Some bioelectrochemical applications of self-assembled films on mercury

The homogeneous, defect-free surface of a hanging mercury drop electrode (HMDE) is used to self-assemble films apt to the investigation of problems of biological interest. The electrochemical behavior in the dark of a film of chlorophyll (Chl) molecules self-assembled on mercury and the peculiar features of the photocurrents recorded upon illuminating this film at different applied potentials are discussed. The limiting flux of hydrated Tl+ ions through the channel-forming polypeptide gramicidin (GR), incorporated in a dioleoylphosphatidylcholine (DOPC) monolayer self-assembled on mercury, is shown to be controlled both by diffusion and by a dehydration step, while the limiting flux of dehydrated Tl+ ions stemming from Tl amalgam electrooxidation is exclusively controlled by the diffusion of thallium atoms within the amalgam. The photocurrents of purple membrane (PM) fragments adsorbed on a DOPC monolayer and on a mixed alkanethiol/DOPC bilayer self-assembled on mercury are examined to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The photocurrents on the mixed bilayer are interpreted on the basis of a simple equivalent circuit and their pH dependence is analyzed to estimate the pK(a) values of the transitions releasing protons to, and taking up protons from the solution.