Electron transfer and adsorption of myoglobin on self-assembled surfactantfilms: An electrochemical tapping-mode AFM study

We have studied myoglobin (Mb) on graphite basal plane and on self-assemble d didodecyldimethylammonium bromide (DDAB) mono- and multilayers with in si tu tapping-mode AFM, cyclic voltammetry, and differential capacitance measu rements. On graphite, Mb molecules adsorb and aggregate into chainlike feat ures. The aggregation indicates an attractive interaction between the adsor bed molecules. In contrast, the molecules are randomly distributed on the D DAB layers. The adsorption on DDAB changes drastically the domains and defe cts in the DDAB layers due to a strong Mb-DDAB interaction. On both the bar e and the DDAB-coated electrodes, the protein undergoes a fast electron-tra nsfer reaction involving Fe3+ + le(-) <-> Fe2+ in the heme group. The struc ture of the DDAB film is potential dependent. At low potentials, the film i s in a solidlike phase. When the potential is raised to similar to 0 V, the film transforms into a liquidlike phase via a first-order phase transition . The liquidlike phase may be responsible for the fast diffusion of Mb thro ugh the DDAB layers.