CHARACTERIZATION OF CYTOCHROME-C ALKANETHIOLATE STRUCTURES PREPARED BY SELF-ASSEMBLY ON GOLD

Composite monolayer structures comprised of cytochrome c strongly adsorbed to alkanethiolate self-assembled (SA) monolayers on sputter-deposited gold film electrodes, i.e., cyt c/HOOC(CH2)(n)S/Au (n = 5,10,15), were examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). Monolayer coverage of cytochrome c in a stable, functional, electroactive state was obtained in neutral phosphate buffer of low ionic strength for the thickest film (n = 15). Somewhat lower electroactive coverages were generally observed with the thinner SA monolayers (n = 5, 10). XPS revealed similar total coverage of adsorbed cytochrome c on all of the monolayers, however, suggesting the presence of some cytochrome c in a nonelectroactive conformation on the two thinner monolayers. The voltammetric responses were well-behaved and reasonably ideal although evidence for some dispersion of the formal potential value was apparent that is attributed in part to defect sites in the SA monolayer. The surface formal potential of cytochrome c, +215 mV vs NHE, is nearly identical to values previously reported for cytochrome c bound to physiological membranes. Electron transfer within the cyt c/HOOC(CH2)15S/Au structure appears to be strongly nonadiabatic, with a rate constant (k(et)-degrees) of 0.4 s-1, a postulated electron-transfer distance of ca. 28 angstrom, and a tunneling decay factor (beta) of 1.0 angstrom-1. The reorganization energy for electron transfer was estimated to be 0.35 (+/-0.15) eV. The potential usefulness of these composite monolayers for investigating biological electron transfer is highlighted.