| Abstract
| - Using nanometer thick tunneling barriers with specifically attached cytochrome c, the electron-transfer rate constant was studied as a function of the SAM composition (alkane versus terthiophene), theω-terminating group type (pyridine, imidazole, nitrile), and the solution viscosity. At large electrode−reactantseparations, the pyridine terminated alkanethiols exhibit an exponential decline of the rate constant withincreasing electron-transfer distance. At short separations, a plateau behavior, analogous to systemsinvolving −COOH terminal groups to which cytochrome c can be attached electrostatically, is observed.The dependence of the rate constant in the plateau region on system properties is investigated. The rateconstant is insensitive to the mode of attachment to the surface but displays a significant viscositydependence, change with spacer composition (alkane versus terthiophene), and nature of the solvent (H2Oversus D2O). Based on these findings and others, the conclusion is drawn that the charge-transfer rateconstant at short distance is determined by polarization relaxation processes in the structure, rather thanthe electron tunneling probability or large-amplitude conformational rearrangement (gating). The transitionin reaction mechanism with distance reflects a gradual transition between the tunneling and frictionalmechanisms. This conclusion is consistent with data from a number of other sources as well.
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