Ordered electrochemically active films of hemoglobin, didodecyldimethylammonium ions, and clay

Ordered films were made by casting aqueous dispersions of didodecyldimethylammonium bromide (DDAB)-clay composite and the protein hemoglobin (Hb). Cyclic voltammetry of Hb-DDAB-clay films on pyrolytic graphite (PG) electrodes showed a pair of well-defined and nearly reversible peaks at about -0.06 V vs NHE in pH 5.5 buffers, characteristic of the four Hb heme Fe-III/Fe-II redox couples. These composite films showed slow, small changes in electrochemical properties over 10 days storage in buffers, and excellent stability during further storage. Interlayer clay spacing estimated by X-ray diffraction suggested that the Hb was partly, but not fully, intercalated between clay platelets. Only a fraction of the DDAB surfactant in the films resided in bilayers. Soret absorption band positions suggested that Hb retains its native conformation in these films. The average formal potential of Hb heme Fe-III/Fe-II couples in DDAB-clay films shifted linearly between pH 4.5 and 11 with a slope of -52 mV pH(-1), suggesting that one proton is coupled with each electron transfer. Trichloroacetic acid was catalytically reduced by Hb-DDAB-clay film electrodes.