Adsorption and interfacial electron transfer of Saccharomyces cerevisiae yeast cytochrome c monolayers on Au(111) electrodes

We have studied the adsorption and electron-transfer dynamics of Saccharomyces cerevisiae (yeast) iso-1-cytochrome c adsorbed on Au(111) electrodes in aqueous phosphate buffer media. This cytochrome possesses a thiol group close to the protein surface (Cys102) suitable for linking the protein to gold without drastic protein unfolding. A comprehensive approach, based on linear sweep and differential pulse voltammetry, capacitance measurements, X-ray photoelectron spectroscopy (XPS), in situ scanning tunneling microscopy (STM), and microcantilever sensor (MCS) techniques has been used. The voltammetric data display a thiol reductive desorption signal corresponding to close to monolayer coverage. Reductive desorption is also reflected in a capacitance peak. Voltammetric signals from the heme group in both native and partially denatured states could also be detected. XPS shows clear Au-S bond formation, but this observation is not conclusive for aqueous buffer conditions, as the protein is extensively unfolded under ultrahigh vacuum conditions needed for XPS. In situ STM discloses clear sub-monolayer coverage to molecular resolution. Imaging is robust in a 0.2 V electrochemical potential range negative of the equilibrium potential of YCC, where the protein is electrochemically functional. The MCS data show tensile differential stress signals when YCC is adsorbed on a gold-coated MCS, with distinguishable adsorption phases in the time range from <10(2) s to several thousand seconds. Comprehensive approaches to the mapping of adsorbed functional redox metalloproteins toward the single-molecule level, such as in the present study, will be important in the construction of nanoscale devices for multifarious biological and environmental screening.