165-6175). According to our analysis, in this configuration, Mb's distal pocket (linked to the "ligand channel") seems to be arrested within the restricted configuration, allowing the rate-determining reversible ET process to be coupled only to the inner-sphere reorganization (minimal elongation/shortening of an Fe-OH₂ bond) rather than the pronounced detachment (rebinding) of water and, hence, to be much faster.
Publication Date: 2014-01-13 PubMed ID: 24369906DOI: 10.1021/jp4101569Google Scholar: Lookup
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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This research paper presents the study of electron transfer (ET) in horse muscle myoglobin (Mb) under extreme conditions. The paper also entails comparisons of these results with previously studied Mb mechanisms in other environments.
Research Context and Methodology
- The researchers immobilized horse muscle myoglobin (Mb) at gold-deposited alkanethiol self-assembled monolayers (SAMs) and exposed them to water or heavy water solutions.
- They used fast-scan cyclic voltammetry (CV), a technique for analyzing how a system’s reaction rate changes over potential, along with a Marcus-equation-based analysis to find the unimolecular standard rate constants and reorganization free energies for electron transfer (ET).
- The tests were conducted under variable temperature and pressure conditions, and the CV signal was observed to be impressively consistent and reproducible.
Findings and Analysis
- The standard rate constant, reorganization free energy, activation enthalpy, activation volume, and solvent kinetic isotope effect (k(H)⁰/k(D)⁰) were systematically recorded and analyzed.
- Results revealed that Mb’s distal pocket (which is linked to the “ligand channel”) seems to be restrained within a limited configuration, suggesting that the primary process in this setup is the inner-sphere reorganization, specifically the minimal changes of an Fe-OH₂ bond.
- The team discovered that their findings were similar to those from a prior study examining cytochrome c immobilized at the gold/HS-(CH₂)₁₁-COOH junction. The rate constant and reorganization free energy were found to be very alike in both studies.
Comparison with Previous Studies
- In the natural physiological environment, Mb forms a dynamic complex with cytochrome b₅, enabling a slower ET.
- This natural process couples the ET with the removal of water from the “defective” heme iron, forming an integral part of the biological repair mechanism for the “met” form of myoglobin.
- Contrarily, the research experiments mirrored a simplified docking scenario of the redesigned (mutant) Mb with cytochrome b₅.
Implications
- The results provided new insights into the coupling process of horse muscle myoglobin (Mb), especially in variations of its docking mechanisms with cytochrome b₅.
- The findings also enriched the understanding of the energetics and dynamics of long-range electron transfers in biomolecular systems.
Cite This Article
APA
Khoshtariya DE, Dolidze TD, Shushanyan M, van Eldik R.
(2014).
Long-range electron transfer with myoglobin immobilized at Au/mixed-SAM junctions: mechanistic impact of the strong protein confinement.
J Phys Chem B, 118(3), 692-706.
https://doi.org/10.1021/jp4101569 Publication
Researcher Affiliations
- Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg , 91058 Erlangen, Germany.
MeSH Terms
- Animals
- Cytochromes c / metabolism
- Deuterium Oxide / chemistry
- Electrochemistry
- Electron Transport
- Heme / metabolism
- Horses
- Hydrogen-Ion Concentration
- Immobilized Proteins / chemistry
- Immobilized Proteins / metabolism
- Kinetics
- Models, Molecular
- Myoglobin / chemistry
- Myoglobin / metabolism
- Pressure
- Protein Conformation
- Temperature
- Thermodynamics
Citations
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