The electron transfer (ET) scheme of cytochrome c (cyt. c) coupled to carboxylic acid-terminated alkanethiol self-assembled monolayers (SAMs) on well-defined gold (111) electrodes is a simplified model system to investigate both long range and intermolecular ET processes. The advantages of an electrochemical approach to investigate the ET mechanism are that one can both regulate the ET path length by using alkanethiol SAMs of varying chain lengths and deconvolute the intermolecular ET event at the interface from the intramolecular ET event. It has been shown that the interactions between cyt. c and the carboxylate termini are electrostatic in nature, analogous to those between cyt. c and negatively charged proteins such as cytochrome c peroxidase. In the present work, the effects of alkanethiol chain length, ionic strength, pH, and viscosity of supporting electrolyte on the ET kinetics were studied. The ET rates through long alkanethiol chains were observed to be slow because electron tunneling through the alkyl chain was the rate-limiting step in the process. On the other hand, the ET rate through shorter chain alkanethiols appeared to be independent of chain length, and the effect of ionic strength and pH on the observed ET rates was insignificant. It is proposed that the rate-limiting ET step through short alkyl chains results from a configurational rearrangement process preceding the ET event, and that its rate is 2.6 × 10 3 s -1 . This "gating" process arises from a rearrangement of the cyt. c from a stable binding form (binding complex) on the carboxylic acid terminus to a configuration (ET complex) which facilitates the most efficient ET pathway. The rate of the configurational rearrangement reaction that precedes the ET reaction was found to be markedly influenced by solution viscosity, but its equilibrium constant was independent of solution viscosity. The change in the configurational rearrangement reaction rate with solution viscosity follows a modified Kramers equation.

A preliminary model is presented that describes the observed surface-enhanced electronic Raman scattering spectrum of self-assembled n-alkanethiol monolayers on roughened Au surfaces. At room temperature, a series of intense resonance Raman spectral lines, superimposed on the normal surface-enhanced Raman vibrational spectrum, are produced when monolayers of CH 3 (CH 2 ) m SH (m ) 8 to 17) are illuminated with laser radiation between 630 and 740 nm. We describe the effects of isotopic substitution, and of varying both the alkane chain length and the temperature of the monolayer films.

The experimental observation of electronic Raman scattering between image potential surface states is described and has been observed from roughened Au and Ag surfaces covered with long-chain alkanethiol self-assembled monolayers. To simulate the effect of surface roughness, interactions of the image state electron are modeled using a potential that incorporates two orthogonal metal surface planes, as would be expected near a step edge. Although the Raman spectra indicate that the image state electron exists within the film, modeling suggests more specifically that the electron is constrained to move within the sulfur headgroup layer. Although it is commonly accepted that the headgroup binding and structure on these surfaces is identical for both the short and the long chain alkanethiol homologues, these results highlight dramatic differences in the Au/S interfacial electronic structure between these two chain length regimes at room temperature.