Electric-Field Control of the pH-Dependent Redox Process of Cytochrome c Immobilized on a Gold Electrode

The pH-dependent redox processes of cytochrome c (cyt c) immobilized on a gold electrode that was coated with a self-assembled monolayer (SAM) of mercaptounadecanoic acid (MUA) were studied by electrochemical methods combined with quartz crystal microbalance (QCM) and surface enhanced infrared absorption (SEIRA) spectroscopy. Variation of the solution pH in the range from 4.0 to 10.0 determines the surface charge of the SAM, for which an apparent pK(a) of 6.0 was determined, whereas the structure of the electrostatically bound cyt c remains largely unchanged. Thus, the pH-dependence of the interfacial redox process reflects the electric-field control of cyt c immobilization which in turn has a pronounced impact on the electron transfer process. In the pH range between 7.0 and 4.0, the electrostatic interactions with the cationic protein are weakened due to the protonation of the carboxyl headgroups of the SAM such that the immobilized protein remains highly mobile and can rapidly adopt the orientation which is most favorable for electron transfer. Thus, the rate constant for direct electron transfer remains unchanged in this pH range, but it decreases upon increasing the pH above 7.0. The dramatic slowdown of the interfacial electron transfer is attributed to the increased strength of electrostatic binding which traps the protein in an orientation that is unfavorable for electron exchange with the electrode. The present study demonstrates that the solution pH is an important parameter that allows for optimizing interfacial electron transfer processes of electrostatically bound proteins.