AVS 53rd International Symposium
    Biomaterial Interfaces Friday Sessions
       Session BI-FrM

Invited Paper BI-FrM1
Probing Relations Between Molecular Orientation and Electron Transfer in Immobilized Metalloprotein Films Using Frequency-Domain, Planar Waveguide Spectroelectrochemistry

Friday, November 17, 2006, 8:00 am, Room 2014

Session: Biomolecular Surface Characterization II
Presenter: S. Saavedra, University of Arizona
Authors: S. Saavedra, University of Arizona
Z. Oraci, University of Arizona
A.F. Runge, University of Arizona
W.J. Doherty, University of Arizona
S.B. Mendes, University of Arizona
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This talk will describe efforts to develop a better understanding of the relationship between structure of a protein film immobilized on an electrode surface and its electrochemical activity, which is a prerequisite to design of protein-based molecular devices in which vectorial, heterogeneous electron transfer is required for efficient operation. The relationship between molecular orientation and electron transfer in immobilized films of redox-active proteins is being investigated using planar waveguide spectroelectrochemistry. This approach has been used to determine the porphyrin tilt angle distribution in a cytochrome c submonolayer adsorbed to an indium-tin oxide (ITO) electrode. However, only about half of the film is electroactive, which makes to difficult to correlate the broad orientation distribution (measured spectroscopically on the entire film) with the electron transfer rate (measured electrochemically on the electroactive portion of the film). To address this problem, a novel form of electroreflectance spectroscopy, potential-modulated, attenuated total reflection spectroscopy (PM-ATR), has been developed. In PM-ATR, the waveguide output is monitored while an ac potential modulation is simultaneously applied to the planar waveguide electrode. Changes in the absorbance as a function of the light polarization, modulation frequency, and amplitude provide information about electron transfer rates, electro-optical switching rates, and molecular orientation. For cytochrome c films on ITO, the electron transfer rate measured using TM polarized light was four-fold greater than that measured using TE polarized light, which is consistent with a shorter tunneling distance for molecules adsorbed in a vertical orientation (probed with TM) vs. molecules adsorbed in a horizontal orientation (probed with TE). These data are the first to correlate a distribution of molecular orientations with a distribution of electron transfer rates in a redox-active molecular film.