Electron Transfer from Electrodes to Myoglobin: Facilitated in Surfactant Films and Blocked by Adsorbed Biomacromolecules

In previous work, greatly enhanced rates of electron transfer were found for myoglobin (Mb) in ordered films of surfactants on pyrolytic graphite (PG) electrodes. Direct electron transfer is now reported for Mb in films of didodecyldimethylammonium bromide (DDAB) on platinum, tin-doped indium oxide, and gold electrodes. Rates of electron transfer in these films were similar on all electrodes. In the absence of surfactant, electron transfer was observed on bare electrodes only when Mb was purified by chromatography, and only on hydrophilic tin-doped In2O3 or PG. Treatment of tin-doped In2O3 or PG electrodes with unpurified protein solutions blocked electron transfer to Mb in the purified solutions. Reflectance-absorbance infrared and X-ray photoelectron spectroscopy revealed proteinaceous adsorbates on electrodes exposed to unpurified solutions of Mb. This adsorbate blocks electron transfer to Mb and to ferricyanide in solution. Results suggest that electron transfer in the Mb-DDAB films may be facilitated partly by strong adsorption of surfactants on electrodes. Surfactant adsorbed at electrode-film interfaces appears to inhibit adsorption of macromolecules from Mb solutions which could otherwise block electron transfer between Mb and electrodes.