Electron Transfer and Adsorption of Myoglobin on Self-Assembled Surfactant Films: An Electrochemical Tapping-Mode AFM Study

We have studied myoglobin (Mb) on graphite basal plane and on self-assembled didodecyldimethylammonium bromide (DDAB) monoand multilayers with in situ tapping-mode AFM, cyclic voltammetry, and differential capacitance measurements. On graphite, Mb molecules adsorb and aggregate into chainlike features. The aggregation indicates an attractive interaction between the adsorbed molecules. In contrast, the molecules are randomly distributed on the DDAB layers. The adsorption on DDAB changes drastically the domains and defects in the DDAB layers due to a strong Mb-DDAB interaction. On both the bare and the DDAB-coated electrodes, the protein undergoes a fast electron-transfer reaction involving Fe3+ + 1eT Fe2+ in the heme group. The structure of the DDAB film is potential dependent. At low potentials, the film is in a solidlike phase. When the potential is raised to ∼0 V, the film transforms into a liquidlike phase via a firstorder phase transition. The liquidlike phase may be responsible for the fast diffusion of Mb through the DDAB layers.