Thermal Fluctuations Determine the Electron-Transfer Rates of Cytochrome c in Electrostatic and Covalent Complexes

• Published in: Chemphyschem 2010-04, Vol.11 (6), p.1225-1235
• Main Authors: Ly , Hoang Khoa, Marti , Marcelo A., Martin , Diego F., Alvarez-Paggi, Damian, Meister, Wiebke, Kranich, Anja, Weidinger, Inez M., Hildebrandt, Peter, Murgida, Daniel H.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Chemistry; Cytochromes c - chemistry; Electrodes; electron transfer; Electron Transport; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General and physical chemistry; Immobilized Proteins - chemistry; Kinetics; Molecular biophysics; molecular dynamics; Molecular Dynamics Simulation; monolayers; Oxidation-Reduction; Physical chemistry in biology; proteins; Spectrum Analysis, Raman; Static Electricity; Surface physical chemistry; Surface properties. Adsorption; Thermodynamics; time-resolved spectroscopy
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200900966

The heterogeneous electron‐transfer (ET) reaction of cytochrome c (Cyt‐c) electrostatically or covalently immobilized on electrodes coated with self‐assembled monolayers (SAMs) of ω‐functionalized alkanethiols is analyzed by surface‐enhanced resonance Raman (SERR) spectroscopy and molecular dynamics (MD) simulations. Electrostatically bound Cyt‐c on pure carboxyl‐terminated and mixed carboxyl/hydroxyl‐terminated SAMs reveals the same distance dependence of the rate constants, that is, electron tunneling at long distances and a regime controlled by the protein orientational distribution and dynamics that leads to a nearly distance‐independent rate constant at short distances. Qualitatively, the same behavior is found for covalently bound Cyt‐c, although the apparent ET rates in the plateau region are lower since protein mobility is restricted due to formation of amide bonds between the protein and the SAM. The experimental findings are consistent with the results of MD simulations indicating that thermal fluctuations of the protein and interfacial solvent molecules can effectively modulate the electron tunneling probability. Interfacial electron‐transfer processes of cytochrome c covalently or electrostatically bound to electrodes coated with self‐assembled monolayers (SAMs) of ω‐functionalized alkanethiols are studied by time‐resolved surface‐enhanced resonance Raman spectroscopy and molecular dynamics simulations (see picture).