Correlating molecular orientation distributions and electrochemical kinetics in subpopulations of an immobilized protein film

Understanding the relationship between the structure and electrochemical activity of a protein film immobilized on an electrode surface is a prerequisite to the rational design of protein-based bioelectronic devices. In a monolayer of horse heart cytochrome c (cyt c) adsorbed to an indium-tin oxide (ITO) electrode, only about half of the film is electroactive, which makes it difficult to correlate the broad orientation distribution (measured spectroscopically on the entire film) with the electron transfer rate constant (measured electrochemically on the electroactive portion of the film) [Runge, A. F.; Mendes, S. B.; Saavedra, S. S. J. Phys. Chem. B 2006, 110, 6732-6739]. To address this problem, a novel form of electroreflectance spectroscopy, potential-modulated, attenuated total reflectance (PM-ATR), is used to monitor changes in the absorbance of a cyt c while a modulated potential is simultaneously applied to the ITO-coated, planar waveguide electrode. From measurements as a function of the light polarization and modulation frequency, electron transfer rate constants for differently oriented subpopulations of cyt c molecules are obtained. The apparent rate constant measured using transverse magnetic (TM) polarized light was 3.3-fold greater than that measured using transverse electric (TE) polarized light, while the rate constant measured electrochemically was intermediate between the TM and TE constants. These data are consistent with a shorter heme-electrode tunneling distance for molecules adsorbed in a vertical orientation (probed with TM) versus molecules adsorbed in a horizontal orientation (probed with TE). This is the first study to correlate a distribution of molecular orientations with a distribution of electron transfer rate constants in a redox-active molecular film.