Heme protein-clay films: Direct electrochemistry and electrochemical catalysis

Stable protein-clay films were fabricated by casting an aqueous dispersion of protein and clay on pyrolytic graphite electrodes. Myoglobin (Mb), hemoglobin (Hb), and horseradish peroxidase (HRP) in clay films gave a pair of well defined, quasi-reversible cyclic voltammetric peaks at about -0.28 V vs SCE in pH 5.5 buffers, characteristic of the protein heme Fe-III/Fe-II redox couples. Square wave voltammograms (SWV) of the protein-clay films gave good fits by nonlinear, regression analysis to a model that featured thin-layer SWV and formal potential dispersion, providing average apparent heterogeneous electron-transfer rate constants, k(s), and average formal potentials, Edegrees'. UV-vis and reflectance absorption infrared spectra showed that the proteins in clay films retained near-native secondary structures. X-ray diffraction revealed that Mb-clay and Hb-clay films feature ordered layered structures with Mb and Hb intercalated between clay layers. Incorporated HRP induced disorder in the clay films. Oxygen, trichloroacetic acid, nitrite, and hydrogen peroxide were catalytically reduced by all three proteins in clay films.